The earth’s atmosphere is the sustainer and, to some extent, protector of life on our planet. But to the astronomer, it often seems to be a severe handicap, affecting the quality of telescope images, as well as blocking some parts of the electromagnetic spectrum, and preventing radiation from space from reaching the surface of the earth. It is not surprising, then, that since the early 1960’s, much effort has been expended on finding ways of observing the heavens from above the limits of our atmosphere.
The atmosphere and image quality
The limitations that the earth’s atmosphere imposes on ground-based astronomy become apparent even in the optical (visible) region of the spectrum. The atmosphere is completely transparent between wavelengths of 4,000 and 7,000 A, but observation of very faint sources of light is hampered by the background of natural airglow and interference from man-made sources, such as city lights. The atmosphere is also in a state of constant movement because of temperature and humidity changes, which cause the telescope image to dance around or spread out. The result is that the resolution of a telescope is always less than it should theoretically be. All these factors lead to unfavorable “seeing,” or conditions, for observations of a good quality from beneath the atmosphere. “Seeing”—the viewing conditions—limits the useful size of large optical telescopes. The 200-inch (5-meter) Hale telescope on Mt. Palomar is theoretically capable of a resolution of 0.025 arc seconds, but invariably falls well short of this figure because of the “seeing” condition; the resolution is usually around 0.8 arc seconds.
The limitations imposed by the atmosphere were reduced dramatically when the American space shuttle placed the Hubble Space Telescope into an orbit about 380 miles (610 kilometers) above the earth’s surface. This telescope has a resolution of 0.066 arc seconds at 6,330 A wavelength, much better than an earth-based telescope of an equivalent size. It can observe objects 50 times fainter than can be seen using the best telescopes on the ground.
Absorption of radiation by the atmosphere
Many objects in the sky emit radiation over a range of the electromagnetic spectrum. Due to the absorption of parts of the spectrum by various atmospheric atoms and molecules, however, ground-based observations are restricted to selected regions of the spectrum. The parts of the atmosphere that are transparent to electromagnetic radiation are called “windows,” the most important for plant and animal life being the optical window between 4,000 and 7,000 A—the wavelength range of visible light. (This fact provides the reason why most animals have evolved sight organs that are sensitive to these wavelengths, and these alone.)
The other prominent and astronomically useful windows are in the radio region and in the near infrared region (between 8 and 13p,m, and 17 and 20p.m). The ultraviolet, gamma-ray, and X-ray regions of the spectrum are completely absorbed by the earth’s atmosphere. Astronomers observing from below the atmosphere are therefore unable to obtain full information on the spectrum of an object in the sky because of the opaqueness of the atmosphere at certain wavelengths. The advent of rockets, satellites, and space probes has done much to overcome this difficulty and to increase our knowledge of the universe.
Solving the problem
To minimize the effects on observing conditions, astronomers have built observatories far away from cities to avoid background light, and have sited them at high altitudes where the air is thin, unpolluted, and relatively dry. The last condition is especially important for observations in the infrared region of the spectrum, for which atmospheric absorption depends on the water content of the atmosphere. Some sites, such as those in Northern Europe, are so wet that the 17 to 20/im infrared “window” becomes nearly opaque, making observations impossible. Sites such as Mauna Kea in Hawaii, however, at an altitude of about 13,796 feet (4,205 meters), or the South Pole, are ideal for observations through this “window.”
The careful choice of observatory sites may improve the optical quality of the image as well as the transmission of certain regions of the spectrum, but it is not the ideal solution because many spectral regions are completely invisible from the earth. A better solution is to take the observing equipment above the obstructing layers of the atmosphere to a height at which turbulence and atmospheric absorption become negligible.
Before the development of orbital satellites, three methods of observing from above the atmosphere were used—from aircraft, balloons, and sounding (research) rockets. Aircraft observatories can take instruments to altitudes of about 12 miles (20 kilometers)—high enough to open up many more of the “windows” in the infrared part of the spectrum. Helium-filled balloons, capable of reaching heights of 30 miles (48 kilometers), enable ultraviolet as well as infrared measurements to be made. Sounding rockets can carry small payloads up to altitudes of 300 miles (482 kilometers), or even higher. At these heights, the experiments are clear of the obstructions of the atmosphere, although observations can be made for only about 12 minutes.
All of these methods are more economical than launching a satellite, but they all also have many drawbacks, the most serious being the limited observing time. Nevertheless, they do enable experiments to be placed at high altitudes quickly (as would be needed for observations of novae or flare stars) if a satellite is not in orbit to undertake the observations. They are also useful as a means of testing equipment before placing it in orbit.
Before the first satellite, Sputnik 1, was launched in 1957, most of our knowledge of the universe had come from research with sounding rockets. These carried simple experiments to measure the conditions in the upper reaches of the atmosphere. The real breakthrough in rocket technology came with the work of the Germans in the 1930’s, resulting in the development of the A-4 rocket (more commonly called the V-2). This, in turn, became the basis of most subsequent rocket programs. The United States launched 67 V-2’s from White Sands, New Mexico, between April 1946 and June 1951, primarily for upper atmosphere research and astronomy. One of these, launched in 1946, carried the first experiment to record the solar spectrum below 3,000 A, an ultraviolet region that is absorbed by the ozone layer in the earth’s atmosphere.
Since these early beginnings, many hundreds of sounding rocket launches have taken place throughout the world. Many of the payloads have been directly intended for astronomical research, such as experiments for recording the spectra of the sun and stars in the ultraviolet and X-ray regions. Sounding rockets still have a useful place in astronomical research, although the real mass of data has come since the advent of satellites in orbit.
The benefits of using satellites and probes
Apart from improving on the telescope image and observing a greater range of the electromagnetic spectrum, there are other advantages of placing astronomical instruments above the atmosphere that merit the vast amounts of money spent on this aspect of space research. Satellites can provide continuous observing time well above any cloud cover. The advantage of a satellite over sounding rockets is well illustrated by the Orbiting Astronomical Observatory (OAO) launched in 1972. In half a day it furnished data equivalent to that gathered from 40 sounding rockets over a 15-year period.
Satellites also enable scientists to send experiments into a particular environment they wish to study. For example, the Explorer I satellite measured belts of intense radiation from particles captured in the geomagnetic field about 600 miles (1,000 kilometers) above the Earth (the Van Allen belts), which were totally unknown from ground-based observations.
The disadvantages of satellites
Satellites and probes have to operate with a high degree of reliability, because after being launched, they are “connected” to the ground control only by a radio link. All on-board systems must therefore function efficiently—and continue to function—for the mission to be a success. The space environment is very hostile, and satellites are subjected to extremes of temperature, intense solar radiation, and bombardment from micrometeorites and fast particles. The satellite engineer, therefore, has to design the satellite to function for many years without fail in this environment. The extensive development and rigorous “space” testing of a satellite prior to its launch means that the cost of building one is very high—as much as $50 to $60 million—which does not include the cost of the launch.
Communicating with spacecraft
The communication package on a spacecraft or rocket payload is possibly its most important feature, because without it, the mission would be virtually useless. Communicating with a spacecraft is a two-way process: ground control sends up commands to alter the status of the spacecraft, such as pointing it at different parts of the sky or controlling the start and end of a particular experiment; the spacecraft, conversely, sends back information called telemetry data to the ground station. This information can take the form of scientific data from an experiment, or data on the status of the spacecraft, such as temperature measurements.
The communication payload of the Voyager spacecraft (1979) is a fine example of a system developed for the transmission of large amounts of data over vast distances of space. It also illustrates many of the problems that communication engineers face when designing spacecraft systems. The transmitter on board the Voyager craft had a maximum output of only 23 watts (about the same power as a refrigerator light bulb). It could, however, transmit information accurately and at very high speed over 435 million miles (700 million kilometers) back to earth (40 minutes journey time). This meant that the power at the receiving dish of the satellite tracking station was exceptionally small (about 1014 watts/m2), and therefore required the most sensitive ground equipment.
The system was designed to transmit information at the rate of one complete picture every 48 seconds. This translates into about 107,000 bits per second. One picture consists of 800 by 800 picture elements, or “pixels,” each one being an eight-bit binary number that indicates the light intensity level of that part of the picture. This rate of sending back data is 14,000 times faster than the Mariner IV mission to Mars in 1964, when only 20 pictures were sent back to Earth in 7 days. Voyager’s communication system, like all those of scientific and communications satellites, was designed to transmit data with a high degree of accuracy, because the information was of such vital importance. The error rate for Voyager was one bit in 10,000, or 0.01 per cent. If it were possible to use the information capacity of the Voyager spacecraft on a commercial communication satellite, it would enable every American citizen to speak to a Soviet counterpart simultaneously—more than 500 million telephone conversations. A typical astronomy satellite is able to transmit data at a rate of 20,000 bits per second—about one-fifth the rate of the Voyager system—over a distance of roughly 18,500 miles (30,000 kilometers).