Space Exploration  
 

   In the seventh decade of the 20th century, people from earth built a machine that set two of them on the moon. These explorers had come from a contentious world rapidly growing in population and in the technological power that had made the moon trip possible. They returned to that world, leaving behind a plaque containing the declaration:

HERE MEN FROM THE PLANET EARTH
FIRST SET FOOT UPON THE MOON
JULY 1969, a. d.
WE CAME IN PEACE FOR ALL MANKIND

   Their brief visit to the moon marked the end of one story of space exploration and the beginning of another -- an end because this trip, designated Apollo 11, was the successful culmination of a decade-long effort to land people on the moon, a beginning because it gave people confidence to go on to greater tasks. It has been predicted that humankind will go on to explore and inhabit other planets, other moons, and asteroids; and that eventually they will modify and control the conditions of the entire solar system. This article will discuss the technology that made possible the successful Apollo 11 mission as well as various unmanned space activities. More recent developments are covered in the articles Space Operation Military; Space Shuttle; and Space Station.
 

A Moonship: Apollo/Saturn

   A space vehicle consists of two main parts: the launch vehicle and the spacecraft itself. The launch vehicle is the rocket that lifts the space vehicle off the ground and thrusts it into orbit around the earth or into some other path of flight, called a trajectory. It is usually made up of a series of rockets, called stages, stacked one atop another. The spacecraft is the part that carries the payload, that is, the person and/or objects needed to fulfill the purpose of the mission.
   The launch vehicle used for the Apollo 11 mission was the Saturn V, which had three stages. The Apollo spacecraft consisted of three principal parts: the command module, the service module, and the lunar module. This same configuration, the combination of the Saturn V rocket and the Apollo spacecraft, was used for all U.S. manned lunar missions. Before the lunar landing, various components of the Apollo/Saturn configuration had been tested in Apollo missions that orbited the earth or looped around the moon without landing on the moon's surface.
 

Saturn V

   Before the launch or liftoff from the earth, the three massive stages of the Saturn V rocket were topped by the lunar landing module parked within a skirt (called the adapter) forward of the third stage, the service module containing an engine and many instruments, the command module housing the astronauts, and an emergency escape rocket at the pinnacle. This assembly, including the propellants (fuel and oxidizer), weighed nearly 3 million kilograms (6 million pounds). The huge first stage, the S-1C booster rocket, weighed 2 million kilograms (5 million pounds) with its propellants; when empty it weighed only about 130,950 kilograms (288,700 pounds). Its five F-1 engines weighed only 9,026 kilograms (19,900 pounds) each, although each was capable of supplying ' million kilograms (1 million pounds) of thrust to lift the Apollo vehicle. The S-1C consisted chiefly of propellants, their thin metal tanks, combustion chambers, and pumps.
   The main function of the first stage rocket was to combine the 647,300 kilograms (1,427,000 pounds) of kerosine (fuel)  and 1,500,300 kilograms (3,308,000 pounds) of liquid oxygen (oxidizer) at a high rate to provide the powerful thrust needed to launch the space vehicle and to overcome the pull of the earth's gravity. The complexities of such a rocket are many. The propellants must be fed into the several engines simultaneously. They must be kept burning evenly and stably in the combustion chambers. The base of the vehicle and the propellant tanks must be prevented from overheating. The engines must be gimbaled for directional control during launch so as to prevent winds and slight misalignments of the rocket's thrust from moving the launch vehicle off course. (Each rocket engine is mounted on a gimbal, a device with two axes of rotation that intersect at right angles to allow free movement in any direction.)
   The second stage rocket, the S-2, had at its base a cluster of five J-2 engines, each capable of providing 90,720 kilograms (200,000 pounds) of thrust. Four of the engines, equally spaced about a central fixed one, could be gimbaled through a square pattern of 7 for flight control.
   A single J-2 engine provided enough thrust to power the third stage, the S-4B. This stage weighed only 11,340 kilograms (25,000 pounds) empty and 118,840 kilograms (262,000 pounds) when filled with propellant -- this only about 1/20 of the overall weight of the first stage. A third-stage rocket need not be as heavy or powerful as the earlier stages; by the time the third stage's propellants are combined, the earlier stages have already used up their propellants, performed their task of thrust for acceleration, and have been discarded. The multistaged rocket is a very efficient design for placing a heavy load in orbit because it allows each successive stage to be discarded after its propellants have been used up, thereby lightening the overall load and permitting higher velocities. (See also Rocket.)
 

Instrument Unit

   Attached to the top of the third stage of Saturn V was an instrument unit 0.9 meter (3 feet) high and 9.8 meters (28.6 feet) wide. Arranged around the inner face of this ring was the navigation, guidance, and control equipment needed to direct the rocket into earth orbit and then into the maneuver called translunar injection. This maneuver sends the command, service, and lunar modules on the trajectory to the moon. The ring also carried telemetry and tracking equipment, electrical power units, and a cooling system.
The instrument unit directed Saturn V through the many steps of the mission. It kept check on the workings of all components and generated electrical signals to the rocket-engine controls to alter the direction of thrust in order to keep the vehicle on the path into orbit that would use the least propellant. The instrument unit could detect emergencies, such as the failure of an engine, and could then direct the remaining engines functioning in a cluster to compensate for the failure by burning the propellants for a longer time. It activated the small, 32.6-kilogram (72-pound) engines that separated the stages when it was time to discard a stage.
   Altogether there were 41 rocket engines in Saturn V. Each of the small, solid-propellant rockets (of which there were eight), provided almost 39,900 kilograms (88,000 pounds) of thrust for 0.6 seconds to separate the first stage from the vehicle; four rockets, each providing 9,520 kilograms (21,000 pounds) of thrust, were used to separate the second stage. There were also small rockets, called ullage rockets, which could be fired to provide a slight forward acceleration to shift propellant in a tank in order to make it flow smoothly into the pumps. All of these rockets could be controlled by the instrument unit.
   The heart of the instrument unit was an inertial platform, a gyroscopic device that maintains its orientation in space. This device measures change of motion of the vehicle along three axes or directions (that is, it detects any change of motion). A computer records these changes and adds them up once each second to calculate the position and velocity of the vehicle. A device called a guidance signal processer is associated with the computer; when the position and velocity are determined, the processer issues signals to correct the flight path. (See also Gyroscope; Inertial Guidance.)
   Had the inertial platform failed, for example, by loss of electrical power, the guidance system in the Apollo command module could have taken over and directed the launch vehicle automatically. After ignition of the second stage, the astronauts themselves, with the aid of a chart, could guide the whole vehicle by aligning its direction of thrust. Laboratory simulations of such guidance had indicated that the Apollo pilot would be able to "fly" the launch vehicle into orbit, under most conditions, almost as accurately as the automatic system of the instrument unit.
 

Apollo Spacecraft

   The Apollo spacecraft had an overall length of 25 meters (82 feet) and included the escape tower, the command module (CM), the service module (SM), and the adapter skirt, which protected the lunar module (LM) nested within it during the flight through the atmosphere. The modules were extremely complex. The CM alone, exclusive of wires and structural components, had nearly two million functional parts. It contained 24 kilometers (15 miles) of wire and had a panel display that included 24 instruments, 566 switches, 40 event-indicators, and 71 lights. The entire spacecraft employed 50 rocket engines: 12 reaction-control units on the CM and 16 each on the SM and LM, the descent engine and the ascent engine on the LM,  the rocket used to jettison the escape tower, and three-pitch-control motors. (All but the pitch-control rockets, which used solid propellant, burned the hypergolic propellants nitrogen tetroxide and hydrazine. Hypergolic propellants ignite spontaneously when they are brought together.)
 

Command Module

   The CM, basic living and working quarters for the astronauts journeying to the moon, consisted of a pressurized inner shell encased in an outer heat shield. The inner shell was formed of a layer of aluminum alloy that resembled a honeycomb between sheets of another form of aluminum alloy. The outer shield was formed of brazed stainless-steel honeycomb holding an internal layer of phenolic epoxy resin. The thickness of the resin ranged from 17.8 millimeters (0.7 inch) at the apex of the cone of the module to 68.6 millimeters (2.7 inches) at the broad, blunt reentry face; the variation of thickness was required to withstand the varying intensities of heat that would be caused by friction on the surface of the module as it reentered the earth's atmosphere upon return from the moon. On heating, such a resin layer melts and vaporizes to form a gas sheath that limits the heating of the surface and dissipates the heat. This process is called ablation.
   The CM contained a complete inertial navigation system for guidance and control. The system consisted of an inertial platform similar to the one in the instrument unit, a computer that compared flight data with stored information on correct flight paths and then issued control data, and a scanning telescope and sextant to be used by the crew to take navigational fixes by sighting the stars. In an emergency the crew could have used this system to return to earth and would have needed only on-board information.
Both the CM and the SM used a set of small rockets to adjust the altitude or pointing direction of the craft. The SM rockets could also be used to change the velocity slightly. The rocket set was a redundant system. (A redundant system is one that has secondary parts that can take over in case of malfunction of some primary part.)
   An environmental control system regulated the atmospheric pressure and temperature of the CM, keeping a pressure of 0.35 kilograms per square centimeter (5 pounds per square inch) of pure oxygen at a temperature of 22C. to 24C. (70F. to 75F.). Space has no atmosphere and hence no oxygen for breathing and little external pressure. The body fluids of a person exposed to space would boil because the external pressure is too low to keep them in a liquid state. The astronauts' spacesuits were also controlled by the environmental control system when they were donned in the module for critical phases of the voyages.
   The system removed carbon dioxide exhaled by the astronauts by filtering the atmosphere through lithium hydroxide; it removed odors by means of charcoal filters. It also collected and stored water produced by fuel cells, supplied it to the astronauts for drinking and to glycol evaporators for cooling, and poured the excess overboard through the urine-dump valve. The system controlled the temperature of various pieces of equipment by regulating heat exchangers, radiators, and evaporators. The environmental control system could operate safely for 14 days.
   The CM also housed controls for the service module, the attitude-control rockets, a variety of subsidiary systems, and communications. It included a system to detect emergencies, send signals, and activate the launch-escape system. There was another system that sent caution and warning signals in case of any malfunction or irregularity in equipment. Finally there were the parachutes and recovery aids to be used during the reentry and landing on the earth.
 

Service Module

   Fuel cells in the SM supplied power most of the time. In these cells hydrogen and oxygen gas reacted to produce electricity and water. Tanks in the SM stored the hydrogen and oxygen. Silver-zinc oxide storage batteries provided power for the CM during reentry and after landing. Separate batteries powered explosive devices to separate the CM and SM and to separate the Saturn V third stage.
   One of the most important major systems of Apollo 11 was located in the SM. This was the service propulsion system (SPS), with an engine that slowed down the spacecraft as it passed behind the moon for the first time (by firing in a forward direction) in order to place the spacecraft in orbit around the moon. The SPS engine would have powered the CM to rescue the LM should the latter have been in a bad orbit after it separated for descent to the moon. The engine also powered the spacecraft back to earth and made any large midcourse corrections necessary. It responded either to automatic firing commands from the guidance and navigation system or to crew commands. It fired at a thrust of 9,300 kilograms (20,500 pounds), could be fired many times, for short or long periods, and was gimbaled for directional control. It carried redundant propellant valves and firing circuits to ensure firing should one set fail. (Because of a rupture in an oxygen tank on the 1970 Apollo 13 flight, the SM fuel cells could not power the SPS engine, so the LM descent engine had to be fired instead to cause the spacecraft to loop around the moon and head earthward.)
 

Lunar Module

   The two states of the LM, the ascent and descent stages, made up a self-contained spacecraft and launch vehicle with a total weight of 15,060 kilograms (33,205 pounds). Both stages were built of intricately ma  chined and welded aluminum-alloy panels and beams. The entire ascent stage was encased in a single aluminum skin and a shield to protect the craft from heat radiation and the impact of micrometeoroids. The shield was formed of many layers of thin aluminized mylar plastic sheet. The ascent stage, 3.6 meters (12 feet) high and roughly 4.3 meters (14 feet) in circumference, contained the pressurized crew compartments supplied with pure oxygen. Its environmental control system controlled cabin conditions and serviced the water and oxygen for the portable life support systems. It also contained silver-zinc batteries for electrical power and equipment for communication with the earth and with the CM, a redundant reaction-control system of four clusters of four 45-kilogram (100-pound)-thrust engines mounted on outriggers 90 degrees apart, a warning system similar to that in the CM, tracking and docking lights and a docking ring atop the vehicle, and a variable-thrust rocket engine. The navigation, guidance, and control system of the module had an inertial unit that automatically received range data from a radar for the main guidance computations during descent.
   Each of the four legs of the LM contained a crushable aluminum shock absorber. Dish-shaped pads 94 centimeters (37 inches) in diameter spread the weight of the module on the lunar surface to prevent the legs from digging in. Four legs were used; that number gives the best stability for most landing conditions, such as small crater holes and limited slopes, and for the weight allotable to each leg.
   The descent stage contained the braking engine and its propellants as well as a platform and ladder for climbing down to the lunar surface after landing. It also bore the scientific equipment to be used by the astronauts in their exploration of the lunar surface.
The ascent engine delivered a thrust of 1,590 kilograms (3,500 pounds). It was controlled by the navigation, guidance, and control system except during emergencies and during the final docking maneuver when the LM rejoined the CM in lunar orbit. This engine was the most vital component of the entire spacecraft, for the astronauts relied on it to lift them off the surface of the moon and get them back into lunar orbit for the return trip to the earth. Therefore, it was a rugged engine of simple design. Rendezvous radar, with a range from 24 meters (80 feet) to 740 kilometers (400 nautical miles), was also part of the LM's gear.
 

Voyage To The Moon

   On Wednesday, July 16, 1969, at 9:32 a.m. (United States eastern daylight time), the American astronauts Neil A. Armstrong, Edwin E. Aldrin, Jr., and Michael Collins blasted off from the National Aeronautics and Space Administration (NASA) John F. Kennedy Space Center in Florida. The purpose of their mission was stated simply: "Perform a manned lunar landing and return." Their historic trip was the Apollo 11 mission.
   The astronauts rode atop a pillar of fire as the first-stage engines ignited for launch. The massive space vehicle roared into the sky, at first ponderously, then with greater and greater velocities. When the propellants had been used up, the engines were shut off and the first stage was separated from the rest of the Saturn V, falling into the Atlantic Ocean about 630 kilometers (340 nautical miles) downrange 9 minutes after launch. The second stage took over to boost the vehicle to an altitude of 187 kilometers (101 nautical miles) and to nearly orbital velocity. When separated, it too followed a ballistic path, dropping into the Atlantic 4,260 kilometer (2,300 nautical miles) from Kennedy Space Center. The third stage S-4B then powered the vehicle to orbital velocity at only slightly greater altitude and was directed into a circular parking orbit. Only 12 minutes had elapsed since blastoff. (The formal countdown preceding launch had taken four days. During the countdown all systems were closely checked over, propellants were pumped into the tanks, and final preparations were made.)
   As the space vehicle made one and a half revolutions around the earth in a circular parking orbit, the ground controllers and the astronauts checked all systems of the vehicle for a last time before the trip to the moon. Until the moment of translunar insertion they could have altered the flight plan, causing the Apollo 11 to remain in earth orbit or preparing it for an immediate emergency landing. Once they ignited the third stage for the second time, however, they committed the Apollo vehicle to a flight toward the moon, and at least one loop around the moon in a so-called free-return trajectory. (Such a trajectory would use only corrections made by the spacecraft's small control rockets to loop around the moon and return to the earth.)
 

On a Path to the Moon

   The instrumentation unit of the third stage computed the direction and timing for translunar insertion and the exact duration of burning of the third stage J-2 engine. The engine finished burning at an altitude of about 300 kilometers (190 miles), having accelerated the spacecraft to a velocity of 39,100 kilometers (24,300 miles) per hour -- almost the velocity required for escape from the earth's gravitation field. For about 10 minutes the ground stations tracked the vehicle to determine whether it was following a good trajectory. The technicians at the NASA Mission Control Center in Houston, Texas, gave the astronauts permission to prepare their spacecraft for lunar orbit. The third stage then made adjustments necessary for the maneuvers that were to follow.
About 30 minutes later, three hours after liftoff, the astronauts directed the combined command and service modules (CSM) to separate from the rest of the vehicle, turn around 180, and dock with the LM, which was nested within the adapter skirt that had joined the third stage and the CSM. They connected the power, oxygen, and signal cables of the two spacecraft, the CSM and LM, then caused the LM to be pressurized with oxygen from a surge tank. A short time later small springs boosted the joined CSM and LM free of the third stage with a separation velocity of one foot per second. A brief burst from the rocket on the SM moved the spacecraft a safe distance away from the S4-B stage. The last propellant in that stage was dumped out through the J-2 engine nozzle, propelling the S4-B into a trajectory beyond the moon and then forever into orbit around the sun. The astronauts settled down for the 73-hour trip to the moon.
   Still working against the earth's gravitational pull, Apollo 11 slowed to a speed of 3,936 meters (12,914 feet) per second at a distance of 6,700 kilometers (22,000 miles) from the earth. By the time the astronauts went to sleep their spacecraft had slowed to nearly half that speed.
   During the second and third days of the flight, the astronauts tended the life-support system, transmitted television pictures to the earth showing their view from the spacecraft and the interior of the CM, and rested. By about 11 p.m. Apollo had slowed to 910 meters (2,990 feet) per second and had reached the point where the gravitational field of the moon is dominant over that of the earth.
 

Orbiting the Moon

   On July 19, the fourth day of the flight, with the moon as a spectacular backdrop, the astro   nauts began the first critical maneuver of the mission -- insertion into orbit around the moon. At 1:13 p.m. the spacecraft passed behind the moon and out of radio contact with earth. At 1:28 p.m. the service propulsion engine on the SM was ignited for 6 minutes to place the spacecraft in an elliptical orbit that was 113.7 kilometers (61.3 nautical miles) from the moon's surface at its closest point and 312.5 kilometers (168.8 nautical miles) from the moon at its furthest point. (Placing the spacecraft in orbit was accomplished by slowing its velocity enough to allow the spacecraft to be captured by the pull of the moon's gravity.) The spacecraft had been turned around so that the SM engine preceded it. Thus, when the engine was ignited, it acted as a brake because its rocket exhaust gases were expelled in a forward direction, causing a reaction thrust against the vehicle. Without this braking action, the spacecraft would have swept past the moon into a solar orbit.
   After surveying their landing site as they passed over it and after sending a 35-minute telecast to earth, the astronauts directed the SM engine to ignite again, this time to slow the spacecraft enough to make the orbit circular.
 

The Lunar Landing

   On the morning of July 20, the three astronauts donned their spacesuits. Neil Armstrong and Edwin Aldrin, the two astronauts who were to land on the moon, transferred to the LM, checked over its systems, and released the landing legs before directing the LM to separate  from the CSM. At 3:08 p.m., as the LM began its descent to the lunar surface, Michael Collins, the pilot of the CSM, became the first person to be alone in orbit around the moon.
   The LM descent engine was fired so that its exhaust acted as a brake (as had the SM engine earlier) to transfer the craft into the descent orbit, which lasted for half a revolution around the moon. The firing was automatically controlled by an inertial unit in the navigation, guidance, and control system; this unit also performed the main guidance computations on the basis of range data automatically fed to it by a radar. The engine was fired again at an altitude of 15,240 meters (50,000 feet). The two astronauts stood side by side in the LM facing their controls and looking out small ports as they made the final approach, the vertical descent that started at an altitude of 900 meters (3,000 feet). Armstrong took over the controls when he saw that the automatic guidance system was about to land the LM in a boulder-filled crater the size of a football field.
   At 4:18 p.m. -- 102 hours, 45 minutes, 39 seconds after launch from earth -- Astronaut Armstrong announced to Mission Control "The Eagle has landed." The LM Eagle had touched down on the moon's surface, landing on a level, rock-strewn plain near the southwestern shore of the arid Sea of Tranquillity. For the next 6 hours the astronauts were busy making the adjustments necessary to prepare the LM for ascent from the lunar surface in case of emergency and donning their suits and backpacks to prepare for a walk on the moon.
   The first human footprint was planted on the lunar crust at 10:56 p.m. On earth an estimated 600 million people in 43 countries around the world watched Armstrong take his first step. They heard him declare "That's one small step for a man, one giant leap for mankind." Armstrong's movements were televised and transmitted virtually instantaneously to this vast audience, nearly one fifth of the world's population, by means of a small TV camera attached to a leg of the LM. Countless more people heard his words on radio.
   The two astronauts set up a passive seismometer (an instrument that can record ground vibrations) and a laser reflector and collected 20.4 kilograms (45 pounds) of rocks and soil to take back to the earth for analysis. They were in high spirits as they did their work and they hopped and loped about to display the ease of movement possible in lunar gravity, which is only one sixth that of the earth.
   The astronauts wore spacesuits that weighed 84 kilograms (185 pounds) on the earth but only about 14 kilograms (31 pounds) on the moon. Each suit carried its own atmosphere and offered protection against total vacuum, extremes of temperature (the moon was  -101C. ( -150F.) in the shadows), and possible puncture by micrometeoroid impact.
   After two hours the astronauts returned to the LM, their backpack supplies having been depleted. Before going to sleep, they began to prepare for the one operation that had never been simulated realistically on earth, the ascent from the moon's surface, which was to occur the next day.
 

The Trip Home

   The lift-off from the moon came at 1:55 p.m. (eastern daylight time) on July 21, ending the astronaut's stay of 21 hours 37 minutes. There was a moment of high tension, in the LM and on earth, as the ascent engine, a 1,590-kilogram (3,500-pound)-thrust rocket in the upper stage of the LM, began to fire -- if that engine had failed, the two astronauts would have been stranded. As the LM ascent stage climbed into the long orbital path toward a rendezvous with the CM, orbiting 111 kilometers (69 miles) above the moon, it left behind an American flag that the astronauts had  planted on the moon, as well as the descent stage of the LM with a special memorial plaque, as mementos of the visit.
   About four hours later the CM and the LM met in lunar orbit. The CM docked with the LM. Armstrong and Aldrin rejoined Michael Collins in the CM. The hatch was shut and the LM was jettisoned. About an hour after midnight, when the CSM was behind the moon, the astronauts directed the service propulsion engine to fire again to propel the CSM out of moon orbit and into the trajectory for return to earth.
   Reentry took 14 minutes. Splashdown was at 12:51 p.m., July 24, 1969, in the Pacific Ocean 1,528 kilometers (825 nautical miles) southwest of Honolulu, Hawaii. A helicopter transferred the astronauts to the carrier U.S.S. Hornet, and quarantine. They remained in quarantine for 18 days.
 

A Vital Link

   A key role in any mission of outer space is played by the worldwide network of facilities used to keep track of the spacecraft, to command its automatic equipment, and to communicate with the astronauts aboard, if it is a manned vehicle. NASA's Manned Space Flight Network depends on a worldwide chain of stations equipped with 9-meter- (30-foot-) wide radar antennas located on land, on ships, and on airplanes. When the spacecraft reached a distance of 16,000 kilometers (10,000 miles) from the earth, more powerful radio astronomy antennas located in Madrid, Spain; Goldstone, Calif.; and Canberra, Australia, began tracking it.
 

From Sputnik To Apollo 11

   On Oct. 4, 1957, the Soviet Union launched into orbit the world's first artificial satellite, Sputnik 1. It was followed by the launching of Sputnik 2 on Nov. 3, 1957. The second satellite carried a payload of more than 450 kilograms (1,000 pounds), including a live dog for biomedical experiments  and scientific instruments for the study of solar radiation and cosmic rays. The Soviet accomplishments came as a shock to most Americans, for they had become complacent in the knowledge that the United States was the world's leader in applied science and technology.
   The immediate response of the United States to the challenge presented by the Soviet achievements was to launch the first U.S. satellite, Explorer 1. This satellite, placed in orbit on Jan. 31, 1958, weighed 14 kilograms (31 pounds), including 5 kilograms (11 pounds) of scientific instruments. Data transmitted by these instruments made possible an important scientific discovery -- the identification by James Van Allen of a belt of electrically charged particles surrounding the earth. A second U.S. satellite, Vanguard 1, which had been under development since 1955, was successfully launched on Mar. 17, 1958.
   Intelligence reports from the U.S.S.R. indicated that the launching of a manned Soviet spacecraft should be expected and that flights to the moon might be the ultimate goal of the Soviet space experiments. In an effort to catch up with the Russians, the United States Congress passed the National Aeronautics and Space Act, signed by President Dwight D. Eisenhower on July 29, 1958. This act created the National Aeronautics and Space Administration (NASA), a powerful civilian agency that had the task of planning and guiding the United States' venture into space. NASA plunged into the development of the first project for manned space flight, Project Mercury, which had been under study in the laboratories of the National Advisory Committee for Aeronautics, the backbone of the new agency.
   Meanwhile the Russians again captured the attention of the world with a series of impressive unmanned moon shots. Luna 1, a lunar probe, was launched on Jan. 2, 1959. It passed within 6,000 kilometers (3,728 miles) of the moon and went on into solar orbit, becoming the first artificial satellite of the sun. (Solar orbit is also called heliocentric orbit.) Luna 2, launched in September of that year, was the first probe to hit the moon. By early October the Russians had obtained photographs of the far side of the moon. The photographs had been sent by Luna 3, which was successfully orbiting the moon.
   The Americans continued planning. On Feb. 5, 1959 NASA established a working group on lunar exploration whose responsibility was to define a program of manned and unmanned circumlunar vehicles, lunar satellites, and lunar surface explorers. On March 5 a test model of the F-1 engine, the type that would eventually power the launch of Saturn V rockets, was fired, producing over 450,000 kilograms (one million pounds) of thrust, the greatest thrust ever attained by a single rocket chamber. In April NASA held a conference to discuss the manned space program beyond Project Mercury with the goals of interplanetary manned vehicles, extended operations in a space station, and military space operations. In that month NASA also announced the basic plan for a deep space network, the series of tracking stations equipped with large radio telescopes to pick up signals from space vehicles, to be located around the world to give 24-hour coverage of any mission to outer space.
   The names of the first seven astronauts were made public: Alan B. Shepard, Jr., Virgil I. Grissom, John H. Glenn, Jr., M. Scott Carpenter, Walter M. Schirra, Jr., Donald K. Slayton, and L. Gordon Cooper, all officers in the U.S. Navy or the Air Force. An important turn was the decision by the Department of Defense that army plans for development of the Saturn rocket should be cancelled since they lacked military justification. The way was clear  for the transfer of the team of engineers led by Wernher von Braun to NASA auspices from the Army Ballistic Missile Agency in Huntsville, Ala., where they had been designing the Saturn rocket. On June 3 construction of the first Saturn launch facilities began at Cape Canaveral, Florida (named Cape Kennedy from 1963 to 1973). By the end of 1959 the plans for a three-person spacecraft and the steps to its development for a circumlunar mission had been outlined, and it had been decided that one version of the Saturn rocket would be used for earth orbital missions and that a more advanced version (the Saturn V) would later be used for the lunar flight.
   By 1960 the framework for a mission of circumlunar flight had been laid and NASA asked presidential approval for Project Apollo with a 1969 deadline for lunar orbit. President Eisenhower's Scientific Advisory Committee studied NASA's proposal and concluded that "man in space cannot be justified on purely scientific grounds" and that a goal of a lunar landing as early as 1975 would be prohibitively expensive. On these grounds President Eisenhower refused NASA's request.
 

The Decision

   On Apr. 12, 1961, the Soviet astronaut Yuri A. Gagarin became the first person in space, orbiting the earth aboard Vostok 1. President John F. Kennedy voiced a sentiment shared by many Americans when he stated that day that he was tired of the United States being second in space. American morale was boosted on May 5, 1961, when Alan B. Shepard, Jr., became the first American in space, making a successful suborbital flight aboard the spacecraft Freedom 7 for Project Mercury. Shepard's flight set the stage for the decision about the future of the U.S. space effort, announced by the president on May 25 in the following words:

Now it is time to take longer strides -- time for this nation to take a clearly leading role in space achievement, which in many ways may hold the key to our future on earth.... We go into space because whatever mankind must undertake, free men must fully share.... No single space project in this period will be more impressive to mankind, or more important for the long-range exploration of space; and none will be so difficult or expensive to accomplish.... I believe that this nation should commit itself to achieving the goal, before this decade is out, of landing a man on the moon and returning him safely to earth.

   The following order of importance had been given to the reasons for a manned lunar landing several months earlier in a report to the president by his science adviser, Jerome B. Wiesner:

1) national prestige,

2) national security,

3) opportunities for scientific observation and experiment,

4) practical nonmilitary applications, and

5) possibilities for international cooperation.

   There was also a practical consideration motivating President Kennedy's decision to set a 1969 deadline for lunar landing. He had received information from NASA indicating that the United States had a good chance of getting to the moon before the Russians.
   Although the Soviet Union's achievements in space had been possible because of the advanced state of technology of their big rockets, it was speculated that the Soviet Union could not mount a lunar-landing mission without developing an even larger new rocket launch vehicle. Informed observers in the United States believed that if the Russians had not embarked on the development of such a rocket by the spring of 1961, it would be possible for the United States, with its great industrial capacity, to match or surpass Soviet achievements within eight years. The U.S. goal was to beat the Russians in big-rocket construction and score a space "first" of historical and political significance.
 

Choosing a Path to the Moon

   The choice of the best route to the moon and back was one of the most important problems to be resolved by NASA administrators before actual building and testing of the hardware of Project Apollo could begin. It was an important choice because it would influence the design and cost of all components. Three approach schemes were considered.
   The direct approach would have involved the fewest steps and the fewest parts and supplies, but it would have required a massive rocket more powerful than any yet designed to boost the vehicle from the earth directly to the moon. The vehicle would have been large and heavy, and hence difficult to control, and all but impossible to simulate with a flight-research vehicle on earth.
In the earth-orbital-rendezvous mode, the launch or booster problem would have been solved by using two Saturn rockets, one to launch the space vehicle and another, launched into a slightly lower orbit, to carry extra fuel. When both were in orbit, the velocity of the fuel-carrying rocket would be increased to bring its orbit up to that of the space vehicle, a maneuver called a Hohmann transfer. When the two rockets were side by side, the extra fuel would be transferred to the space vehicle. Then the second rocket would be jettisoned and the space vehicle would continue on to a lunar landing following the same course of events as the direct approach mode.
   The lunar-orbit-rendezvous (LOR) mode was proposed by a NASA engineer, John Houbolt, who wanted to make the Apollo lunar mission less expensive and more reliable by employing a separate lunar landing vehicle instead of taking the entire space vehicle down to the moon's surface as would have been done with either of the other landing schemes. He reasoned that the main spacecraft could be left in lunar orbit while the astronauts descended to the lunar surface in a small landing craft that would later ascend and rejoin the mother ship, the main living quarters, in a rendezvous operation. The LOR scheme was eventually adopted by NASA because it had the following advantageous features.

1) It would be much cheaper to launch, requiring only one Saturn rocket, because the overall weight would be much lower than for either the direct approach or the earth-orbit-rendezvous approach. The weight would be lower because the ascent stage of the LM only need be powerful enough to boost the lightweight LM into lunar orbit, whereas in the other two modes the ascent stage would have to boost the entire vehicle off the lunar surface and back to the earth.

2) The return vehicle and associated propulsion system could be designed more specifically for the purpose of return to the earth, and therefore could be simplified and made more dependable.

3) The lunar-landing vehicle need only cope with the low gravitational pull of the moon (one sixth the gravity of the earth) since it could be tucked away for protection until well out of the main pull of the earth's gravity and could be discarded after it had done its task. Therefore, it could be fragile and lightweight. Moreover, it could be designed exclusively for the task of lunar landing and ascent with optimum efficiency.

4) Complete crew training and checkout for the lunar-landing, lunar launch, and rendezvous-docking operations would be possible in the actual flight vehicle. (Such testing was done with the Apollo missions 8-10 preceding the lunar landing of Apollo 11.)

5) A safe return of the primary vehicle would be possible in the event of a lunar landing accident.
 

Precursors of Apollo

   Much testing of space vehicles and of the reactions of people aboard the vehicles in space had to be done before the flight tests of the Apollo mission. From 1961 through 1963 Project Mercury demonstrated that humans could be proficient as space pilots in flights lasting up to 34 hours, each carrying one astronaut. The Gemini program of ten flights in 1965 and 1966 was a transition between the Mercury and the Apollo projects. It involved longer earth-orbital flights of up to two weeks in duration, each flight carrying two astronauts. Its purpose was to supply data on several unknown factors of travel in space, such as the behavior and performance of astronauts during prolonged flights and the physiological effects of prolonged weightlessness and of possible radiation in space. Gemini also had to perfect the techniques of orbital rendezvous and docking essential to Apollo operations.
 

The Nationwide Effort

   The design, manufacturing, testing, and final asembly of the thousands of components that made up the Apollo/Saturn system involved hundreds of thousands of workers. At the peak of the effort 420,000 workers employed by 20,000 industrial and university contractors were actively participating in the development of Apollo. Millions of hours of engineering time were re  quired to design the rocket and work out the technical complexities of its manufacture. Developing the rocket required that test machines be built that were often even more complex than the launch vehicle itself. These machines simulated the effects of vibrations on electrical parts and other sensitive parts (there was even a rig to shake a fullscale model of Saturn V). Test stands were built to run each engine for hundreds of hours on the ground.
   Besides the rockets and propellants, many other major systems had to be developed and tested as fully, systems such as the inertial guidance and navigation systems, and tracking systems, and the life-support system. The work was done by separate laboratories, companies, and plants spread throughout the country and organized by NASA. The chief engineers of NASA were responsible for the overall plan and for the myriad details. They were called systems engineers because they had to define the technical requirements of the parts that constitute a system and devise each system -- that is, decide how to bring together the needed equipment, technical information, facilities and people for each major task.
   Despite all the intensive preliminary testing and the exacting precautions taken, a tragic mishap occurred on Jan. 27, 1967, when a flash fire occurred on the CM of an Apollo spacecraft that was being tested on the launch pad. Three astronauts, Virgil I. Grissom, Edward H. White II, and Roger B. Chaffee, perished in that fire. Investigations and alterations in the project following the accident delayed the first manned flight of an Apollo spacecraft until Oct. 11, 1968, when Apollo 7 orbited the earth 163 times. The astronauts aboard Apollo 8 (Borman, Lovell, and Anders) became the first people to orbit the moon when that craft made 10 lunar orbits in December 1968. Apollo 9, which carried the LM into space for the first time and performed tests in earth orbit, and Apollo 10, which took the LM to within 14 kilometers (9 miles) of the lunar surface, were the final steps preceding Apollo 11.
 

Exploring The Moon

   The Apollo 11 astronauts did not go to a strange, completely unknown territory in their voyage to the moon, for the moon had already been surveyed extensively by many space probes that did not carry humans. These unmanned precursor flights established many facts about the moon -- facts that are important not only because they helped to ensure the safety of Apollo 11, but also because they are valuable from the scientific point of view.
   Interest in the moon reflects the central goal of the space program to date -- to explain the origin of the solar system. Such a purely scientific orientation was given to the programs of the Ranger space probes, which crash-landed on the moon, and to the Surveyor probes, which soft-landed, when these programs were formulated in 1960. The Soviet Luna probes were also primarily scientific. With the formal inception of Project Apollo, the primary objective of U.S. unmanned space probes to the moon was changed to that of providing lunar environmental data needed to design the Apollo systems. The lunar probes were intended to show the forms, bearing strength, and other physical properties of the moon's surface in typical zones, particularly in the maria. They accomplished these purposes with brilliant success. In 1966 and 1967 the Lunar Orbiter probes photographed 99 percent of the lunar surface that faces the earth, concentrating on the equatorial belt, the zone destined for the first manned landings.
 

Unmanned Probes

Ranger

   The first lunar probes, Rangers 1-6, failed for various technical reasons. As Rangers 7-9 approached the moon before crash-landing, their television cameras transmitted more than 10,000 television pictures of the lunar surface. The resolution of the best pictures was 2,000 times better than the resolution of photographs taken through telescopes based on earth. (Resolution is a measure of the camera's ability to separate objects or features that are close together.) The pictures revealed a surface that has been rounded and degraded by repeated and superimposed cratering. Scientists deduced that the sur  face is covered by a fragmented layer more than 100 feet deep in places. Ranger 7 revealed relatively flat and open stretches in the maria, suitable landing sites for Apollo. Ranger 9 hit very close to the point at which it had been aimed, a site within the crater Alphonsus, where telescopes and radiometers had previously indicated the existence of hot spots. Ranger's pictures showed in greater detail the dark, so-called halo craters long known from telescopic photographs. The comparative newness of some of these craters supported the interpretation that the halos are rings of ejecta or volcanic ash. The most important deduction concerned the fine structure of the surface. Scientists inferred that unconsolidated material probably covers the greatest part of the lunar surface to a considerable depth, but they could not determine whether the material originated on the moon or was externally derived. Nor could the Ranger pictures add much new information about the composition of the lunar surface materials; that was a task for the Surveyor probes.
   Special computer techniques were developed during the lunar probe program to give accurate form to television pictures transmitted from the spacecraft. To cope with small distortions in the geometry and lighting of the pictures introduced by the electronic characteristics of the television cameras, it was decided to rectify the pictures by giving them a numerical form. Each sample along a scan line was represented by one of 64 numbers, which stood for gray levels, or variations in brightness, and the whole picture was formed by a rectilinear array of the samples. Adjustments in the assignment of values could be made instantly by an on-board computer. Without this technique it would not have been possible to gauge the slope and brightness of any lunar feature. By using this method the engineers converted Ranger pictures to more than 60 precise topographic maps that were used to determine the amount of tilting spacecraft would experience upon landing. The so-called "digital enhancement" and computer analysis of television pictures transmitted from spacecraft have become a basic tool of space science. Moreover, they have been taken up in medical and industrial radiography and in microscopy, astronomy, and reconnaissance -- an example of technological "spinoff."

Luna

   The first device to send pictures directly from the moon's surface was an instrument capsule ejected onto the surface by the Soviet space probe Luna 9 just before it landed on Feb. 3, 1966. The space probe itself crashed, but the capsule made a soft landing. (Soft landing means that the object, instead of crashing, lands intact and functioning.) The landing demonstrated that the lunar surface could support the weight of a spacecraft. (It had been feared earlier that the surface material might be too soft or loose, causing a landing object to sink or tip over.) Two months later another Soviet space probe, Luna 10, became the first artificial object to orbit the moon. For 57 days Luna 10 transmitted a broad range of scientific observations. It was found that the moon passes through a cometlike tail created by the effect of the solar wind on the earth's magnetosphere, the outer part of the earth's atmosphere. (Further discussion of the magnetosphere will be found in Atmosphere and Earth.)
   A major feat was accomplished in September 1970 by the unmanned Luna 16, which landed on the moon, collected a sample of lunar material, blasted off, and returned a capsule containing the lunar material to a safe landing on earth. Although the small size of the sample (a few ounces) limited its scientific value, the device demonstrated the feasibility of unmanned sampling missions.
In November 1970 the unmanned Luna 17 landed on the moon in the Sea of Rains, where it deposited a solar-powered vehicle, Lunokhod 1. During lunar days the unmanned vehicle roved slowly over the lunar surface, covering an area of 74,000 square meters (800,000 square feet), as it traveled 10 kilometers (6 miles) in a total of 6 months of activity during 10 months of operation. Lunokhod 1 was controlled remotely by operators on earth who could view its prospective path via TV and send commands via radio. Lunokhod 1 transmitted to earth 200 panoramas, 20,000 photographs of the lunar surface, and analyses of the physical and mechanical properties of lunar soil at 500 points and of chemical properties of the soil at 25 points. A second Soviet vehicle, Lunokhod 2, was placed on the moon by Luna 21 in January 1973.

Surveyor

   The first true soft landing on the moon was made on May 30, 1966, by the United States spacecraft Surveyor 1. This was the first lunar craft to have retrorockets brake its velocity as it descended so that it would settle gently on the surface; the same landing principle was later used for the Apollo LM. By July 13, the beginning of the 14-day-long lunar night, Surveyor 1 had transmitted 11,150 pictures of the lunar surface and of space (10,388 were transmitted on the day it landed). During the first 12 days on the moon Surveyor 1's instruments responded properly to more than 100,000 commands from the control center at the Jet Propulsion Laboratory in Pasadena, Calif.
   The landing of Surveyor 1 and subsequent successful Surveyor landings assured the designers of the Apollo project that the lunar surface in the maria could support the LM's weight and that there were safe landing areas. The various flights carried different combinations of the following instruments: television cameras equipped with color filters, extendable pivoted arms with a scoop at the end of each for sampling loose surface material, small bar magnets mounted in various positions, alpha-particle scattering devices, strain gauges on the legs of the spacecraft, thermal sensors, movable shadow shields, gyros and accelerometers, gas jets, and radar systems. These instruments delivered extremely detailed information for each landing site, including data on topography and surface structure; cohesion, shear, and bearing strength of surface material to a depth of several inches; strength and density of specific rocks; surface temperature; radar reflectivity; reaction to gas jets; surface photometry, color, and polarization; and much more. Surveyor pictures delivered basic information about the optical and microwave characteristics of the earth and its atmosphere and about the sun's corona.
   The Surveyor observations confirmed that the lunar surface has been churned, pulverized, and differentiated, as Ranger's pictures had indicated. From alpha-scattering experiments performed by a Surveyor device it was concluded that no common material on earth matches the chemical composition of the surface material at Mare Tranquillitatis, the Surveyor 5 landing site near the lunar equator. In this experiment pieces of the radioactive material curium-242 carried by Surveyor 5 emitted alpha particles that gently bombarded the lunar material and were reflected. From differing reflections of particles (including protons), the chemical composition of the lunar surface material could be deduced. The same test performed by Surveyor 6 indicated that the composition of the surface at its landing site, near a mare ridge in Sinus Medii almost exactly in the center of the visible lunar disk, differs slightly but significantly from the composition at the Surveyor 5 site.

Lunar Orbiter

   Instead of surveying very specific, very small areas of the lunar surface as Surveyor probes had done, Lunar Orbiter spacecraft in 1966 and 1967 presented breathtaking and virtually all-encompassing views of the moon and of the earth. As they orbited the moon, each of the first three Orbiters photographed in detail the equatorial landing sites for Apollo. Lunar Orbiter 1 photographed "earth rise," showing the earth covered by swirls of clouds as it rose above the horizon of the moon.
   Lunar Orbiter 3 successfully made a 13 change in the plane of its orbit and gave data to confirm coefficients for a mathematical model of the moon. Lunar Orbiter 4, in a highly inclined orbit that almost passed over the poles of the moon, photographed 99 percent of the side of the moon that faces the earth and 60 percent of the far side. It revealed the magnificent Mare Orientale Basin, 970 kilometers (600 miles) wide and ringed by mountains 6,100 meters (20,000 feet) high. The basin probably was formed by the impact of a giant meteorite or comet nucleus. The sharpness of its features leads many planetologists to believe that the Mare Orientale Basin is younger than other large lunar basins.
   Lunar Orbiter 5 gave tantalizing views of the lunar valley that has been named after the 18th-century German selenographer Johann Schroter. A rille meandering down the course of this valley looks like a dry river bed on earth. Schroter valley is near the relatively young crater Aristarchus, which shows red glows that have been interpreted to indicate volcanic activity.
 

Mascons

   During the summer of 1968 P. M. Muller and W. L. Sjogren, both of the Jet Propulsion Laboratory, announced the discovery of concentrations of dense material beneath the great circular maria of the moon. These dense bodies, named mass concentrations or "mascons," produced gravitational anomalies (variations in gravitational pull) that caused Lunar Orbiters to depart slightly from their predicted orbits; it was through the detection of these slight orbital departures that the existence of mascons was discovered. Mascons are of importance in theories of the history of the moon because they indicate that the moon is far more rigid than the earth. On earth, bodies as dense as mascons (relative to the surrounding material) would subside until the gravitational anomaly disappeared; that is, they would come to equilibrium with their surroundings, a process known as isostatic adjustment. However, although mascons indicate that the moon has been cold and rigid for a long time, other evidence (such as the existence of basaltic lunar rocks) indicates that the moon has produced molten lava-like material and therefore could not always have been rigid. Despite the wealth of Apollo data, much lunar exploration and study will be required before scientists can fully explain such puzzles as mascons. (See also Moon, Origin and History of.)
 

Flights After Apollo 11

   Apollo 11 was followed four months later, in November 1969, by the equally successful Apollo 12 flight, which landed in a mare called Oceanus Procellarum (Ocean of Storms). Apollo 13, in April 1970, was aborted. The astronauts had to return to earth without landing on the moon because their mission nearly ended tragically when an explosion in the service module caused a massive power failure and loss of oxygen. The SPS engine was disabled and the LM descent engine had to take over one of the SPS engine's most vital functions -- changing the trajectory so that the spacecraft would loop around the moon, in this case to return to earth as quickly as possible. The astronauts splashed down safely after a journey fraught with peril. Later investigation showed that lapses during checkout had led to the near disaster of Apollo 13. As a consequence of it NASA added $15 million worth of new safety measures to the Apollo spacecraft for future missions. These improvements included a redesigned oxygen tank in duplicate, an extra battery to provide more emergency power, and 40 pounds more drinking water.
   Apollo 14 in February 1971 made a successful landing on the Fra Mauro formation, a highland region along the eastern rim of the Ocean of Storms, south of the moon's equator. The rugged terrain there is thought by geologists to be older than the immense and flat maria where Apollo 11 and 12 had landed, and rock samples brought back by the Apollo 14 astronauts offered new clues to the origin and history of the moon. The astronauts took pictures of the undulating hills, craters, and large boulders they found. They also used a rickshaw-like tool cart to carry the dozen scientific instruments they placed in a network on the moon's surface.
   In July 1971 Apollo 15 landed in the lunar plain called Palus Putredinis (Marsh of Decay), which is near the lunar Apennine Mountains and the gorge called Hadley Rille. Riding in the battery-powered Lunar Rover vehicle, the Apollo 15 astronauts explored the flatlands, the slopes of the 4,600-meter-high (15,000-foot-high) mountains, the edges of craters, and the rim of Hadley Rille, which is 1.6 kilometers (1 mile) wide and 180 to 360 meters (600 to 1,200 feet) deep. Astronauts Scott and Irwin gathered about 77 kilograms (170 pounds) of lunar rocks, including one sample that is believed to be a piece of the primordial lunar crust, between 4 billion and 4.5 billion years old. They bored holes as deep as 2.35 meters (7 feet 9 inches) into the crust in order to obtain cores of layered subsurface material and to emplace thermometers needed to determine the internal temperature of the moon. They also used a color television camera to transmit pictures to earth. While the extravehicular activity took place on the lunar surface, various X-ray, magnetic, and chemical data were being gathered by instruments aboard the command module orbiting the moon.
The exploration of the lunar highlands, begun by Apollo 14, was continued by the Apollo 16 mission in April 1972. Despite the accidental destruction of one experiment, Apollo 16 returned to earth an even greater mass of scientific data.
   The last Apollo flight, Apollo 17 in December 1972, carried the first geologist to land on the moon, Harrison H. Schmitt, and was considered the most successful mission scientifically. The astronauts deployed several experiments designed to answer questions raised by earlier Apollo flights.
 

Post-Apollo Planning

   The Apollo 11 moon landing took place early in the first term of President Richard M. Nixon. Urged by some of his advisers, President Nixon came close to halting the Apollo program with that triumphant mission -- a move that would have avoided the risk of failure in a very risky enterprise, quickly shunted technical human labor from the program into other fields, and cut the NASA budget nearly in half at a stroke. But President Nixon accepted the risks of further Apollo flights and chose a moderate course of shifting toward a decade-long program of developing a new and less expensive means of delivering payloads into space, the so-called space shuttle. This transition took three years of his first term, and it involved a long technical and political debate on the appropriate space course for the nation in the 1970's and early 1980's. In choosing this course President Nixon weighed the advice of an advisory group that he set up as he began his first term.
 

Space Task Group

   Early in 1969 President Nixon appointed a group of advisers to consider the future of the U.S. space program. This Space Task Group (STG) was chaired by Vice-President Spiro T. Agnew and consisted of the president's science adviser, the administrator of NASA, and the secretary of the air force (representing the secretary of defense). Observers included the under secretary of state for political affairs, the director of the Budget Bureau, and the chairperson of the Atomic Energy Commission. Two months after the Apollo 11 moon landing, the STG presented its report, "The Post-Apollo Program: Directions for the Future." The report outlined several goals the United States should emphasize that answer the objections of critics of the space program. In answer to those who believe that the vast sums spent on the manned space flight program could better be spent on improving the conditions of society, the STG listed as apparently the first objective "the application of space technology to the direct benefit of mankind" through such services as surveying earth resources, monitoring weather and air and water pollution, controlling air and ocean traffic, and improving communications. In answer to the military-minded who jealously eye NASA's budget and who have misgivings about the relative openness (nonsecrecy) of NASA's programs, the STG listed as another objective "the operation of military space systems to enhance military defense." A third, scientific objective was stated as the "exploration of the solar system and beyond" to increase humankind's knowledge of the universe. The report suggested that new systems and technology for space operations should be developed with emphasis upon commonality, reusability, and economy. The United States was advised to promote a sense of world community through a program that would provide the opportunity for broad international participation and cooperation. The report also stated: "As a focus for the development of new capability, we recommend the United States accept the long-range option or goal of manned planetary exploration with a manned Mars mission before the end of this century as a first target." Balance between manned space flight programs and unmanned programs was emphasized, as was balance between exploitation of existing equipment and techniques and development of new techniques.
   The STG recommendations invited debate over the future of the space program. Should the U.S.S.R. reveal a portentous space station, the president could authorize immediate development of an American space station. Should the Soviet Union indicate willingness to cooperate in developing an international orbiting laboratory, lunar base, and manned Mars expedition, such developments could be shifted well into the 1980's and 1990's.
 

NASA's Proposals

   The STG recommendations differed considerably in emphasis and detail from the recommendations of NASA, released at the same time in a report entitled "America's Next Decade in Space." NASA recommended a far more ambitious and costly program than did the STG.
   The NASA report promoted the goal of manned planetary explorations for the 1980's as a focus for the space program. NASA held that just as the Apollo project served as an umbrella over the myriad aspects of development of the program in the 1960's, a project involving preparation for exploration of Mars would cover a series of new developments. These would include space stations in earth orbit and lunar orbit, reusable launch vehicles and nuclear stages for earth-moon shuttles and for the Mars spacecraft, and bases on the moon and on Mars, as well as new byproducts usable in other fields. Just as the Ranger, Orbiter, and Surveyor probes scouted the moon and the Mariners probed Mars and Venus in the 1960's, Mariner and Viking spacecraft would scout Mars in the 1970's and new unmanned Voyager spacecraft would probe Jupiter and the more distant planets of the solar system in the 1980's. (By 1983, much of this program had been accomplished; see below.) The program proposed by NASA would, together with the continuing exploration of the entire solar system, embrace sustained efforts in space astronomy, physics, and biology, relying on space stations and the greater payloads permitted by cheaper launch vehicle operations. It foresaw new applications in communications, navigation, air traffic control, management of earth resources, and control of our environment.
Unlike the STG, which made the primary objective of the space program the practical application of space technology, NASA leaders believed that such uses would naturally follow new technology and, hence, need not be the main objectives of future plans. Instead, NASA believed that the United States could and should continue its preeminence in space exploration in the 1970's and 1980's, serving national scientific, economic, social, and political purposes.
   For the most part President Nixon followed the STG recommendations and set about shifting the focus of NASA, though the Apollo moon-landing programs still commanded attention. He pushed the applications of space technology that use automatic spacecraft, notably the Earth Resources Technology Satellite (ERTS) program; authorized the launching of a Skylab space station in the year after the last Apollo mission; negotiated a joint U.S.S.R.-U.S. manned flight for 1975; maintained a balanced program of limited unmanned exploration of the inner and outer planets of the solar system; kept military space development a major activity of the nation; and called for the development of a new manned space transportation system (the shuttle) and a spacecraft development philosophy to meet the STG's recommendation for "emphasis upon commonality, reusability, and economy." He set a course covering a full decade of the nation's efforts in space, leveling off space expenditures under heavy criticism from opponents of such technical enterprise yet maintaining a set of strong and basic technical and exploratory programs covering the whole range of objectives set forth in the National Space Act of 1958.
 

U.S.-Soviet Cooperation

   In 1962, President Kennedy and Premier Nikita Khrushchev of the Soviet Union exchanged letters that led to some space cooperation between the United States and the Soviet Union, chiefly in biomedicine and meteorology. However, the race for the moon kept this cooperation on a small scale. After Apollo 11, on Oct. 10, 1969, the NASA administrator opened new discussions on space cooperation with the president of the Soviet Academy of Sciences, particularly concerning a joint flight of U.S. and Soviet spacecraft made possible through common rendezvous and docking gear. In 1970 U.S. and Soviet delegations met in Moscow to work out the engineering plans for a joint manned flight of Soyuz and Apollo spacecraft -- the ApolloSoyuz Test Project (ASTP) -- and created five joint working groups in space research. When this accord had taken hold in good working relationships, President Nixon and Premier Alexei Kosygin signed an executive agreement during the president's trip to Moscow in May 1972, confirming its terms for five years.
   U.S. and Soviet engineers exchanged many visits in preparing the common hardware and operational procedures for the joint Apollo-Soyuz Test Project. On July 15, 1975, the Soyuz 19 spacecraft was launched from the Baikonur Cosmodrome in Kazakhstan, and 7 hours later a Saturn IB rocket launched Apollo 18 from Cape Canaveral. After maneuvering into the proper orbit, the two spacecraft joined on July 17. The two Soviet and three U.S. crew members exchanged visits and also performed scientific experiments, including the first observations (by the Apollo astronauts) of stellar radiation in the extreme ultraviolet range. After nearly two days together, the spaceships separated and returned to earth in their customary manner, the Soyuz landing on Soviet territory and the Apollo in the Pacific Ocean.
   This flight achieved three major results. It

1) demonstrated the joint equipment and procedures for making space rescues, a capability that is advisable for the subsequent frequent manned orbital flights in the 1980's and beyond;

2) set a precedent of international inspection and cordial visitation in space stations, similar to that established on earth by Antarctic stations; and

3) laid the foundations for U.S.-U.S.S.R. cooperation on manned missions to the planets.
 

Exploring the Solar System

   During the 1960's and early 1970's a number of probes to Mars and Venus charted the territory of the inner planets; networks of Interplanetary Monitoring Probes (IMP) revealed much about the interplanetary environment of solar wind, particles and fields, and cosmic dust; and Pioneer and Mariner spacecraft began the exploration of Jupiter and Mercury. The immediate vicinity of the earth and moon received the attention of many automatic and manned spacecraft which delineated their environment, particularly the earth's radiation belts and their interaction with the solar wind, in great detail. (See also Solar System.)
   Mariner 4, launched Nov. 28, 1964, opened Mars exploration with a flyby and seemed to show a moonlike planet, heavily cratered, extremely thin of atmosphere, and bleak to any chance of life. These results caused the attention of scientists to shift to the outer planets and, for a time, to Venus. Mariner 5, launched on a flyby June 14, 1967, confirmed the view that Venus is a densely atmosphered and broiling hot place. The flyby Mariner 7, launched Mar. 27, 1969, quickly rekindled interest in Mars, however, because it showed or suggested a rich variety of conditions on the planet's surface. And Mariner 9, an orbiter of Mars, revealed enormous canyons, studded with gigantic volcanoes, patterned with channel markings suggesting water, and capped by water-bearing polar ices apparently composed chiefly of carbon dioxide -- a finding later revised by the Viking spacecraft.
 

The Mariners

The extraordinarily successful Mariner series of probes to Venus and Mars during the 1960's formed the technical foundation for many of the coming missions. These spacecraft, like the related Ranger and Survey-or moon-survey craft, were designed and developed by the Jet Propulsion Laboratory for NASA. They used a characteristic combination of features: a platform controlled in all three axes by small cold-gas rockets; solar cell panels for primary power; a fixed, high-gain antenna for transmitting data to stations of the Deep Space Network; a star tracker; a temperature-controlled compartment for electronics and flight controls; and small rockets for making midcourse corrections. In the latest Mariners, instruments ride a movable platform below the main octagonal bus (platform) structure. Louvers and aluminized mylar blankets on the bus control temperature.
   These strongly interdependent pieces of equipment have been made highly proficient, particularly the electronic systems associated with instruments, controls, and communications. Although the number of individual electronic parts in the Mariner series has been reduced by the use of integrated circuits, the complexity of functions they perform has increased. Guidance and control of the spacecraft have been refined to such an extent that Mariner 7 was controlled by signals transmitted by the Deep Space Network to within 200 kilometers (approximately 124 miles) of the nominal aiming point, that is, the point in space at which the Mariner was aimed when it was launched on its trajectory to Mars. Over the flight path of 315 million kilometers (196 million miles), this meant an accuracy better than one part in a million. Such control permitted engineers on earth to place spacecraft in orbit around Mars in 1971.
   The early Mariners explored space and the planets in several ways. By tracking the Mariners very accurately, scientists were able to refine measurements of the dimensions of the orbits of the inner planets, increasing the accuracy of all future missions. Some of the Mariners have measured interplanetary radiation; Mariner 2 detected a flux of radiation associated with a solar flare. Mariners have also measured micrometeoroid density in the regions through which they passed and have determined that Mars and Venus lack  magnetic fields and radiation belts. As they passed into and out of the occulting disks of Mars and of Venus (that is, as they became hidden behind the planets and then reappeared), the Mariners probed the atmospheres of those planets with radio beams. In this type of probing, radio signals of precisely controlled frequency are transmitted from earth to the spacecraft, which then retransmits the signals to earth. The density and chemical character of the atmosphere alter the radio waves and make the spacecraft appear displaced from a predicted point. Scientists then interpret the apparent shifts of position of the spacecraft in terms of temperature, density, and pressure, relating these factors to models of atmospheric chemistry so as to deduce the composition and character of the atmosphere.
 

Venus

   For Venus such measurements confirmed estimates made by radar that the surface is very hot, about 425C. (800F.), and that the atmosphere is primarily composed of carbon dioxide with a surface pressure 100 times that of earth's atmosphere. Similar measurements were made by instruments in a capsule dropped into Venus' atmosphere by the Soviet spacecraft Venera 4 at virtually the same time in October 1967 that Mariner 5 flew by the planet.
   In May 1969 two Soviet spacecraft, Venera 5 and 6, and in December 1970 Venera 7, probed the atmosphere of Venus. Venera 7 made the first "soft" landing on Venus, relaying to earth a surface temperature of 450C. (843F.). A follow-up craft, Venera 8, landed in July 1972 on the illuminated side of the planet, radioing back data to earth for nearly 50 minutes. In February 1974, Mariner 10 passed Venus on its way to Mercury and obtained a sequence of excellent ultraviolet photographs of Venus' cloud patterns, revealing the circulation of the planet's atmosphere. It also determined that there is hydrogen in the upper atmosphere, probably from the solar wind.
   Venera 9 and Venera 10, two Soviet spacecraft that were landed on Venus in October 1975, sent back the first photographs ever taken of the surface of Venus. Each transmitted data for about one hour before being inactivated by the intense heat and pressure, and each obtained a panoramic view revealing details of the surface as much as 100 meters (300 feet) away. Venera 9 showed a landscape littered with sharp rocks. These are evidently "young" rocks, with no signs of weathering by the dense atmosphere. Soviet scientists believe this shows that the interior of Venus is still geologically active. At the Venera 10 landing site the terrain consisted of smooth, worn rocks. Neither spacecraft showed the sandy or dusty surface that had been expected. Perhaps the most surprising finding was that it was possible to take such good pictures at all, since it had been thought that very little light penetrates the thick clouds of Venus.
   In the regions above the cloud layer, minor constituents of the atmosphere can be measured by improved ultraviolet and infrared spectrometers on flyby and orbiter spacecraft. Measurements extending down to the surface, including measurements made on clouds, require atmospheric probes, improved in sensitivity and accuracy over the instruments used in the early Soviet Venera probes. In 1978 two Pioneers were launched to study the weather and topography of Venus: Pioneer Venus 1 to orbit the planet and Pioneer Venus 2, containing instrumented probes, to study the Venusian atmosphere from top to bottom.
   Dense cloud cover makes mapping of Venus' surface practical only by radar. The geological properties of Venus should resemble those of the earth in many ways, but just how the erosional properties of the hot, dense atmosphere of Venus affect the planet has not been determined. The high temperature of Venus' surface probably modifies the geological processes in its crust. Sampling the surface material directly and comparing it with that of the earth, the moon, and Mars would obviously be of great interest. Landing seismometers on the surface should reveal geological processes in Venus' crust and its internal structure.
 

Mars

   For Mars, probes have shown a very thin atmosphere, 100 times less dense than earth's at the surface, with a composition like the atmosphere of Venus, primarily carbon dioxide. The minor constituents of the atmosphere include nitrogen, water, oxygen, and carbon monoxide, as well as an unexpectedly large amount of argon.
   No signs of life have been detected on Venus and no clear signs on Mars. The extreme heat and pressure on Venus' surface make it unlikely that life will be found there. The Martian atmosphere contains carbon monoxide, a gas lethal to earth life forms, and lacks more than a trace of nitrogen, the dominant constituent of earth's atmosphere. The Viking landers have returned data that do not deny or confirm life on Mars.
   Before Mariner 4 went to Mars, there was endless speculation about civilizations on the mysterious Martian surface, so changeable to the eye of the optical astronomers who viewed it through telescopes on earth. Mariner 4 flew within 10,000 kilometers (6,000 miles), revealing craters that were promptly compared with the moon's craters, and ending talk of little green men. Mariners 6 and 7 in 1969 flew within 3,000 kilometers (about 2,000 miles) of the surface and revealed more of the distinctive character of the planet. Mariner 6 returned 74 television pictures of Mars; Mariner 7 sent 126 pictures, 33 of them clear closeups. These pictures show a complexly cratered surface with a sharply demarked polar cap. Certain regions appeared to have undergone some subsurface erosion or slumping.
   The first spacecraft to orbit another planet was Mariner 9, which went into orbit around Mars on Nov. 13, 1971. It was followed two weeks later by the Soviet Union's Mars 2 and shortly thereafter by Mars 3. The principal task of Mariner 9 was to map photographically, from distances as close as 1,380 kilometers (860 miles), some 70 percent of the Martian surface. Actually it mapped the entire surface. In the first pictures the planet's features were obscured by a dust storm. Later photographs showed that the Martian terrain was far more varied and interesting than had been thought. Huge volcanoes were seen, and a giant canyon longer than the continental width of the United States. Most intriguing were the widespread signs of erosion, which were difficult to explain by any agency other than running water. The spacecraft also obtained very clear pictures of Mars' two natural satellites, Phobos and Deimos.
   Mars 2 and 3 carried, in addition to cameras, a variety of scientific instruments, including infrared and ultraviolet sensors to measure the chemical composition of the surface and atmosphere. Before entering its orbit, Mars 3 ejected a landing capsule, which reached the Martian surface but ceased to transmit data after 20 seconds. Four more Soviet craft, Mars 4-7, were launched toward Mars in the summer of 1973 but failed to achieve most of their objectives.

Viking

   The Viking series of Mars probes was announced by NASA in February 1969, after years had been spent attempting to design landing craft that can withstand the long periods of heating that are necessary to sterilize them. The Viking consists of a bus that can drop a lander and then go into a synchronous or "stationary" orbit above the lander, where it can relay data from it to earth. (A synchronous orbit about a planet is an equatorial orbit whose direction and period are the same as the direction and rotational period of the planet. Thus, the satellite remains above the same spot on the planet.) The lander descends with the aid of a supersonic parachute and a retrorocket, such as that used by the Surveyors, and then views its surroundings with stereoscopic color television cameras. In addition it sets out a little chemical laboratory to sample and test the soil for organic compounds, instruments for detecting atmospheric constituents and conditions, and equipment to examine the soil for water, free or bound in crystals. An arm reaches out as much as 3 meters (10 feet) to take soil samples for biological and mineral analysis.
   The two Vikings, which were launched in 1975 and landed safely on Mars in 1976, are the most complex unmanned spacecraft yet developed. The landers had to be designed to withstand 6 g at launch and 20 g on planetary entry and landing (g, acceleration due to gravity, equals 981 cm/sec2 or 32 feet/sec2; see below). They also had to be designed to function 90 days on the planet's surface while being powered by two radioisotope thermoelectric generators, the radiation from which is a hazard, necessitating shielding of the more sensitive instruments.
   To meet international guidelines on planetary "quarantine" (no contamination of a planet for 20 years commencing with the first landing), the landers were subjected to dry heating at 125C. (257F.) while inside a "biocanister" for 20-30 hours. The canisters remained on the spacecraft when they were launched but they were jettisoned before entry into the Martian atmosphere.
As the landers descended to the Martian surface, they stayed in communication with the Deep Space Network so that engineers on earth could pinpoint their location and align the orbiter sections' communications equipment. The information returned from Mars has been of great scientific importance, such as the fact that the residual northern ice cap is mainly water ice, but scientists do not yet know how to interpret the puzzling results of the biological experiments.  The evidence for past or present life on Mars remains inconclusive.
 

Mercury

   The planet closest to the sun, Mercury has been baked by solar radiation since the formation of the solar system. Knowing the effects of this intense radiation on Mercury should lead to a better understanding of how the sun has affected the earth. Thus, Mercury may repay careful examination by space probes.
   The 1973-1975 Mariner 10 Venus/Mercury mission opened the direct exploration of Mercury. The spacecraft was placed in a solar orbit calculated to bring it near Mercury every 176 days, and in three flybys (the first on Mar. 29, 1974) it obtained excellent photographs of the surface. Mercury was found to resemble the moon with its many craters, and also to have scarps (long lines of cliffs) that may indicate shrinking at some time in its history. A weak magnetic field was found, contrary to expectations, and a very thin atmosphere consisting mainly of helium was detected.
 

Uranus

   On Mar. 10, 1977, the Kuiper Airborne Observatory (KAO), an aircraft equipped by NASA as an astronomical laboratory, detected a series of rings around the planet Uranus. Further observations in 1978 and 1986 confirmed the existence of at least 11 such rings.
 

Probing The Outer Planets

   Project Viking symbolized the long-term commitment of the U.S. space program to seek clues to the origin and evolution of life. Probes to the outer planets, particularly to Jupiter, emphasize another basic aim of the program -- to understand the origin and evolution of the solar system.
   Theories of the origin of the solar system all face one pivotal question: What was the chemical composition of the original matter from which the sun and the planets were made? When the inner planets formed, the heavier matter apparently was nucleated (that is, it clustered about nuclei), while the original gaseous constituents were dissipated by the solar wind, which is believed to have been ten million times stronger in the early stages of the solar system than it is today. The four large outer planets differ radically from the inner planets, being composed chiefly of hydrogen, helium, methane, and ammonia. The masses of Jupiter, Saturn, Uranus, and Neptune range from 14 to 318 times that of earth and each generates such a tremendous gravitational field that it is hard to see how they could have lost any matter. At least Jupiter and Saturn, scientists agree, may have retained the original elements of the primordial stuff of the solar system in the original proportions. Very little is known about Pluto. Meteorites and the rocks of the earth and of the moon have been analyzed and the composition of the sun has been studied spectroscopically to shed light on this question, but the compositions deduced from these analyses do not agree. Because Jupiter is the most massive planet it is thought to be the most likely to have retained its original matter. Investigation of Jupiter will probably prove critical not only to the study of the evolution of the solar system but also to understanding the formation of the universe.

Scientific Objectives

   A key question in cosmology concerns the formation of chemical elements. Answers to this question may be found by determining the abundance ratios of isotopes, for example, the ratio of carbon-12 to carbon-13. The Space Science Board of the National Academy of Sciences recommends examining all the outer planets to establish the relative abundances and isotopic ratios of hydrogen, helium, carbon, and heavy elements up to mass 40. The Board's full list of objectives reveals the maturity attained by space science since the early days of space exploration when the small and simple Explorer 1 satellite, bearing a Geiger-Mueller tube for counting radiation, detected the now famous Van Allen belt. The objectives are as follows:

1. Investigate the appearance, size, mass, magnetic properties, and dynamics of each of the outer planets and their major satellites.

2. Determine the chemical and isotopic composition of the atmospheres of the outer planets.

3. Determine whether biologically important organic substances exist in these atmospheres.

4. Describe the atmospheric motions and the temperature-density-composition structure.

5. Make a detailed study for each of the outer planets of the external magnetic field and particle population, associated radio emissions, and the interactions of particles and waves in the magnetosphere.

6. Determine the mode of interaction of the solar wind with the outer planets, including the interaction of satellites with a planet's magnetosphere.

7. Investigate the properties of the solar wind and the interplanetary magnetic field at great distances from the sun both near the sun's equatorial plane and far from it, and search for the outer boundary of the solar-wind flow -- that is, find the boundary of the solar system.

8. Measure the composition, energy spectra, and flux density of cosmic rays in interstellar space, free of the effects of the solar wind.

   Scientists believe it will be necessary to determine the composition of at least four of the five outer planets before a sound theory of the origin of the solar system can be formulated. Uranus and Neptune, the Uranian planets,  contain less hydrogen than the Jovian planets, Jupiter and Saturn, and so show less similarity to the sun.

Jupiter

   The exploration of Jupiter from space began with the launching of the United States' Pioneer 10 probe in March 1972. The spacecraft passed without harm through the asteroid belt and flew to within 130,000 kilometers (81,000 miles) of the planet, sending back data on Jupiter's magnetic and radiation fields as well as photographs and spectroscopic observations. The data indicated that the magnetic field was more extensive and differently shaped than had been thought. A very intense electron belt was found. More exact information was also obtained about the masses and sizes of some of Jupiter's satellites, as well as a more precise value for the mass of Jupiter itself. A second Pioneer, Pioneer 11, launched in 1973, flew within 42,000 kilometers (26,000 miles) of Jupiter and passed through the ring system of Saturn in 1979.
   Analysis of the Pioneer data shows that Jupiter probably consists almost entirely of liquid hydrogen, with some helium and other elements in its atmosphere. The interior of the planet remains very hot from the time of its formation, and it still radiates energy about twice as fast as it receives it from the sun.
   The U.S. space probes Voyager 1 and 2, launched in 1977, flew by Jupiter in 1979. Voyager 1 passed Saturn in 1981; Voyager 2 in 1980. They examined at close quarters the swirling cloud patterns of these giant gassy planets, their major satellites, and the rings of Saturn.
   Satellites orbiting Jupiter are planned. One proposed Jupiter orbiter would circle the planet once each 35 days. The orbit would be very eccentric. About 90 percent of the time the spacecraft would be over a million miles from the planet, and sometimes as far as two million miles; but it would also approach Jupiter's surface within 100,000 miles -- a very good distance for observing the planet, while safe from its radiations. The orbiter would make the equivalent of ten flybys in an earth year, enough to permit good viewing of the cloudy atmosphere, and mapping of the bow shock, magnetosheath, magnetopause, energetic particles, trapped radiation, and magnetic belts within the magnetosphere. The eccentric orbit would also allow the spacecraft to encounter many of Jupiter's satellites. A spacecraft in proper orbit could encounter Io, Europa, Ganymede, and Callisto at least twice each in a year's time.
Asteroids and Comets. Asteroids and comets may represent primordial materials from the solar system's formation, so it will be important to sample them. Flybys and indirect measurements will probably come first. A landing on an asteroid will not be difficult, and a sample could thus be returned to earth for analysis. Spacecraft can try to penetrate a comet nucleus and sample it for onboard analysis; but that may prove a difficult mission because of the effect of the comet's dust and gases on the spacecraft.
 

Studying The Sun

   It is hard to realize that only a generation ago physicists had developed no more than a rudimentary theoretical picture of the solar processes that excite magnetic storms in the earth's upper atmosphere. In the 1930's scientists in England had developed a theory for expansion of the sun's corona into space and explained aspects of magnetic storms. By the early 1950's a German physicist had suggested that outward streaming plasma from the sun (that is, the expanding corona) accelerated ions in comet tails. (A plasma is any collection of high-temperature charged particles.) By 1957, as the space age began, the American physicist E. N. Parker began publication of a series of papers that developed a theory of the "solar wind," as he called it, a name that is now widely used. His theory, confirmed by space-probe studies, predicted a continuous flux of particles accelerated to supersonic velocities by a process of hydromagnetic acceleration in the high-energy solar corona, rather than a static interplanetary medium penetrated by tongues of plasma as others had proposed. (Hydromagnetic acceleration is acceleration by magnetic fields.) The Luna 1 probe in 1959, Mariner 2 in 1962, and Explorers 10 and 12 (operating in the earth's magnetic field) gave solid experimental backing to Parker's theory. This direct experimental substantiation of the theory of solar wind represents a foremost achievement of space science.
   Such theoretical work, discovery of the earth's radiation belt, and early experimental success with interplanetary probes precipitated a massive study of "solar weather" and its influence on the earth. A decade of exploration with such vehicles as Explorers, Orbiting Geophysical Observatories, IMP (Interplanetary Monitor Probes), Pioneer probes, Orbiting Solar Observatories, and other vehicles developed a basic picture of solar weather phenomena, the interplanetary medium, the particle and field environment of earth, and their structure and dynamic interrelations. Skylab, launched in 1973 (see above), carried a battery of solar instruments including a coronagraph, several ultraviolet spectrographs, and X-ray telescopes. It produced a large amount of new information, particularly about the solar corona. The Helios 1 and 2 satellites, designed mainly by West German scientists and launched into solar orbit by the United States in December 1974 and January 1976, respectively, also produced important data. Solar Maximum Mission, launched by NASA in 1980, was an unmanned spacecraft equipped with six telescopes to study sunspots, flares, and other solar features.
   At the sun, energy surges upward from the turbulent photosphere as acoustical and hydromagnetic waves, enhanced over active solar regions. Plasma wells outward into space under hydromagnetic acceleration in the corona, drawing a magnetic field trapped within it. The sun's rotation winds the field lines into Archimedean spirals in what has been called the garden-hose effect. The solar wind is a good indicator of what is going on inside the sun. As one physicist has remarked, the solar wind's "smell" (composition, temperature, velocity, and frozen-in magnetic field) "tells us what's cooking in the solar oven." Near the earth the solar wind has a speed of about 350-400 kilometers (200-250 miles) per second. During a big solar flare this speed may jump to 700 kilometers (430 miles) per second. The wind consists largely of hydrogen and helium ions (that is, protons and alpha particles), with some heavier ions. It is electrically neutral, with enough negative electrons to equal the charge of the positive ions.
   Solar cosmic rays, produced in flares, tend to follow the spiral field of the solar wind. Galactic cosmic rays, on the other hand, curve around the field lines in interplanetary space and are scattered away from the sun. Therefore, the stronger the solar wind (as during periods of maximum suspect activity), the weaker the galactic cosmic ray flux at earth. Solar flares also interact with normal solar wind flows to produce shock waves within the wind and accelerate particles in it. These and many other interactions can be mapped and studied only by spacecraft.
   The solar wind produces a shock wave on the earth's magnetic field, called the magnetosphere, compressing it on the sun side and forcing out a long tail on the dark side  along the ecliptic plane well beyond the orbit of the moon.
Spacecraft in solar orbit, interplanetary probes, earth-orbit satellites, moon and Mars orbiters, and outer-planet missions together will give an immediate, three-dimensional reading of solar weather events, helping to refine existing theory and relating solar events to weather changes on earth. The analysis of the solar wind will be extended to every planet, even out beyond Pluto, perhaps as far as 50 astronomical units from the sun, where the wind forms a boundary with the interstellar medium.
 

OTHER ASPECTS
 

Space Astronomy

   The heavens are best viewed from artificial satellites orbiting the earth above the obscuring effects of our atmosphere. The United States first launched V-2 rockets in 1946. Since then, astronomy and astrophysics have played an important role in space activities. Orbiting laboratories, such as those of the HEAO series, gather information about galactic and extragalactic objects using all parts of the electromagnetic spectrum. A 9.5-inch (240-cm) telescope (the Edwin P. Hubble Space Telescope) was launched by the space shuttle in April 1990. Astronauts of the space shuttle Endeavour corrected some deficiencies in the telescope's optical system during a visit in December 1993. (See also Space Astronomy.)
 

Earth Applications

   Orbiting satellites are now used for telephone and television communications, as well as for military reconnaissance, meteorology, geography, oceanography, geophysics, atmospheric physics, navigation, air traffic control, and surveying. Other applications are in mapping, hydrology, demography, prospecting, and the monitoring of air and water pollution. (See also Mineral Deposits of the Earth; Navigation: Modern Methods of Navigation.)
 

Weather Prediction

   Instruments carried aloft by balloons, planes, rockets, and orbiting satellites have given a clearer picture of earth's weather than ever before. Agencies such as NASA, the World Meteorological Organization (WMO), and the European Space Agency (ESA) coordinate the data gathered by satellites to produce day-to-day maps of weather patterns worldwide. Powerful computers may one day analyze data in order to predict weather beyond the one or two days possible now. (See also Meteorology and Climatology: Contemporary Weather and Climate Concerns.)
 

Communications Satellites

   In 1964 the International Telecommunications Satellite Consortium (Intelsat) was set up to establish a single global commercial communications satellite system. To date, six generations of Intelsat satellites have been launched; these vehicles now handle most of the world's international communications. (See also Communications Satellite; Military Communications.)
 

Navigation and Traffic Control

   A series of satellites to aid navigation on the world's oceans and inland waterways was first launched in the early 1960's. Schemes for directing air and ground traffic by satellite are in various stages of testing and development in a number of countries.
 

Surveying Earth Resources

   Since 1972 satellites of the NASA Landsat series have surveyed the earth's food, forest, and mineral resources. Seasat, launched in 1978, was designed for scientific ocean observation. (See also Remote Sensing.)
 

SPACE BIOLOGY

   Space biology is the science of life beyond the environs of the earth. The term as it is used in this article encompasses

1) those sciences that delineate extraterrestrial environments in relation to life;

2) those that are concerned with the medical and biological problems involved in getting humans into space or on celestial bodies and returning them safely to earth; and

3) those sciences that deal with problems relative to the determination of extraterrestrial life, the avoidance of contamination of extraterrestrial bodies, and the avoidance of contamination of the earth with materials brought back from space.
 

Space Medicine

   Space medicine is a direct extension of aviation medicine to the space environment. In general usage the term encompasses the medical, physiological, and psychological problems associated with flight into the new and hostile environment; selection and care of space travelers; the human problems associated with movement of the vehicle; the development of means to support, project, and maintain travelers in space or on celestial objects; and problems associated with the traveler's return to earth. The roots of space medicine lie within many diverse categories of science, several of which, until recently, seemed remote from the sphere of medicine. Background information on the general problems of space medicine will be found in the article Aviation Medicine.

Space Equipment

   The term "space," as applied to the environment beyond the earth's atmosphere, is a misnomer which came into use before the true situation was known. Actually, the universe beyond the upper reaches of our earth's atmosphere is not empty at all but contains many particles of matter in one form or another, representing most of the elements of the periodic table. The flux of particles, largely protons and electrons, emanating from the sun and sweeping by the planets is called the "solar wind," a term hardly consistent with the idea of empty space. Also present in space are all of the energies of the electromagnetic spectrum. Some of the matter of macroscopic size -- meteoric particles, for example -- has been accelerated to very high velocities.

Earth's Atmosphere:

   Life on earth has evolved beneath the very effective protective and life-supportive canopy afforded by the earth's atmosphere. The protective qualities of the atmosphere are due to its mass and chemistry. The life-supportive qualities result from the presence of oxygen, nitrogen, carbon dioxide, and water vapor, along with minor constituents, and the pressures they exert.
Since most of the problems of space medicine arise from the presence of the earth's atmosphere (because of resistance it produces) or the absence of the earth's atmosphere (because of the loss of its protective and life-supporting qualities), it is important to consider the structure of this atmosphere. More than 99 percent of the mass of the earth's atmosphere lies within the first 32 kilometers (20 miles) of altitude. Thus it forms a relatively thin layer equal to about only 1/400 of the diameter of the earth -- comparable in relative thickness to the peel of an apple. The plant and animal populations of the earth, almost in the entirety, dwell in the innermost one-fourth of this very thin layer. They encounter various difficulties as they try to go high into it or above it. The remaining one percent of the atmosphere extends outward in rapidly decreasing density. Functional space begins about 240 kilometers (150 miles) above the earth, at which level most atmospheric resistance (drag) to a space vehicle has ceased to exist.
Temperature. On earth, an object's temperature is in part determined by the surrounding temperature. At about 74 to 77 kilometers (46 to 48 miles) of altitude the atmosphere becomes so rarefied that heat can no longer be transferred to it. Consequently, the temperature of any object is the result of a balance between the heat radiated to it by the sun or from another source and the heat it radiates away.  Therefore, above this level, temperature depends upon the absorptive or reflective characteristics of an object. It depends upon the brightness, or the dullness, or the color of the surface. Temperature of an object -- for instance, a space vehicle -- also depends upon how great an area is exposed to the sun or other heat-radiating source, and whether the portion whose temperature is being measured is on the shadow or sunny side, or whether the entire object is in the shadow of the earth or other body. The distance from the radiation source is obviously of importance.

Radiation:

   Various types of radiation hazards are associated with space flight. Most important are geomagnetically trapped radiation (Van Allen belt), solar-flare radiation, and cosmic radiation. As space flight progresses, two other types will probably have to be reckoned with: radiation from nuclear power sources and radiation produced by extremely high-speed passage of a vehicle through the sparsely scattered hydrogen atoms of space.
   Geomagnetically trapped radiation consists of charged particles of high energy, protons and electrons, trapped in the earth's magnetic field, forming the Van Allen belt, a doughnut-shaped region of varying density. The inner zone, formed of protons, is at an altitude of about 3,200 kilometers (2,000 miles), with its lower boundary as low as 480 kilometers (300 miles); the outer zone, formed of electrons, ranges from 16,000 to 160,000 kilometers (10,000 to 100,000 miles) in altitude. The shape of the belt is strongly influenced by the solar wind.
   Radiation of high energy associated with solar flare phenomena may pose a great hazard to crewmembers participating in extended space flight. During random solar chromospheric eruptions, streams of particles, some of which are in the billion electron volt (GeV) range of energy, are ejected by the sun. A solution to the problem depends on accurate prediction of solar eruptions and/or shielding concentrated in such a manner that it could be protective during the period of exposure but would not be of prohibitive weight.
   Cosmic radiation consists of a more or less constant low flux of extremely energetic particles from sources probably beyond the planetary system. The particles consist of a mixture of stripped-down atomic nuclei ranging from protons upward in the atomic scale through iron and possibly heavier elements. Present assessment evaluates this form of radiation as probably the least of the three hazards. The kinds of radiation produced by nuclear power sources and from the high-velocity impact of hydrogen and other atoms during high-speed travel are too poorly defined for detailed consideration.

Environment of the Moon:

   In comparison with the earth, the moon is a small body, having a diameter of approximately 3,480 kilometers (2,160 miles). As it is also of low density (about 3.33 times the density of water whereas the earth's density is about 5.5 times that of water), its mass is only about 1/80 that of the earth. Consequently, its gravitational force, 1/6 that of the earth's, has been unable to retain an atmosphere. Any remnant of an atmosphere that it may have is only about one trillionth as dense as that at the surface of the earth and probably consists only of the heavy gases, such as xenon, krypton, and argon. Without an atmosphere and without an electromagnetic shield, the moon's surface is exposed to the entire electromagnetic spectrum of solar and galactic radiation and to larger particles of cosmic rubble. There is no water vapor, no weather, and thus no erosion as we know it on earth. The moon rotates about the earth in an elliptical orbit every 27.32 days. Consequently, there is about half an earth-month of daylight and a similar period of darkness. Since there is no atmospheric amelioration, surface temperatures may reach more than 120C. (250F.) in some areas during daylight and quickly descend to below -130C. (-200F.) during darkness.

Medical Problems of Space Travel

   The high thrust and rapid increase in acceleration rate of the rocket as it lifts off the launch pad and rises to an orbital trajectory place a strain on the bodies of astronauts within a spacecraft. In multistaging the acceleration rate increases as rocket fuel is expended and suddenly ceases when the propellants are used up and the engines are cut off for each successive rocket stage. This succession of bursts of speed is more tolerable to the body of a passenger in the spacecraft than a single, prolonged, and necessarily higher acceleration would be.
   It has been demonstrated that human tolerance to acceleration is determined to a significant extent by the amount of body distortion produced by the force, particularly in the form of blood displacement. Consequently, tolerance is improved when distortion or displacement is prevented by proper body positioning, cushioning, and body restriction.
   The relationship of the direction of the forces producing acceleration (g) to the body axis is also important. (The symbol g stands for the unit of measure for accelerating force. A body experiences an accelerating force of 1 g when it is being accelerated at the rate the earth's gravity accelerates free-falling bodies near the earth's surface, that is, 980 centimeters/second2, or 32 feet/second2.) When the forces are applied parallel to the long axis of the body (head to foot), the effect on the circulation of blood is great. Contoured or body-molded support covering much of the body gives much added protection. A subject protected in this manner has tolerated peak loads of 25 g for periods as long as 40 seconds without serious difficulty. The term 25 g used in this manner would represent a force of the subject's body against the cushion of 25 times his or her body weight. Studies have been made of people subjected to high accelerative forces while supine in a water immersion capsule. Such subjects have stood as much as 31 g for periods up to five seconds when the force was applied in a direction from back to chest.
   Above the atmosphere, where stabilizing by means of wings or fins is no longer possible, there is a tendency for vehicles to spin about their long axis or to tumble end over end. The human body cannot tolerate either rapid spinning or tumbling for any appreciable period of time. Neither can it adapt well to such aberrant motions. In consequence, a stable vehicle is an important consideration. However, in order to conserve fuel, periods of slow drifting in space have been allowed in space flights without any harmful effects.

Weightlessness:

   After orbit has been achieved beyond the atmosphere and thrust has ceased because of fuel burnout, the vehicle and everything on board reaches the weightless state, also referred to as zero-g. This situation is most easily understood if one takes the vehicle as the frame of reference. Since everything in this frame (that is, the vehicle, everything in it, and everything that might be dumped from it) experiences exactly the same acceleration, then no net acceleration is produced within this frame. Within this frame, all objects float freely (weightlessly) with respect to one another. The weightless state continues until the craft speeds up, rotates, tumbles, or encounters drag or some other form of resistance through reentry into the atmosphere or through the application of some force -- for instance, the retrothrust of rockets used to slow the vehicle.
   During the weightless state fluid forms spherical globules because of surface tension phenomena. The globules float  free of the walls of the containers. Thus, open containers are useless for fluid or food taking. Substitutes used have included squeeze bottles, pressure containers, bite-sized pellets of solid food, and plastic bags in which dehydrated food can be reconstituted and squeezed into the mouth, much as toothpaste is squeezed from a tube.
   In the past, there has been controversy about the maintenance of orientation in the weightless state, as the sensory apparatus of the inner ear and other portions of the body are gravity-oriented. However, during the American space flight series of projects Mercury, Gemini, and Apollo, including the 14-day Gemini 7 flight, orientation was no problem, although a few astronauts experienced brief periods of nausea. In the Soviet series there were incidents of minor illness and disorientation. It is theorized that observation of the horizon, horizon simulation instruments, or body cues afforded by the restraints compensated for the loss of the gravity vector. The performance of tasks requiring fine movements, such as the handling of controls, when the gravity vector is absent seems to be no problem.

Reentry into the Earth's Atmosphere:

   The problem of reentry into the earth's atmosphere following space flight is quite serious, as it involves a relatively rapid slowdown from speeds in the neighborhood of 28,200 kilometers (17,500 miles) per hour if from earth orbit, or higher speeds if the spacecraft is returning from the moon or interplanetary space.
   Positioning of the astronaut's body, cushioning, restraint, rapidity of the change of velocity, and other factors are important in the determination of human tolerance to decelerative forces. Many reentry flight profiles have been studied in centrifuges and track decelerators. These studies demonstrated that with proper reentry trajectories a pilot returning from orbit is able to stand the forces without serious effect. If the rate of onset is slow enough to allow the body's circulatory apparatus to adjust itself, deceleration of 3 g can be maintained for as much as one hour, whereas accelerations above 8 g without a special type of protection soon result in blackout or difficult breathing. Typical spaceshuttle operations during launch and reentry will stay within the 3-g figure.
   Biomedical factors telemetrically monitored during the space flights include pulse and respiration rates, blood pressure, body temperature, heartbeat, and respiratory movements. Urine and fecal wastes have been collected and have been analyzed after return to earth. In addition, detailed medical examinations have been made. Most physiological changes appear to revert to normal in a few hours or days. The most dramatic such change is seen in pulse rate, which occasionally becomes quite high, especially during EVA (extravehicular activity), but rapidly returns to normal. Following a few of the flights, orthostatic hypotension, a tendency toward faintness as the astronauts first stand up after emerging from the capsule, has been noticed. This is the same effect, however, that patients have when first standing after long confinement in bed. After the Soviet Soyuz 9 17-day-long flight, cosmonauts Andrian Nikolayev and Vitaly Sevastyanov found adjustment to the earth's gravity to be quite difficult. Their heads, limbs, and other portions of their bodies felt extremely heavy to them. The sensation lasted for five to eight days following their return from the long flight. It may have been caused by the decrease in blood volume observed during weightlessness or by the deconditioning of veins which was found in Soyuz 9 cosmonauts. Decalcification of bones was also found, and poses a potential danger for long flights such as those to Mars and other planets.
   Weight losses in longer flights have been as high as 3-4 kilograms (7-8 pounds). To date weight losses have been regained within a few days. Astronauts are trained to overcome disorientation caused by spinning and tumbling.
 

Life-Support Systems:

   The complexity of the life-support systems and their size and weight are dependent upon the type of mission, the time period covered by the mission, and the number, size, and weight of occupants of the space vehicle. For flights of relatively short duration, most of the life-support materials, such as food, water, and oxygen, can be carried in containers. The containers must, of course, be as light as possible and of the smallest possible capacity, as weight costs for each pound of payload, depending on the mission time and distance, may require hundreds, thousands, or tens of thousands of pounds of fuel and structure in the booster assembly for the launching. In the present state of space flight, resupply during space operations would be extremely difficult; consequently, each item for either astronaut or vehicle must be conserved as far as possible or used over and over again, as in the case of recycling in closed ecological loops. If an item of necessity can have more than one use, that, too, is a method of saving weight.
Daily requirements for a 68-kilogram (150-pound) man performing tasks of the type an astronaut is required to perform are usually considered approximately as follows: food, 0.50 kilograms (1.1 pounds); oxygen, 0.86 kilograms (1.9 pounds); and water, 2.3 kilograms (5.0 pounds). The same body in the course of a day gives up as waste material approximately, carbon dioxide, 1.0 kilograms (2.2 pounds); water, 2.5 kilograms (5.6 pounds); and solid wastes, 0.09 kilograms (0.2 pounds). The apparent discrepancy between water intake and water output is the result of the synthesis by the body of a certain amount of water (about 10 percent) in the course of metabolism of food. The astronauts are supplied with a food equivalent of 2,500 calories per person per day. During most flights they have consumed slightly less. Water supply of 2.7 kilograms (6 pounds) per person per day has proven ample.
 

Extravehicular Activity (EVA):

   To perform many future space tasks, astronauts will be required to work outside their spacecraft. EVA experiments during space flights have shown the practicality of working in space and have demonstrated many of the human problems involved. Some of the difficulties include the extreme work load induced by the relative inelasticity of the pressurized suit and gloves; the freezing of moisture inside the face-plate produced by a combination of perspiration from work and the low temperature of space, especially during periods of travel in the earth's shadow; and the difficulty of working and maneuvering in the weightless state, complicated by the implications of Newton's principle of action and reaction.
 

Closed Ecological Systems:

   Since the problem of resupply of materials and energies for space operation is always extremely difficult, recycling of water, the breakdown of carbon dioxide for its oxygen content, and reutilization of the basic constituents of food have been proposed.
In other words, for extended space travel it is desirable to perform the functions of the earth's ecological system -- in which animals, plants, the atmosphere, the seas, and the earth utilize one another's waste products, furnishing in return the basic constituents required for life. If these complex processes could be duplicated in miniaturized form in a space vehicle, many of the problems of extended space travel could be solved. Studies are under way to duplicate some of the ecological processes. For instance, there has been considerable scientific research in the use of certain  nonpoisonous algae as gas exchangers -- exchanging respired carbon dioxide for oxygen. The usually stated formula for this reaction is: carbon dioxide + water + light, through the photosynthetic process of green plants, give starch or sugar + oxygen. This reaction immediately suggests the possibility of using body wastes as nutrients for the algae and receiving from the life processes of the algae not only oxygen but food. Broad-leaf plants have also been studied and indicate some promise. Closed, self-sustaining ecological systems may be possible by the year 2000.
 

Exobiology

   Exobiology is the science of extraterrestrial life. Synonyms are astrobiology, planetary biology, and cosmobiology. Recent advances in radioastronomy, balloon and rocket probing, and manned space flight and in organic chemistry, zoology, and botany have brought phases of the science to a project basis.
   The objectives of the study of extraterrestrial life are manifold. Most often considered is the enhancement of our knowledge of earth life through study of the overall framework of the universe. Inquiries would concern the origin of the universe and its components, paleontology, comparative evolution, comparative biology, botany, zoology, chemistry, physics, anthropology, and sociology. The possibility of an exchange of knowledge with intelligent life elsewhere is often mentioned. Knowledge about the possibility of the inadvertant contamination of celestial bodies through rocket probes or visits by humans, which might confuse later studies, is necessary. Conversely, knowledge is also required concerning the possibility of the contamination of the earth and its indigenous plant and animal population by returning voyagers.

Search for Extraterrestrial Life

   The search for extraterrestrial life must be separated into two areas. The first is a search for life in our planetary system, and the second is a search for life beyond our planetary system. Distances, the limitation imposed by the speed of light, propulsion energies, and logistics force the division. Reaching other planets in our system is possible in the foreseeable future, whereas reaching planets outside our system does not seem likely. Efforts to discover life beyond our sun's planets must be limited to listening for intelligent signals on radio bandwidths that intelligent beings might use. Searching for life in our solar system is much less complex.    Temperature, radiation, physical and chemical profiles, as well as proximity, mark the moon, Venus, and Mars as the most likely subjects for study in this generation. All three lie within the ecosphere, the zone of the solar system in which conditions such as light, temperature, and the availability of fluid water do not preclude life of the carbon-hydrogen-oxygen type. American and Soviet space probes to Venus and Mars had not detected any definite signs of life on either of those relatively nearby planets, although the interpretations of the data returned by the Viking landers on Mars do not yet and likely will not rule out the possibility that some primitive, unknown biological activity takes place on that planet. Nor did samples brought back from the moon include any organic matter. However, further searching for extraterrestrial life forms will certainly be a part of future space exploration.
 

Problems of Contamination

   Contamination of celestial objects by earth probes or vehicles carrying humans, animals, or plants is a source of concern, as is the possibility of a returning probe or vehicle contaminating the earth. It has often been pointed out that organisms that are normally held under control by nature's system of checks and balances might act very differently if the system were to be disturbed. According to this thesis, contamination of a celestial body with earth organisms in an environment free of defense barriers, such as are present on earth, might result in overgrowth and altered growth. This then could destroy any potential for study of virgin conditions.
   Returning contaminating materials to earth where no natural defense mechanisms have evolved might produce similar problems and others involving our public health, agriculture, and other areas. Studies are being made on methods of preventing contamination.
 

HISTORY OF SPACE FLIGHT

   People have always dreamed of escape from the earth and of travel to other parts of the universe. As early as a.d. 160 two stories of flights to the moon were written in Greece. Lacking scientific knowledge, the author, Lucian of Samosata, failed to differentiate between a flight through the air and a trip through space. In Lucian's True Story  a seagoing vessel is blown to the moon by a storm; in his Ikaromenippus the hero flies to the moon with the aid of the wings of large birds. After the Middle Ages, Lucian's stories were printed several times in Europe. In the Orient, as well, people were speculating about travel to the moon. A Chinese scholar, Wan Hu, is said to have had the moon as his goal when he built a rocket chair in 1500. He strapped himself into the chair to which he had attached 47 skyrockets; then, holding a kite in each hand, he ordered the rockets ignited. The chair disappeared in flame, and with it Wan Hu.
   The scientific advances of the Renaissance on which a theory of space travel could be based are connected with the names of Nicolaus Copernicus and Johannes Kepler. Copernicus proved that the sun, and not the earth, is the center of the solar system. Kepler showed that the planets in the "Copernican system" move in elliptical orbits about the sun, but, since the concept of inertia was unknown in his time, he believed that there had to be a motive power originating in the sun to keep the planets moving.
 

Newton

   The principles of mechanics that made space flight scientifically feasible were set forth in the 17th century by Sir Isaac Newton in his law of universal gravitation and laws of motion. The publication of his Principia in 1687 was a tremendous step forward in science. This work, one of the greatest intellectual achievements of all time, formed the basis of what is called classical mechanics, the mechanics to which there was no rival until the appearance of quantum and relativistic mechanics in the early 20th century, and on which all computations of trajectories and other dynamic problems is still based. One could say that artificial satellites were predicted in the Principia. Newton discussed in detail what would happen to cannon balls fired horizontally from the top of a high mountain. He pointed out that a cannon ball fired with a certain velocity would describe a trajectory with a certain degree of curvature and that the ball would hit the earth a certain distance away. Then, he said, if we double the velocity of the cannon ball it will strike the ground a much longer distance away and the curvature of its trajectory would be less. Now, if we could increase the muzzle velocity at will, we would obtain longer and longer ranges with trajectories of less and less curvature. We might shoot halfway around the earth. And if the velocity is high enough, the curvature would become so slight that it would match the curvature of the earth's surface below so that the cannon ball would go all the way round the earth. Its path would never intersect the surface of the earth; it would be in orbit. After developing this thought, he pointed out  that the moon's orbit around the earth depends on the same natural laws, namely an inertial velocity that is bent into a closed orbit by the earth's gravitational pull.
   Science was infinitely far ahead of engineering -- the art of applying and utilizing scientific principles -- in Newton's time and for nearly 200 years after. Toward the end of the 19th century, engineering had progressed to a point at which inventors interested in space flight could begin to apply Newton's principles. Concepts began to be developed for astronautical vehicles in a manner that was characterized by skillful application of fundamental principles to practical technical schemes.
 

Space Pioneers

Tsiolkovsky

   Many of the ideas that are basic to rocket design and space travel were set forth in 1898 in a report on the "Investigation of Interplanetary Space by Means of Rocket Devices." The author of this report, a Russian schoolteacher named Konstantin Eduardovitch Tsiolkovsky, was the first to derive many of the fundamental mathematical equations of rocketry. Tsiolkovsky proposed using liquid propellants instead of solid propellants for rockets, starting with alcohol and liquid oxygen and then proceeding with liquid hydrogen and liquid oxygen, one of the most powerful chemical propellant combinations (exceeded only by hydrogen-fluorine and hydrogen-ozone). He proposed multistaging in order to achieve high velocities. The body shape he envisioned was round and streamlined for minimizing aerodynamic drag during ascent (see Fig. 1). He proposed to increase the stiffness of these bodies by internal pressurization in order to attain maximum rigidity at lowest weight, since this would permit minimization of the skin thickness of the hull. He suggested surrounding the combustion chamber with two cooling jackets, an inner jacket containing fuel and an outer jacket containing liquid oxygen.
   Guidance and control in Tsiolkovsky's design were to be fully automatic, with a manual emergency system that could override the automatic device. The control devices (jet-vanes) were to be activated by doubly gimbaled gyroscopes. Utilizing the inherent stability of fast-spinning gyros, any rotation of the spaceship about one of its three principal axes (pitch, yaw, and roll) would be detected by these gyros, which would then release electrical commands to the jet-vane deflection motors, bringing about changes proportional to the ship's deviation from its correct position. The modern all-inertial guidance system used in missiles and rockets is based on the same principle. The launching mechanism of Tsiolkovsky's spaceship is also of interest. He assumed horizontal takeoff. The spaceship was partly inserted into the front end of a completely automatic rocket sled from which the spaceship was to start when the sled had reached the highest possible speed. Although today Tsiolkovsky's importance is recognized, his work received little notice during his lifetime.

Goddard

   An American rocket pioneer, Robert Hutchings Goddard, was responsible for much of the earliest experimentation and research in rocketry. In 1914 he received a patent that includes mention of a solid-propellant combustion chamber that can be modified for continuous combustion using a liquid fuel and a liquid oxidizer. Goddard's first publication was a report entitled A Method of Reaching Extreme Altitudes , printed by the Smithsonian Institution in 1919. In this work he discussed rockets as instrument carriers for the exploration of the upper atmosphere and calculated the size necessary for a rocket capable of going to the moon. The report consisted mainly of a theoretical treatment of various aspects of rocket propulsion, theories that were put to the test with the world's first flight of a liquid-propellant rocket, launched by Goddard on Mar. 16, 1929, in Massachusetts. Three years later he launched the first instrument-carrying rocket, which included a barometer, a thermometer, and a camera to record readings at maximum altitude. From 1930 to 1941 he continued research and development of rockets; receiving 214 patents, many for components that have become standard in rocket design. His work received little attention because it dealt with liquid-propellant systems most applicable for rockets of large size and long range, rockets for which there was no need at the time.

Oberth

   A contemporary of Goddard's in Germany, Hermann Oberth, provided much of the theoretical knowledge essential to modern rocketry. In 1923 his pamphlet Die Rakete zu den Planetenrumen ("The Rocket into Interplanetary Space") presented many theoretical and speculative aspects of rocket flight and space travel, going beyond much of the work of Tsiolkovsky and Goddard, whose writings were unknown to him until his own calculations were nearly completed. Oberth envisioned a two-stage, liquid-propellant human-carrying rocket.

Hohmann

   The publication of Oberth's book encouraged Dr. Walter Hohmann, also in Germany, to publish in 1925 studies that he had begun in 1910 in which he gave a systematic analysis of departure from the earth, return to the earth, free coasting in space, and circumnavigation of other worlds. He was the first to suggest using the atmosphere as a brake during reentry.
 

World War II and Postwar Developments

   During World War II the ideas of Oberth, Goddard, and Tsiolkovsky were put to use in building the first ballistic missile, the German V-2, under the direction of Wernher von Braun. The V-2 was used by the Germans to bombard England. It was the progenitor of the rockets used to boost people into space.
   After the war the United States and the Soviet Union both appropriated many V-2 rockets and V-2 designers and technicians for their programs of rocket development, concentrating on military weapons. The work of 130 German scientists and engineers who accepted an invitation to come to America in 1945 culminated later in the development of the Saturn V, the biggest and most powerful descendant of the V-2.