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 (6¹ 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 (1¹ 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 22°C. to 24°C. (70°F. to 75°F.).
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 6¹ 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 -101°C.
( -150°F.) 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
10¹ kilometers (6¹ miles) in a total of 6 months of activity
during 10¹ 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 7¹ 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 425°C. (800°F.),
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 450°C. (843°F.). 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 125°C.
(257°F.) 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 120°C. (250°F.) in some areas during daylight and quickly descend to below -130°C. (-200°F.) 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 Planetenräumen ("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.