Necessity for Space Observations
Knowledge in astronomy is based on what is
learned from radiation that reaches the earth from distant objects and
regions in space. Such radiation does not flow unobstructed between its
origin and the earth. Along its path it is scattered by dust and electrons,
absorbed by atoms, and deflected by magnetic fields; finally, it must pass
through the earth's atmosphere before reaching a telescope. Besides its
major atomic constituents, the atmosphere contains varying quantities of
ozone, water vapor, and dust, each of which has its distinctive effects
on the incident radiation. Also, the atmosphere is not static; variations
in temperature and pressure create constant motion. This effect produces
the "twinkling" of stars which, while beautiful to look at, is possibly
the most serious limitation on ground-based optical astronomy.
The earth and its atmosphere are themselves sources of radiation. Some radiation -- radio waves from broadcasting stations for example, and visible light from cities -- results from human activities. Some, however, such as the faint glow from aurora-like activity, is intrinsic to the atmosphere.
The need for astronomical observations from space is related to a transformation that took place in astronomy between about 1900 and 1930. To this period belong 1) the development of the general theory of relativity and its observational verification in a few cases; 2) the discovery of cosmic rays and the realization that their origin is not the solar system; 3) the development of the concept of our local galaxy, the Milky Way, as a vast assemblage of stars and the awareness that galaxies exist far beyond our own; 4) the discovery of the recession of the galaxies and the development of the idea of the expanding universe; 5) the discovery of white dwarf stars and the realization that they represent a collapsed state of matter and that other such states could exist; 6) the recognition of the enormous energy release in supernovae, and the development of the idea of exploding stars.
Each of these developments represented a radical change from what had been known or thought; collectively, they caused as great an upheaval in astronomy as had ever occurred. As scientists tried to exploit and extend the new ideas, the limitations imposed by the atmosphere became increasingly frustrating. It was fortunate for astronomy that, at about the same time, the technology was developed to carry instruments far above the atmosphere, by means of sub-orbital rocket flights and orbiting satellites. The launch of an orbiting satellite usually implied that its payload or instrument package could not be returned intact to earth, but with the advent of the Space Transportation System (the shuttle), instrument retrieval will again be possible.
Divisions of Space Astronomy
Space astronomy provides information concerning
virtually all aspects of traditional (ground-based) astronomy and has spawned
besides new astronomical disciplines that owe their existence to the serendipitous
discoveries made from space. The principal objectives in the various disciplines
are listed in the table. Roughly, the field divides into three areas. The
first, comprising infrared, optical, and ultraviolet astronomy, has been
traditionally the domain of ground-based optical astronomy. The advantages
of space optical astronomy are avoidance of atmospheric absorption at infrared
and ultraviolet wavelengths, and elimination of atmospheric distortion
at visible wavelengths. The second area is high-energy astronomy, which
comprises extreme ultraviolet, X-ray, gamma-ray, and cosmic-ray astronomy.
The oldest of the three space disciplines, high-energy astronomy requires
space observations because of the virtually total atmospheric absorption
of high-energy photons and the destruction of high-energy cosmic-ray particles
by the atmosphere. Finally, the area of space solar physics is distinguished
by the fact that it is not defined by a wavelength interval of observation,
but rather by a particular object of study: the sun. The study of the sun
from space encompasses all the techniques of the other two space astronomy
disciplines, but stands apart because the sun's proximity allows a level
of observational detail unequalled in them.
The relative development of the various space astronomy disciplines varies enormously. Whereas, for example, space ultraviolet and X-ray astronomy are developmentally on a par with ground-based astronomy, fields such as space infrared astronomy will come into their own during the shuttle era. For this reason, the number and quality of observations in space astronomy cover an extremely wide range, and are in some cases still very limited in scope.
The history of high-energy astronomy illustrates
the role of the atmosphere in astronomical observations and the need to
eliminate its effects as far as possible. The discovery of cosmic rays
was linked to early progress in aeronautics; indeed, the history of cosmic-ray
study has paralleled that of high-altitude research generally. Cosmic-ray
research has been performed from mountain tops, high-altitude balloons,
high-flying aircraft, and rockets and satellites. Cosmic rays are charged
particles of energy much higher than was believed present in the universe
before their discovery. Speculations as to their origin, which is still
not known with certainty, led to the inference that there must be a magnetic
field throughout our galaxy and to studies of the interaction of the cosmic
rays with that field and with interstellar matter. These studies in turn
led to the realization that gamma rays -- photons (quanta or particles
of electro-magnetic radiation) with energies comparable to those of the
cosmic rays -- could be generated in large numbers and detected in the
vicinity of the earth. Like cosmic rays, gamma rays are strongly absorbed
in the atmosphere and are best studied by spaceborne instruments. Gamma-ray
astronomy also generated the current search for cosmic X rays, which are
photons of energy lower than gamma rays. (See also Cosmic
Rays; Electromagnetic Radiation; Gamma
Ray Astronomy; Interstellar Matter).
Many of the important results of space astronomy
were obtained by observing X rays from space. These results appear to indicate
astronomical conditions whose existence was hitherto unsuspected.
X-ray astronomy is truly a child of the space age; the first results came from a rocket flight in 1962, and not for several years was it generally apparent the X-ray observations represented an important aspect of astronomy.
X-ray astronomy comprises the study of photons in the energy range from 0.2 to 200 keV (thousand electron volts) originating from astronomical objects. At energies below 0.2 keV -- corresponding to wavelengths greater than 60 angstroms (Å) -- absorption in the intersellar medium severely limits the distance to which objects can be studied.
Distribution of Sources
The essential result of X-ray astronomy is
the discovery that virtually all celestial objects of interest to more
traditional astronomers emit in this energy range and that, more remarkably,
there exist objects that radiate in this energy range with enormous power.
For example, whereas the sun radiates only about one millionth of its energy
in X rays, astronomers know of the existence of stars that are between
1,000 and 100,000 times more powerful radiators than the sun, and whose
radiated energy is emitted primarily in the form of X rays. They also know
of the existence of galaxies and groups of galaxies that are very strong
sources of X rays. Such great release of power implies the existence of
objects or conditions previously unknown in the cosmos. X-ray astronomy
thus allows the study of both the range of conditions under which the more
"mundane" solar-type X-ray emission takes place, as well as the much more
exceptional conditions that lead to enormously enhanced levels of X-ray
The great majority of sources known by the late 1970's were found by the pioneering American X-ray satellite Uhuru (Swahili for "freedom"), the American satellite SAS-3, and the European satellites Ariel V and ANS. More recently, the two large American X-ray satellites of the High Energy Astrophysical Observatory series, HEAO-1 and HEAO-2 (also known as the Einstein Observatory), have increased the number of known celestial X-ray sources to the point that X-ray astronomy today encompasses virtually the entire range of celestial objects studied by the more traditional optical astronomy -- from "mundane" stars to the most distant quasars. HEAO-2 was particularly instrumental in bringing X-ray astronomy into the mainstream of astronomy. Built around a large imaging X-ray telescope, it obtained for the first time arcsecond resolution images of the sky at X-ray wavelengths and revolutionized understanding of cosmic X-ray sources.
Generally speaking, X-ray sources in the sky are distributed as two primary components. The first consists of a large number of sources concentrated along the Milky Way, particularly in the direction toward the constellation Sagittarius, which includes the center of our galaxy; this distribution simply reflects the distribution of stars within our galaxy, and includes the brightest known X-ray sources. The second component is much more uniformly distributed; such a distribution is characteristic of objects that are very distant from our galaxy, and in fact the second component is thought to consist largely of objects that lie outside the Milky Way. Most of the X-ray sources, both in our galaxy and beyond, have been identified with objects well known from radio and optical studies.
Supernovae and Pulsars
Many of the supernova remnants, such as the
Crab nebula, Tycho's supernova, and the Cygnus loop, are observed as X-ray
sources. These objects are the very old remains of stellar explosions,
ranging in age from a few hundred years to many tens of thousands of years.
They extend over large regions of space and are very bright in the radio
and optical ranges. In most cases, the origin of the radiation can be traced
to the effects of the initial explosion, in which a vast wave of enormous
energy is created, expanding out into space and heating the interstellar
gas to millions of degrees. In addition, the explosion may leave behind
a neutron star, an object only a few miles across yet equal in mass to
the sun. Internally, its matter has been reduced to its most basic constituent,
the neutron, the individual atoms having lost their identity because of
the great density. In the case of the Crab nebula this object has a high
rate of rotation (30 times per second) and an intense magnetic field. This
combination is apparently a very efficient generator of very high-energy
cosmic rays, which spread through the nebula and radiate large amounts
of radio, optical, and X-ray energy via the "synchrotron" process as they
gyrate in the magnetic field that holds them in the nebula. It is speculated
that these cosmic rays seep into the interstellar medium to contribute
to the overall flux of cosmic rays throughout our galaxy, some of which
are detected on earth. The young neutron star also emits pulses at a rate
of 30 times per second. Many such rotating neutron stars are observed in
our galaxy with characteristic pulsed emission in the radio range; that
in the Crab nebula is the only one known to an X-ray source. These objects,
first observed in 1967 by radio astronomers in Great Britain, are called
pulsars. (See also Neutron Star; Nova;
Pulsar; Radio Astronomy.)
The evidence is that the majority of the
high-luminosity galactic X-ray sources represent a hitherto unrecognized
kind of stellar system, namely, a binary system -- two stars rotating around
each other -- in which one member is a common star and the other a collapsed
star or black hole. This latter object is of intense interest to astronomers
and physicists. A black hole is the "ultimate" state of matter, one in
which the crush of gravity has overcome all other forces, even the nuclear
forces. The star literally disappears; shrinking to the point at which
its gravitational field pulls back any radiation produced at its surface;
hence the name "black hole." Since no radiation can leave its surface,
it cannot be seen; it is the ultimate in "blackness." This is not to say
that there are no external manifestations of this object. It has an intense
gravitational field, and it can impart some of its rotational energy to
other bodies. Furthermore, any matter in its vicinity will be sucked into
the object, releasing an enormous amount of gravitational energy. The proximity
of a massive star provides a small amount of material that falls toward
the black hole and, in falling, is heated. It is the hot gas that is the
source of the X rays.
The evidence for these objects is that the X-ray intensity in many of the sources is highly variable on a time scale of less than a second. Two of the known X-ray sources pulse periodically with rates of the order of one second. This can only occur in very small objects such as neutron stars or black holes. Also, the X-ray sources are seen to eclipse, typically with periods of days. This is direct evidence of a binary system with two close stars. Finally, at optical wavelengths, in many cases, a hot massive star is seen, and in other instances a more common star, much like the sun, is seen.
The best evidence for a black hole comes from the object Cygnus X-1. Judging from optical data, the mass of the X-ray source is at least 10 times that of the sun, much greater than is possible for a neutron star. This and other evidence has led to the belief that the object is very likely a black hole.
The discovery of these objects is important in several ways. First, they make possible the study of matter in intense gravitational fields -- the realm of the general theory of relativity. Second, these binary systems represent a new aspect of the evolution of stars. The conventional calculations consider only a single star; when two stars are in a binary system, their life history is very different. In particular, the stars can actually transfer most of their matter back and forth. This exchange of stellar matter results in combinations of stars that were not hitherto believed to be possible. (See also Black Hole; Gravitational Collapse; Relativity.).
Galaxies and Galactic Clusters. A large number of external galaxies are observed to be strong X-ray sources. There seem to be two distinct kinds of source. First, there is a class of X-ray sources associated with active galaxies; in particular, the giant radio galaxies (NGC 5128), Seyfert galaxies (NGC 4151), and quasars (3C 273). In each case, the X-ray power is comparable to the optical power emerging from the nuclear regions of the galaxy.
There also appears to be X rays coming from extensive regions in clusters of galaxies. Galaxies occur in clusters containing as many as several thousand individual members. Our own galaxy, the Milky Way, lies in the outer regions of the Virgo cluster, the center of which lies some 13 million parsecs distant in the direction of the Virgo constellation. (One parsec equals 3.26 light-years).
X-ray emission has been observed from the central regions of a number of the richest of the known clusters, among them the Virgo, Perseus, and Coma clusters. In Virgo, the X-ray emission is centered on the galaxy M-87. The emission region is of te order of 200,000 parsecs in diameter, almost 10 times the size of the galaxy. In the Perseus cluster, the emission region is almost one million parsecs in diameter. Perseus illustrates the complexity in clusters generally; the cluster is the seat of low-frequency radio source, apparently the result of cosmic rays that pervade the entire cluster. Also, individual galaxies in the cluster show radio emission from a region that trails behind the galaxy proper as though the particles that give rise to the radio emission are being swept behind by a gigantic wind. This, in fact, is the likely explanation, namely, that an outflow of material -- gas and cosmic rays -- sweeps past the galaxies, producing radio trails in certain of them. The X-ray emission originates in the midst of this, possibly as a result of radiation from a very hot gas. It is likely that in the clusters the space between the galaxies is as complex as the interstellar medium: the space between the stars within a galaxy. (See also Galaxies; Interstellar Matter.)
The above discussion focused on the more
extraordinary objects seen in the sky. Einstein Observatory data reveal
that the normal stars of our galaxy also emit X rays. Based on what is
known of the sun, it should not be surprising that stars -- more precisely,
solar-like stars -- are X-ray emitters, can been seen in the photograph
of the a Centauri system. What has been enormously surprising is that the
sun is a relatively puny X-ray source compared with other solar-type stars
and that stars which are not at all solar-like nevertheless copiously emit
X rays. The expectation that solar-type stars should emit X rays is based
on present understanding of the sun's X-ray emission; it is thought that
the chaotic turbulence on the sun's surface gives rise to disturbances
that propagate outwards and heat the overlying, very tenuous atmosphere
(resulting in a multimillion degree gas that dominantly emits X-ray wavelengths).
The level of surface turbulence is thought to be virtually independent
of a star's age (except perhaps during its very early youth and its senescence)
but strongly related to the total luminosity of the star; the latter correlation
is such that the brightest stars (the so-called early-type stars) are expected
to have virtually no significant surface turbulence. The observation that
stars equally bright in the optical as the sun can emit 100-1,000 times
more X rays than the sun then suggests that our notion of how the sun produces
X rays requires revision, and that the level of surface turbulence cannot
alone determine the level of stellar X-ray emission. The further observation
that the optically brightest stars are also powerful X-ray emitters reinforces
this point: either such stars have vigorous turbulent surface motions that
were heretofore undetected, or additional processes must be at work to
produce the hot gas which is evidently present. An important point is that
current theories for the production of solar X rays contain assumptions
that can only be tested by observing X-ray emission from other stars; after
all, the sun has a fixed age, optical luminosity, and rotation rate which
we are not at all free to change.
Gamma-Ray and Cosmic-Ray Astronomy
Observation of the highest-energy photons
and particles began with high-altitude balloon and mountain observatories
during the 1940's. Satellite observations began during the early 1960's.
Significant fluxes of gamma rays from the Milky Way and the galactic center,
as well as of an isotropic extragalactic component, were detected by an
instrument aboard the third of the Orbiting Solar Observatory (OSO-3) satellites
and a wealth of data on gamma-ray emission from the Crab nebula and on
the general gamma-ray background were obtained from the second of the Small
Astronomy Satellites (SAS-2), launched from West Africa in 1972, and from
the European satellite COS-B. These satellites, together with Apollo 15
and 17, mapped and measured the spectrum of the diffuse portion of the
extragalactic gamma rays. The first extragalactic discrete source to be
identified (by COS-B) was the quasar 3C 273. An extensive survey by COS-B
of galactic gamma-ray sources yielded more than three dozen such objects.
Both COS-B and HEAO-3 (launched in 1979) are observing cosmic rays. These
data provide a direct probe of some of the sites (such as pulsar atmospheres)
at which high-energy photon production and cosmic-ray acceleration takes
place, and a clue also to the formation of the different chemical elements
in the universe. The National Aeronautics and Space Administration (NASA)
designed the Gamma Ray Observatory (GRO) to focus on cosmic gamma-ray emission.
The GRO was launched by the shuttle in April 1991. In addition, in a French-Soviet
collaboration, a gamma ray telescope (Gamma-1) was launched in December
1989. (See also Chemical Evolution; Cosmic
Rays; Gamma Ray Astronomy.)
A more direct way of studying the acceleration processes operating in distant celestial objects is to observe the flux of energetic particles (cosmic rays) impinging on us from space. At sufficiently high particle energies, virtually all cosmic rays observed from space come from without the solar system. These extrasolar particles pervade our galaxy, and bear in their composition and energy spectrum information about the processes that gave rise to them and about the interactions they experienced in traversing the space between their origin and the solar system. Observed since the earliest days of space astronomy, recent studies of cosmic rays have been carried out with the British satellite Ariel VI, and also with HEAO-3. Major results include information on the amount of matter traversed by cosmic rays as a function of their energy (the higher the energy of a particle, the lower the amount of matter traversed by it in our galaxy); the difference in the elemental abundances of extrasolar cosmic rays and solar system matter (cosmic rays show a relative enhancement of the heavier elements); the volume of space in which cosmic rays must be confined (in the low density portions of the interstellar medium, and possibly within the galactic "halo"); and the relative residence time of cosmic-ray electrons and nuclei in our galaxy (both are of the order of about (¥) 107 years).
Optical space astronomy is distinguished
by the fact that photons at these wavelengths can be imaged much as is
done with ground-based telescopes. Although optical space astronomy spans
the wavelength range of ¥300 microns (3 ´ 106 Å) to ¥1,000
Å, most attention has been given to the ultraviolet range shortwards
of 3,000 Å, because atmospheric absorption is virtually total in
this range. In contrast, in the visible range, ground-based observations
are relatively little troubled by atmospheric absorption, whereas in the
infrared there exist "windows," that is, narrow wavelength regions in which
atmospheric absorption is low and so ground-based observations are feasible.
In addition, high-altitude balloons are capable of carrying their instruments
to altitudes where water-vapor absorption is not too serious a difficulty.
Hot stars, as well as other gases at similar
temperatures, emit most of their power in the ultraviolet. The ultraviolet
astronomer can also study the behavior of energetic sources (such as, for
example, quasars) seen by other astronomers, and investigate the properties
of the gases lying between the stars (the interstellar medium). The last
of the Orbiting Astronomical Observatory satellites, OAO-3 (Copernicus),
pioneered the observation of ultraviolet spectra from space, and the International
Ultraviolet Explorer (IUE) satellite, launched in January 1978, has extended
the range of possible observations substantially by virtue of its greater
sensitivity. IUE is an international collaborative venture between the
United States, (represented by NASA), Great Britain, and the European
Space Agency (ESA), and functions much like a ground-based observatory:
observers are stationed in control rooms during the course of their observations,
and are able to direct the taking of data and to immediately view the results
of their observations in preliminary form. IUE is in a geosynchronous orbit
(making it the first such astronomical satellite), hovering over the equator
in such a way that it can be seen at all times by receiving antennas at
NASA's Goddard Space Flight Center (Greenbelt, MD), and for roughly eight
hours per day by receiving antennas of the ESA control center at Villafranca,
Spain. The telescope on board IUE has a 45-cm diameter berylium primary
mirror and feeds one of two available spectrographs; the spectral ranges
covered are 1,150-2,000 Å and 1,900-3,2000
Massive stars, whose surface temperatures
exceed 10,000 K, are subject to very large mass loss via powerful winds;
these winds can carry as much as 10-5 solar masses per year away from the
star, at terminal speeds in excess of 1,000 kilometers per second. Copernicus
found that these winds, thought to be cooler than the stellar surface,
actually contain ions characteristic of gas at much higher temperatures.
These observations have been extensively supplemented by data obtained
from the IUE for a large number of massive stars. Astronomers have concluded
that either the wind itself is hotter than previously thought, or there
must exist a local source of energetic (X-ray) photons able to ionize the
otherwise cool wind. X-ray observations with the Einstein Observatory have
in fact shown that a local source of X-ray photons is present in the outer
atmospheres of these massive stars; whether the wind is, additionally,
"warm" (that is, at temperatures in excess of 105 K) is not settled as
IUE has also allowed extensive studies of the atmospheres of solar-like stars at various evolutionary stages; in such stars, the gas lying between the stellar surface and the star's corona (the chromosphere and transition region) emit copiously in the ultraviolet. A major discovery of IUE has been that ultraviolet emission declines precipitously for evolved low-mass stars (the giants and supergiants) somewhat cooler than the sun, and that this decline is roughly associated with the onset of massive (but low-speed) matter outflow (winds) from such stars. This association has suggested to stellar astronomers that, in analogy to the sun, the mass outflow connected with a stellar wind in evolved stars also acts to depress the temperature rise above the star's surface.
Interstellar Medium. The interstellar medium -- the gas lying in the vast reaches of space between stars -- reveals a number of its properties through the observation of its emission and absorption of ultraviolet light. (See also Interstellar Matter.). The latter observation is made possible by using hot stars (which emit copiously in the ultraviolet) to "backlight" the interstellar medium. In this manner, ultraviolet astronomers have been able to map the spatial distribution of molecular hydrogen (H2) within the Milky Way; particularly significant enhancements of H2 were found in cold interstellar clouds, the presumed birthplace of stars. Similar studies of the spatial distribution of heavier elements (the so-called "metals" such as carbon, oxygen, and silicon) have shown that they are underabundant in the interstellar medium gas relative to their occurrence in stars; it is thought that the "missing" heavy atoms (which ought to be there because the interstellar gas is the primordial soup out of which stars are made) are captured by ubiquitous interstellar dust grains.
The observation of diffuse emission in the ultraviolet has led to the realization that much of the space between stars is filled with gas at temperatures in excess of 105 K, which forms (at high galactic latitudes) an extended "corona" above the Milky Way's spiral disk. The ultraviolet observations thus complement observations in the radio, visible, and X ray, which together suggest an enormously complex interstellar medium structure: cold (¥ 100 K), dense clouds embedded in a warmer (¥ 10,000 K) gas, which is itself surrounded by a yet warmer (> 100,000 K), but very tenuous gas which can reach temperatures of the order of 1,000,000 K.
Infrared Astronomy. Until now, the least-obscured view at wavelengths longwards of 1 micron (10,000 Å) was obtained by telescopes aboard high-altitude aircraft or balloons. A dedicated satellite for infrared astronomy was launched by the shuttle in 1983. This satellite (the Infrared Astronomy Satellite, or IRAS) is intended to provide a full-sky survey in the wavelength range 8-120 microns.
Visible Light Astronomy
Unlike the other wavelength regimes, the
visible domain is not primarily limited by atmospheric absorption; it is
therefore unlikely that any attempts to gain purely light-gathering power
will lead to a space-based optical telescope. In contrast, ground-based
optical astronomy suffers from the "twinkling" and from the scattering
of light by the atmosphere. These problems, particularly worrisome for
observations of extremely faint point sources, are completely avoided by
going above the earth's atmosphere. Space-based telescopes can thus be
built so as to be capable of far greater angular resolution. Concommitant
with greater resolution is an increased sensitivity to faint point sources,
and it is this capability that motivates the construction of space-based
Observations of very distant objects by optical observatories in space are expected to make important contributions to the quest for a better understanding of the overall structure of the universe. The goals here are among the most fundamental in the natural sciences. One is to discover the correct form of the general theory of relativity by looking for the curvature of space. Furthermore, it is believed that by studying distant ("old") objects we can learn about the origin of the universe itself. At the other end of the scale, there is the study of stars so close to earth that their motion in the sky can be detected against the background of more distant, motionless stars and galaxies. For these stars, we can determine the distance and the direction and rate of motion. For nearby binary stars, the mass of the stars can also be determined. Much of what is known about astronomy -- for example, the distances to galaxies -- is based on such measurements. With the sharper images available in a spaceborne telescope, this work can now be extended to objects more distant by perhaps tenfold. The importance of this may be seen from the fact that accurate masses are now known for only about 50 stars.
These research goals have been addressed by the Hubble Space Telescope, which was launched by the space shuttle. With an entrance aperture of 2.4 meters, it collects far fewer photons than the largest ground-based telescopes; but its sensitivity to faint point sources is greater than that of the largest ground-based telescopes, such as that on Palomar Mountain. The telescope is capable of both wide-field imaging (using electronic imagers as a substitute for photographic emulsions) and spectroscopy in the wavelength range from the near infrared to the far ultraviolet.
The study of the sun illustrates very well
the interplay between classical methods of astronomy and space observations.
Because of its proximity the sun can be studied in great detail, and is
the only cosmic body in which certain phenomena can be studied directly.
For example, we can measure cosmic rays from the sun and even pinpoint
their place of origin on its surface. Other cosmic rays arrive at the earth
equally from all directions and never betray their origin.
Space observations of the sun can be traced to predictions made in the 1930's that the sun should be a source of X rays. These predictions were based on the observation of day-night differences of radio transmission, which could be traced to changes taking place in our own atmosphere at altitudes of about 100 kilometers (60 miles), presumably as a result of solar radiation. (The predictions served in part to stimulate the development of high-altitude rockets in Germany.) After World War II, one of the earliest experiments performed from a rocket was the successful observations in 1948 of X rays from the sun by a group from the U.S. Naval Research Laboratory, using ordinary dental X-ray films. From this modest beginning solar physicists have progressed through an increasingly complex array of instruments flown on sounding rockets and satellites, culminating in the Apollo Telescope Mount (ATM), which was flown on the Skylab Orbital Workshop in 1973 and 1974 and, most recently, in the Solar Maximum Mission (SMM) satellite, launched in late 1979. ATM looked at the sun during the waning portion of the solar cycle of activity, while SMM looked at the sun during the peak of the activity cycle. Also flown in space are more specialized instruments for direct solar observation, such as the OSO series of American astronomical satellites; the SOLFLEX and SOLEX solar X-ray spectrometers, flown on Department of Defense satellites; the Japanese solar satellite ASTRO-A, which observed solar flares; a wide variety of small payloads flown on sub-orbital rockets; as well as instruments on planetary probes to monitor the magnetic field and the fluxes of electrons, protons, and heavier particles found in the region between the planets, but having their origin in the sun. The Solar Optical Telescope (SOT), a 1.25 meter diameter telescope, is to be launched by the space shuttle. With a resolution of 0.2 arcsecond and a useful wavelength range of 1,200-10,000 Å, SOT will aim at extremely high spatial resolution observations (including spectroscopy) of the solar photosphere, chromosphere, and transition region.
Solar research in the past 20 years has revealed the nature of the outer layers of the sun's atmosphere. The sun's atmosphere is exceedingly hot (in the range of millions of degrees) an exceedingly complex. Loops, rays, and prominences extend far above the sun's surface. Also revealed are features associated with sunspots that are born, grow, and disappear on time scales of many months. These features, known as active regions, are also the seat of bursts of violent activity -- solar flares -- that erupt from time to time and send out vast amounts of high-energy radiation, and may even disrupt radio communication on earth. This activity is apparently controlled by the sun's magnetic field.
From the earth's surface, we have a partial view only of this array of phenomena. Most of the emitted radiation from active regions, comprising only one millionth of the total power of the sun, emerges in ultraviolet and X-ray radiation that can only be studied from above the atmosphere. The hot gas that emits the radiation emerges from small regions in a band around the sun, and loops above the sun's surface from region to region. It is likely that these loops trace out the magnetic field of the sun. By studying this radiation, solar physicists hope to solve the mystery of how exactly the sun's magnetic field is formed and how energy is transferred from the interior of the sun to its outer atmosphere. The sun's surface temperature is about 6000 K, but reaches millions of degrees a few thousand kilometers higher.
At these high temperatures, matter exists in a form known as plasma. Its atoms have lost many or all of their outer electrons and a magnetic field binds the region together. It was the need to understand the radiation emerging from the sun and other stars that led to the first studies of plasma. These have been invaluable in laboratory plasma investigations and in nuclear-fusion studies. In turn, these studies have led to better understanding of the behavior of plasma and of various solar phenomena. (See also Nuclear Fusion; Physics :Contemporary Physics.)
Another important feature of solar studies is the development of instruments that eventually find broader application in astronomy (and, at times, outside astronomy as well). The X-ray telescope, the operating principle of which is illustrated in Fig. 1, is an example of this. Designed for solar astron omy, this device has also been used to study cosmic X-ray sources. In addition, terrestrial scientists studying laboratory plasmas at high temperatures have discovered that the imaging technology developed by X-ray astronomers is well suited to their observations; for example, observations of exploding pellets during laser fusion experiments have been carried out using X-ray "microscopes" designed along the principles of the astronomical telescope precursors.
TECHNIQUES OF SPACE ASTRONOMY
Two essential problems distinguish space
astronomy from other space applications. These are: 1) the necessity of
stabilization, that is, the ability to point along a single direction in
space for long periods of time with high angular precision; and 2) the
presence in space of a great variety of radiations that can seriously interfere
with the desired measurements. The vehicle requirements of space astronomy
range from the smallest that can be conveniently placed in orbit to the
largest (see Table 1). However, space astronomers require very large instruments
for the same reasons as ground-based observers. Because of this, future
space observatories will require the largest boosters available.
The technology of observation by aircraft and balloon is not very different from observation by satellite. But since aircraft and balloon flights are of limited duration, observations that must be conducted at high altitude must eventually all be performed, for reasons of cost, from satellites. One satellite-borne instrument can gather as much information as hundreds of aircraft or balloon flights.
The first problem named above, that of stabilization,
is the more severe. The earth itself provides basic stability for ground-based
observations. In space, however, no such stable base exists, and payloads
are subject to a variety of perturbing forces such as radiation pressure
and aerodynamic torques. These, in spite of their very small magnitude,
will cause a space vehicle to move in an erratic fashion.
Stabilization requires two kinds of device: mechanisms such as gyroscopes that sense deviations from a prescribed direction in space, and actuators, such as gas jets, that counteract the effects of perturbing forces and keep the vehicle stably aligned.
The degree of stability required depends very much on the discipline and on what kinds of observations are being conducted. To study cosmic rays, which apparently have no preferred arrival direction, it is only necessary to fix the orientation of the vehicle to within degrees of arc. For the observation of stars, however, the precision required frequently lies in the range of arc minutes to arc seconds. The Einstein Observatory, which observed X-ray emission from stars and galaxies, could point to specific objects to within arc seconds. The Small Astronomy Satellite, on the other hand, which was designed to record X and gamma rays; only had capability in the arc-minute range. Vehicles intended to observe the sun are generally designed to point with much higher precision. Fortunately, the sun, being so bright, can provide very accurate tracking signals. After an observation, sightings of known stars are generally taken to determine what the precise orientation of the space vehicle was.
As noted, the radiation environment in space
can seriously interfere with many astronomical observations. The most serious
problem is that of the charged particles trapped in the Van Allen radiation
belts. Observations for X-ray and gamma-ray astronomy and cosmic-ray research
cannot be conducted in the radiation belts. The belts can be avoided by
choice of a low-altitude orbit and a low inclination of the orbital plane
with respect to the equator. For this reason, Uhuru satellite was launched
from a site (in Kenya) almost on the equator. However, this option is available
only for the smallest rockets; launches from sites such as Cape Canaveral
in Florida, where the least achievable inclination is 35°, are more
common. In such an orbit the radiation belts are traversed several times
a day in the region of the South Atlantic, where the earth's magnetic field
is anomalously low. An alternative is to fly above the belts at altitudes
of 100,000 kilometers (60,000 miles) or more. This is also advantageous
for gamma-ray and cosmic-ray observations, since cosmic-ray interactions
in the earth itself produce interfering radiation whose influence diminishes
at large distances.
Effects of the Earth
In optical, infrared, and radio astronomy
the earth itself presents the greatest interference. Close to the earth,
perturbing forces make it difficult to achieve the stability required for
observations in the arcsecond range. These forces include effects of the
earth's magnetic field, of the upper atmosphere (aerodynamic drag), and
of the earth's gravity (gravity gradient torque), all of which decrease
with increasing distance from the earth. The radiation from the earth is
a serious factor for infrared and radio astronomy. In the case of the infrared,
it seems that significant emission occurs even at satellite altitudes.
Another factor is that it is essential to cool the mirrors and the detectors
used in infrared observations, otherwise they themselves are sources of
infrared radiation; and this is difficult to accomplish in the vicinity
of the earth, which is itself a warm body.
For radio astronomy, two effects of the earth are important. As already noted, commercial broadcasting produces signals that cannot be distinguished from cosmic ones. Also, the ionized portion of the residual atmosphere, which is highly variable, absorbs and reflects radio waves, producing a correspondingly variable signal. These effects are so serious that RAE-B (Explorer 49), the second of NASA's small satellites for radio astronomy, was placed in orbit around the moon, completely away from such influences.
Space Shuttle and Manned Observatories
The successful Skylab mission of 1973 ended
a period of manned exploration of space based on the use of the Saturn
5 booster, the vehicle that first carried man to the moon. The Skylab served
as a manned astronomical observatory for the study of the sun. The success
of the OAO, OSO, and HEAO satellites proves that high-quality astronomical
observations can be performed in space automatically. Also, various planetary
probes demonstrate that high-quality images can be automatically registered
and transmitted to earth. It is likely that NASA will develop a mixture
of manned and unmanned space observatories.
NASA counts heavily on the space shuttle for its missions. There are two principal astronomical applications of the shuttle. The first is to equip it as an observatory (Spacelab) to be used for 30 days in space. Unlike Skylab, which remained in orbit (until reentry in 1979) and was revisited on two occasions, Spacelab is returned to earth between flights. The shuttle is also used as a boost vehicle to carry instruments into orbit and leave them there to operate in the same manner as unmanned observatories on the ground do. Some of these payloads are retrieved from orbit during subsequent shuttle flights. By these means expensive instruments can be used almost indefinitely; just as are most ground-based telescopes. See also Space Exploration: Post-Apollo Planning.