Early results from sounding rockets indicated the enormous potential of astronomical observations from above the earth’s atmosphere—a potential that began to be realized with the successful launches of the Soviet Sputnik and American Explorer satellites in the late 1950’s. Since these early launches, thousands of satellites have been placed into orbit. Those designed for astronomical research have yielded much information about the heavens that could not have been obtained from ground-based observatories, principally because the earth’s atmosphere absorbs almost all forms of radiation apart from visible light and radio waves.
The early astronomical satellites
Following the launch of the world’s first artificial satellite, Sputnik 1, on October 4,1957, the Soviet Union’s space effort was devoted mainly to lunar and planetary exploration. Nevertheless, the Soviet Union did launch a few satellites designed for purely astronomical research, such as Sputnik 3 (launched on May 15,1958), which carried detectors for cosmic rays, X rays, and ultraviolet radiation. And more sophisticated scientific satellites appeared with the nation’s Cosmos series (which began on March 16,1962 with Cosmos 7), such as Cosmos 51 (launched December 10,1964), which was equipped with cosmic and ultraviolet radiation experiments, and Cosmos 166, a solar X-ray satellite.
The United States, on the other hand, initiated an extensive series of scientific space probes with the launching of its first satellite, Explorer 1, on January 31,1958. One of the series, Explorer 42 (also called Uhuru, the Swahili word for freedom), was the first satellite devoted primarily to studying celestial X-ray sources. Launched in December 1970, it produced an X-ray map of the heavens and, after six months of observations, had discovered more than 170 discrete X-ray sources—five times as many as were previously known. It also pinpointed the location of Cygnus X-1 (so-called because it was the first X-ray source discovered in the constellation of Cygnus), which is now thought to be a black hole.
Following the lead of the Soviet Union and the United States, other nations began to build satellites, including Britain, Italy, West Germany, Canada, France, Japan, China, and India.
What is a satellite?
Satellites are complex structures, consisting of many thousands of individual components, designed to operate in the harsh environment of space. They are of various shapes and sizes, and each is designed specifically for the mission it has to perform. Nevertheless, the structure of any satellite can be divided into a number of discrete sections—called subsystems— each having a particular function, such as supplying power or providing communications. All of a satellite’s subsystems must work perfectly because it is usually impossible to rectify any fault that may occur after a satellite has been launched.
The power subsystem
Although a satellite requires all of its subsystems in order to function properly, the power subsystem is probably the most important, because all others ultimately depend on it. Most satellites are powered by solar cells that convert sunlight into electricity. On average, a scientific satellite requires about 200 watts of power (equivalent to two domestic light bulbs), which is supplied by 10,000 or more solar cells.
There are, however, problems associated with solar cells. They generate electricity only when exposed to light and so do not operate if the sun’s rays are prevented from reaching the spacecraft, as when it is in the earth’s shadow. For this reason, satellites also carry batteries. Solar cells are also very inefficient; at best, only about 10 per cent of the sun’s energy is converted into electricity. Moreover, the constant bombardment of a cell by sunlight tends to reduce its efficiency, and after many years in orbit it can decrease to less than 10 per cent.
Spacecraft on missions that take them away from the sun, where solar cells would receive too little sunlight to provide a useful amount of electricity, need alternative power supplies. For example, the Voyager 1 and 2 probes to the outer planets carried small nuclear generators to provide the necessary power.
From the scientist’s point of view, the payload is the most important part of a satellite. It comprises the experiment module, carefully developed and extensively tested on the ground and, if the mission proceeds as planned, the device that will gather important astronomical data.
Astronomical payloads are extremely varied, designed according to the tasks they have to perform. They can range from simple aluminum foil sheets for the detection of small micrometeoroids, to a complex telescope assembly. Although, varying greatly, astronomical payloads have one factor in common: they are all detectors.
The function of most detectors is to investigate part of the electromagnetic spectrum—X rays, gamma rays, ultraviolet radiation, and so on—and the structure of a detector therefore depends on the part of the spectrum being studied. In the visible part of the spectrum, the simplest form of detector is a photographic plate or film. To detect gamma rays, X rays, ultraviolet, infrared, or radio wavelengths, however, it is necessary to use other forms of detectors.
The observation of X rays
The detection of X rays presents unique problems, mainly because they cannot be reflected and focused by the same types of mirrors used in normal reflecting telescopes. They can, however, be reflected when the incidence angle (the angle at which the X ray hits a mirror’s surface) is very small (less than 2°). It is this property that has been used to construct X-ray telescopes, which focus the rays onto special detectors. X-ray telescopes are difficult to build, and for this reason the early models did not use mirrors but rather other mechanisms for channeling the X rays into the detector. The simplest of such channeling mechanisms is the collimator, which consists of an array of thin metal plates. A typical collimator of this type cannot locate X-ray sources very precisely, but more complicated designs, such as modulation collimators (which have fine mesh grids instead of metal plates), are considerably more accurate.
To achieve still greater positional accuracy requires the use of a telescope in which the X rays are reflected by the polished surfaces of extremely accurately shaped mirrors. One of the most common of this type is called the Wolter Type I. A very large X-ray telescope was carried by the Einstein High Energy Astronomy Observatory (HEAO-2), launched in late 1978. It had four Wolter Type I telescopes, mounted one inside the other to increase the collecting area.
Once the X rays have been collimated, or focused, they must be detected with some form of counter; X-ray telescopes often have interchangeable detectors. The most commonly used detector is the gas-filled proportional counter, which is suitable for the detection of low-energy X rays. These enter through a large window made from a thin sheet of beryllium or other light (that is, of low atomic number) metal. Inside the main part of the detector, which is filled with gas, are a series of anodes at a high positive potential (about 2,000 volts). Each X ray entering the window collides with atoms of the gas, causing them to release electrons (called photoelectrons). These electrons then move rapidly towards the anode and, in turn, liberate more electrons, causing an “avalanche” of electric charge. The charge is then measured electronically, which gives an indication of the incoming X ray’s wavelength. Proportional counters such as these have been used successfully on the Uhuru, Ariel V, and Einstein satellites.
Other forms of detectors have been developed, all of which rely on the general principle of converting X rays into other, more easily measurable forms of energy. One alternative type is the scintillation detector, which usually consists of a crystal of sodium iodide or cesium iodide. The crystal responds to X rays by generating flashes of light, which are then detected by a photomultiplier tube. Another form of detector is the microchannel plate, which comprises an array of small-channeled electron multipliers. This last type was used on the Einstein satellite to obtain high-resolution images.
The observation of other radiation
The Infra-Red Astronomical Satellite (IRAS), launched in January 1983, was the first satellite designed specifically to survey the sky at infrared wavelengths, a task hitherto undertaken only by special ground-based telescopes. The satellite’s 24-inch (60-centimeter) telescope mirror focused infrared radiation onto photo-conductive detectors, which converted the radiation into an electric current. The entire telescope assembly (including the detectors) was surrounded by a dewar vessel (similar to a vacuum flask) containing 154 pounds (70 kilograms) of liquid helium at a temperature of about 2 K, 2° above absolute zero. This extremely low temperature was essential because if the telescope was not cooled, its own infrared (heat) radiation would obscure the relatively weak radiation emitted by celestial sources. In addition to its main task of the sky survey, another function of the IRAS was to record the spectrum of unusual infrared sources.
Other types of unmanned astronomical spacecraft are the International Ultraviolet Explorer (IUE) and various gamma-ray satellites (notably Cos-B). The IUE, a joint American and European project, was launched on January 26, 1978. Orbiting at an altitude of more than 18,500 miles (30,000 kilometers), the IUE carried an 18-inch (45-centimeter) reflecting telescope, remotely controlled from earth, with a special ultraviolet radiation detector linked to a television camera. Cos-B, launched in 1975, was designed to investigate celestial gammaray sources. Among the data it sent back was evidence to support the idea that there are two main gamma-ray sources.
From the experience gained from its Orbiting Astronomical Observatory (OAO) satellites— especially from OAO-3 (commonly called Copernicus), which was launched in 1972 and carried a 32-inch (81 -centimeter) telescope for ultraviolet observations—NASA launched a 94-inch (2.4-meter) diameter telescope into space in April 1990 using the Space Shuttle. Orbiting about 380 miles (610 kilometers) above the earth, the Hubble Space Telescope is sensitive to a wide range of wavelengths—from 1,100 to 10,000,000 A (1 millimeter)—and can detect celestial objects 50 times fainter than those observable from earth.
Astronomy from manned spacecraft
Man’s outstanding quality, as compared to electronic and mechanical systems, is his ability to make decisions, based on both experience and intuition, that a machine cannot.