The science of space astronomy is relatively young, and yet it has given astronomers some of the most exciting findings about our own Solar System and the high-energy Universe beyond. Were it not for the development of a camera that could be operated across 200 million km of interplanetary space we would know hardly anything about the surfaces of planets such as Mercury and Mars. Remote landers have even scratched at the soil of Mars in a fruitless search for life, for some shred of evidence pointing to man’s own evolution from the cosmos. Alter millennia of romance and mythology the Moon finally yielded to the scientific method. The Queen of the Night was reduced to a laboratory specimen after the most fantastic voyage in the whole of human history. The US Apollo exploration program has returned to Earth 380kg of lunar material for analysis. So far 20kg has been looked at, and even describing that has filled 30000 pages of the scientific literature. Within our Galaxy there are many objects that emit infrared and X-radiation, but this cannot reach the ground because our atmosphere is strongly absorbent. Hence high-energy astronomy has to be carried out above the atmosphere, in space. In this chapter we describe in general terms the aims and methods of space astronomy. The scientific achievements are presented in context in the relevant chapters, and this chapter is therefore mainly concerned with hardware, experimental methods, and techniques. There are no detailed discussions of individual missions because these become dated very rapidly, whereas the general principles do riot evolve so quickly.
The atmosphere of Earth is a serious impediment to ground-based astronomy. Its absorptive and reflective properties limit satisfactory observing to two wavelength windows: the radio range (including the millimetre region) and optical range (and near infrared). Inhomogeneities in our atmosphere blur the images of cosmic objects and prevent large optical telescopes from imaging as sharply as might otherwise be possible. The gravitational field at the Earth’s surface restricts the precision with which instruments can be pointed and moved. The daily rotation of the Earth force observations of all but the polar regions of the sky to occur at the periodic intervals when the object is above the local horizon. Even in very dark sites, residual atmospheric glow limits sensitivity, and. of course, optical observers can study little apart from the Sun in the daytime. Finally, one of the most devastating effects is that of the weather, particularly for optical observing.
Some of these effects can be overcome. Observatories are built in the best sites, high up and free from contamination by domestic noise (lights, traffic and machinery). Speckle interferometry and similar techniques can improve the spatial resolution of optical telescopes to the extent that the discs of a few nearby giant stars are revealed. Computers can take account of the flexure of telescope mounts and instrumentation to some degree. Nothing we can do on the Earth’s surface, however, will permit us to receive cosmic X-radiation directly.
This high-energy part of the spectrum is only accessible to us by going above the atmosphere. As many of the chapters in this book reveal, the invisible wavebands are not merely superficial additions to optical astronomy. They actually permit us to see completely new Objects and phenomena ,the existence of which would have been otherwise unsuspended .If we are to obtain a full understanding of our cosmic environment ,we must fully explore the complete spectrum of radiations produced in the universe.
Space astronomy requires detectors and telescopes to be placed above our atmosphere ,where the instruments can search deep space and test cosmological theories, or look back at earth and allow us to inspect it for what it is :a planet in orbit about a star. We can now make direct studies of other celestial bodies: the Moon, planets, solar wind and planetary magnetospheres, as well as probing parts of interstellar space. All of the planets of the inner Solar System have now been visited arid had their soil analysed, or f have at least been scrutinized by orbiting spacecraft. Men have driven vehicles on the Moon arid there performed tasks similar those of Earth-bound geologists. Much of this explosion in our knowledge and activities is due to technological progress.The scientific principles of space flight date back to the early years of this century, and from there back to Newton. More romantic ideas of space Might oven pro-date that.
The advantages of performing astronomical observations and experiments in space are many, but perhaps the greatest dis¬advantage is the expense. Even a small satellite carrying, say, 50kg of scientific experiments costs tens of millions of dollars to manufacture and place in orbit, even though the actual material out of which the satellite or its pay load is composed need not be particularly expensive. The design, construction, test, launch, data handling and analysis of such instruments involves much organization and careful work over periods of many years. Even then, satellites have occasionally failed to achieve their desired orbits some even, becoming geostationary- on the sea bed From the development of rockets in the Second World War to present-day space activities, the major thrust to the research and development of spacecraft has come from so-called defence programmes. It is doubtful whether more than a handful of satellites would have been launched by now if it had not been for the enormous quantities of money invested in military projects. For astronomical purposes, though, we now have rocket engines available capable of putting people on the Moon and spacecraft out to Jupiter. This last point is important, for Jupiter is the key to the rest of the Solar System, as far as space travel is concerned.
The United States government, for example, proposes to build and fly the Space Shuttle, capable of putting scientists into orbit for weeks or months. These missions are important to astronomers mostly for their relative cheapness, payload-carrying capabilities and their frequency. Furthermore, satellites can be serviced in orbit, thereby prolonging their active lives. This is of note, for most space equipment, left unattended, will either undergo degradation, or fail completely owing to some electrical or mechanical malfunction, after a few years.
Early space astronomers relied heavily upon SOUNDING ROCKETS developed from the V2 rocket in World War II. Small payloads could be exposed to the space environment above most of the atmosphere before plunging back to Earth. Studies earned out in this way revealed the major features of the upper atmosphere and ionosphere, and discovered short-wavelength radiation from the Sun. Space astronomy was changed dramatically at the end of the 1950s by the launching of satellites orbiting the Earth and beyond (USSR Sputnik I, 1957; US Explorer 1; 1958). This gave way to the numerous space probes that have since explored the Solar System. Series of satellites carrying astronomical experiments, such as the US Orbiting Solar Observatories (OSO); Orbiting Astronomical Observatories (OAO). Small Astronomical Satellites (SAS), the UK Ariel satellites, some of the USSR Cosmos series and others have opened up the new wavebands only accessible from space. Sounding rockets are still in use, since they are relatively inexpensive. Indeed, the sum total of exposure time to the X-ray sky (excluding the Sun) by detectors world wide from 1962 to late-1970 was only a few hours. Balloons are also used to lift instruments to altitudes of 40km or more, which allows valuable measurements to be made in the infrared, optical, hard X-ray and gamma-ray parts of the electromagnetic spectrum. Flight times of a week or more can sometimes be achieved from transatlantic balloon flights Certain infrared wavebands can be usefully explored by telescopes carried on high-flying aircraft.
The future of space astronomy depends to a large extent on continued funding by governments and probably on international cooperation. The United States, for example, have launch vehicles such as the Titan-Centaur system, capable of putting spacecraft anywhere in the Solar System. The time taken to reach the outer Solar System is, of course, many years, but such remote regions are in principle accessible using the gravitational pull from Jupiter or Saturn. Large Earth-orbiting payloads can be lofted, carrying several square meters of short-wavelength detectors and associated electronics. Programmes proposed for the near future involve the use of reusable launchers, such as the Space Shuttle, implying a relatively high degree of space activities. One of the most important proposals is that of the Space Telescope: a large diffraction-limited telescope capable of operation over the entire spectrum from the submillimetre infrared down to 90nm. Such an instrument would have diffraction-limited resolution, approximately 100 times the ultimate sensitivity of ground-based instruments and a lifetime of 15 years or more. The successful launch of the Space Telescope will transform astronomy because it is capable of working down to 29 magnitude. Space-borne instruments will supply many of the expected new results over the next few decades, and hopefully some unexpected results.