Ultraviolet, X-Ray, and Gamma-Ray Telescopes
Although much useful and important observational work remains to be done from ground-based observatories, an increasing portion of future astronomical research will be carried out from platforms outside most of the veil that is the earth's atmosphere. Up to the middle of this century nearly all our knowledge about the cosmos had come from studying the visible light of astronomical objects, and the visible and radio windows still constitute our most readily accessible sources of information. Yet much of today's research is centered on the invisible regions of the electromagnetic spectrum, which do not penetrate the earth's atmosphere. To explore these regions-the infrared, the ultraviolet, the X-ray, and' the gamma-ray wavelengths-new techniques and equipment are being developed, which must be flown above the atmosphere in some type of space vehicle.
SPACE VEHICLES
High-altitude aircraft and balloons are the least expensive way of investigating invisible extraterrestrial radiation. Jet aircraft can ascend to about 15 kilometers, while balloons are useful up to about 30 kilometers, above which only 5 percent of the atmosphere remains. Rockets, though their flights are short compared to balloon flights, lasting for minutes instead of hours, can climb five times higher than balloons can. Artificial satellites cost much more than rocket flights, but satellites can continuously monitor events over different regions of the electromagnetic spectrum for long periods of time, an advantage that outweighs their additional cost. Since 1958 the United States has launched hundreds of instrumented satellites that have either orbited the earth or been sent to search the solar system.
Up until about 1983 the means of getting satellites off the su rface of the earth have been rockets. The advent of Space Shuttle has now provided another way to launch satellites. Space Shuttle is a true aerospace launch vehicle in that it takes off like a rocket, maneuvers in earth orbit as do other spacecraft, but lands like an airplane. This launch vehicle was designed to carry heavy loads into space and to be, reusable. Satellites can be carried into orbit in the Shuttle's cargo bay; when the Shuttle is in orbit, the satellites can be lifted out by a retractable arm and placed in their orbits. This also means that satellites can be retrieved from orbit to be brought back to the earth's surface or serviced and returned to orbit. Space Shuttle gives us the capability of carrying pieces of immense spacecraft, including manned space stations, into orbit to be assembled there. Its versatility signals a new generation of space exploration.
An important group of space vehicles has been the observatory satellites placed in orbits several hundred kilometers above the earth. Two of the most sophisticated and costly observatory satellites have been the Orbiting Astronomical Observatories; OAO-2 and OAO-Copernicus.
The Skylab program cost $6 billion. Three crews of three men each spent 171 days in Skylab between May and November, 1973. These astronauts carried out dozens of astronomical, biomedical, and technological experiments. The abandoned station was to remain in orbit for several years; it was hoped that with Space Shuttle astronauts would be able to reuse the station in the future. However, Skylab was dragged down by the atmosphere to a fiery demise over the Indian Ocean, scattering pieces over western Australia on july 11, 1979.
In early 1985 NASA expects to place a 2.4-meter unmanned reflecting telescope named Space Telescope in orbit at an altitude of 500 kilometers. Its optics and instrumentation will be enclosed in a cylindrical tube 13 meters long and 4.3 meters wide. Auxiliary apparatus includes two imaging cameras, faint-object and high-resolution spectrographs, a photometer, and other specialized devices. These analyzing instruments are designed to cover the wavelength range from about 1000 angstroms in the ultraviolet to 8000 angstroms in the near infrared. Data from the telescope will be radioed in digital (number) form through the Goddard Space Flight Center in Greenbelt, Maryland, to the Space Telescope Sciences Institute on the johns Hopkins University campus for processing.
Out in space no atmospheric absorption or turbulence will distort the images produced by the telescope. Thus the telescope should see astronomical sources up to 50 times fainter than those visible from the earth's surface; in terms of distance a faint object can be seven times farther away than could be seen from the surface of the earth. Space Telescope's spatial resolution will be 10 times better than the best earth-based reflectors. With proper maintenance from Space Shuttle, the telescope could operate for at least a decade. Unlike its ground-based counterparts, Space Telescope will scan the electromagnetic spectrum from the deep ultraviolet to the infrared.
Other space observatories are the planetary probes, which are literally the most exotic. Their role is to go to a planet to photograph and analyze from a close flyby, to orbit the planet, or in some cases to land. As examples, the Viking 1 and Viking 2 spacecraft landed on the surface of Mars (we shall discuss them later). Other examples are the Voyager 1 and Voyager 2 spacecraft launched to encounter jupiter, Saturn, and perhaps Uranus and Neptune. Much of our understanding of the nature of the universe is changing-rapidly and dramatically-because of these space observatories.
ULTRAVIOLET TElESCOPES
The ultraviolet portion of the electromagnetic spectrum has been divided by astronomers into three segments, more or less derived from the time in which serious research into them began. First there is the ground-based ultraviolet, from 4000 angstroms to the atmospheric cutoff at 3000 angstroms; next the far ultraviolet from 3000 to 1000 angstroms; and last the extreme ultraviolet from 1000 to 100 angstroms.
Ultraviolet observations began after World War II, in October 1946, when a captured German V-2 rocket carried a small ultraviolet spectrograph to a height of 100 kilometers. During the ascent it recorded the ultraviolet portion of the solar spectrum down to 2200 angstroms. Telescopes, analyzing instruments, and radiation detectors for ultraviolet research are basically the same kinds of instrument used in visible and infrared observations. The principle difference is that a number of types of glass are not transparent to ultraviolet photons but are highly absorbing. Therefore, special materials must be used for lenses and entrance windows into the instrument. The principles of operation are the same as those for visible radiation. Since ultraviolet-sensitive film cannot be retrieved from an orbiting satellite, photoelectric devices have been the primary radiation detectors, so that data could be radioed back to ground stations.
Between 1962 and 1975 eight Orbiting Solar Observatories (050-1 through 050-8) were launched for study of the sun in ultraviolet wavelengths arid X-ray and gamma-ray radiation. In December 1968 the first Orbiting Astronomical Observatory (OAO-2) began sampling the ultraviolet and far-ultraviolet radiation. By the time OAO-2 ended its useful life in February 1973, it had carried out photometry on more than 1000 objects from planets to galaxies. Its successor, illustrated in Figure 5.18, OAO-Copernicus, launched in August 1972, carried an 0.8-meter ultraviolet telescope and three small X-ray telescopes, and was even more active than OAO-2.
In january 1978 the International Ultraviolet Explorer, an orbiting observatory, shown in Figure 5.18, was launched by NASA. This was a joint undertaking by NASA and several western European countries. Its facilities have been used for studies of planets, stars, galaxies, and the interstellar medium in the wavelength range from 1150 to 3200 angstroms. Astronomers conduct their experiments from an elaborate console of controls located at the Goddard Space Flight Center.
X-RAY DEVICES
X-ray astronomers divide "their" portion of the electromagnetic spectrum into two categories: soft X rays, from about 10 to 100 angstroms, and the more penetrating hard X rays, from approximately 1 to 10 angstroms. Both X rays and gamma rays are emitted by regions of space characterized by very high temperatures, low density, and high-speed subatomic particles-that is, wherever there are extreme conditions involving nuclear and atomic reactions. The observed radiation is in part thermal radiation but mostly nonthermal radiation.
Most people are aware that X rays are more penetrating than visible light since they pass through the human body when making X-ray pictures for medical diagnosis. In this great penetrating power lies the difficulty in making telescopes to focus X rays, analyzing instruments, and radiation detectors for X rays; for glass lenses and mirrors do not refract or reflect X rays impinging directly on them. If X rays strike a smooth surface at a very shallow angle, less than a couple of degrees, they will reflect off the surface. This phenomenon has been used successfully to design an all-grazing-incidence reflector, which focuses X rays as an optical telescope focuses visible light. Such an X-ray telescope was flown as the heart of the second High Energy Orbiting Observatory, known as the Einstein Observatory
At the focus of the X-ray telescope is the radiation detector, just as in the case of an optical telescope. Photographic emulsions can be made that are sensitive to X rays and can record an X-ray-picture. For hard X rays special crystalline materials will absorb X rays, converting their energy into photons of visible wavelengths that can be detected with photoelectric devices. And there are solid silicon detectors whose ability to conduct electrical charges is influenced by their absorption of X-ray photons.
Astronomers first began using X-ray detectors in balloons and rockets and in a few unmanned satellites during the 1960s. By 1967 they had discovered some 30 discrete X-ray sources. Then in December 1970 NASA's Explorer 42 satellite (Uhuru) was launched off the coast of Kenya, Africa. By the end of its useful life in 1973 it had scanned nearly the whole sky and had located nearly 200 X-ray sources. The newly discovered X-ray objects were named after the constellation in which they appeared, followed by X-1, X-2, and so on, in the order of discovery. For example, Taurus X-1, the first X-ray object discovered, is the Crab Nebula. Today, with a growing number of X-ray discoveries, it is convenient to designate the source by a catalog number.
A second generation of NASA satellites (the High Energy Astronomy Observatories), designated HEAO1, HEAO-2, and HEAO-3, was launched in August 1977, November 1978, and September 1979, respectively. The three HEAO satellites are designed specifically to study X rays, gamma rays, and subatomic particles (called cosmic rays). The satellites shown in Figure 5.18 are each about 6 meters long and weigh approximately 3000 kilograms. Instruments aboard these satellites were designed to search the sky for discrete and diffuse background sources of X rays and gamma rays, to measure their total energy output and how that varies with wavelength, and to measure the ranges of energy, the composition, and the numbers of cosmic rays.
Proposed for launching in 1987 is a follow-up mission to Einstein Observatory (HEAO-2). The satellite will be put into orbit from the Space Shuttle, which will also service it and eventually retrieve it for modernization. AXAF's 1.2-meter grazingincidence telescope will have 4 times the spatial resolution and at least 20 times the X-ray photon-collecting power of the X-ray telescope in the Einstein Observatory. This space vehicle represents as big an advance for the X-ray region of the electromagnetic spectrum as Space Telescope does for the visible and ultraviolet. Beyond AXAF an even larger telescope facility is in the planning process. To be known as the Large Area Modular Array (LAMAR) X-ray telescope, it will have 10 times the sensitivity to X rays that ASAF does.
GAMMA-RAY DEVICES
This new field of astronomy has burgeoned in the last decade or so from modest beginnings, employing balloons and rockets, to satellites (European and Ameri· can) carrying highly sensitive gamma-ray detectors. Like that of X rays, the great penetrating power of gamma·ray photons makes observation and detection different from those for visible photons. Gamma·ray photons carry the highest energy of any photon. The primary gamma·ray detector used for space astronomy is a crystalline material that absorbs the gammaray photon, converting its energy to a flash of visible light. The visible photons can then be detected by a photoelectric device.
In 1978 came the first evidence of several gammaray spectral lines. The search for additional discrete lines will be greatly extended with the launch of the Gamma Ray Observatory in 1985. The GRO satellite
