Big eye to the sky to dominate astronomy

Press, 31 January 1984, Page 20

Big eye to the sky to dominate astronomy

The international astronomical community have already been advised of the expected performance of the Space Telescope. They have now been requested to suggest preliminary proposals for use of this instrument, which will undoubtedly dominate astronomy during the next two decades.

The Space Telescope has an aperture of 2.4 metres. It is of Ritchey-Chretien design with a f/24 Cassegrain configuration. The normal Cassegrain reflecting telescope has a large concave primary mirror. This reflects the incoming light on to a secondary concave mirror facing it. The secondary is situated inside the focus of the primary mirror and reflects the light back to the viewing position behind a hole in the centre of the primary mirror. The advantage of such a system is that the focal length is increased without having to increase the length of the tube.

The Ritchey-Chretien design has hyperbolic curves on both mirrors. It provides a wider field than available in other types of telescopes. It has the’ advantage that it is free of coma, an optical effect that distorts a star’s image towards the edge of the field of view.

The Space Telescope will be launched from the Space Shuttle in 1986. It will be in earth orbit where it will be maintained by N.A.S.A.’s Space Transportation System. It is expected to operate for two decades. A system of geostationary communication satellites will provide communications to and from the Space Telescope. This system will be known as the Tracking and Data Relay Satellite System.

The launching of the Space Telescope will, for the first time, provide astronomers with access to an observatory operating completely free of the restrictions imposed by the Earth’s atmosphere. There will be an impressive array of cameras and other auxiliary equipment, all capable of making observations ranging from the far ultra-violet to the far infra-red.

Proposals for researches from this observatory will include almost every aspect of current astronomical research. It is impossible to discuss all of these, but some of the most likely proposals to be accepted are mentioned in this article. One of the fundamental problems of astronomy concerns the age and expansion of the universe. There is a relationship between the distance to a galaxy and its speed of recession. The further away a galaxy is the faster its speed of recession. This is known as Hubble’s Law, after the American astronomer, Edwin Hubble, who discovered the relationship in 1929. He showed that a galaxy’s red shift, which measures the speed of recession is proportional to the apparent brightness of the galaxy. The brightness is, of course, dependent on the distance.

The relationship between the speed of recession and distance demonstrates that the universe is expanding. The figure that relates these two factors is called Hubble’s Constant. Its value is about 55 kilometres per

by

second for every million light years of the distance. Comparison of the local rate of expansion in regions comparatively near to us with the rate at very distant regions should give a rough indication as to whether the rate has remained constant since the original “Big Bang.” Light from the remotest galaxies has taken many millions of years to reach us. We therefore see those objects not as they are today but as they were in the very distant past. If the rate of recession has remained constant since the formation of the universe, then it becomes possible to estimate the age of the universe.

The Space Telescope, because of its much greater capability, will reach further into space and record more distant objects. In other words it will be looking back further into time. Should it be found that Hubble’s Constant holds for the more remote regions then it will be possible to refine the age currently placed on the universe. Another proposal that is likely to be considered for the space observatory will involve attempts to see whether any of the nearer stars have planet-like bodies surrounding them. Simple reasoning suggests that the Sun is not alone in our galaxy, in having a retinue of planets. It is generally believed that many such systems must exist around stars similar to the Sun. It has been suggested that the slight displacements observed in the orbits of certain nearby stars may be due to the gravitational pull of one or more planetary bodies disturbing the orbits of these stars. Such a search will always fascinate astronomers since many believe that if such bodies exist then at least some of them are likely to have some form of intelligent life. The coverage of the full spectral range by the Space Telescope will enable more complete studies to be made of the injection of material into space by supernova explosions and other mechanisms. It is known, for example, that some stars have injected shells of matter into space. These are believed to be carried away by stellar winds, possibly in much the same way as the solar wind is enhanced by

F.M. Bateson

material ejected by the sun. However, there is much to be learnt on the behaviour of stellar winds and the dispersion of ejected material into space.

Planetary studies are likely to be included in the programmes simply because of the lack of knowledge from long-term studies of the atmospheric circulation of the planets, and because current plans do not provide for missions to the major planets. Synoptic studies of the atmospheres of the major planets will provide a very much better knowledge of the storm centres that affect their circulation.

These are just a few of the many proposals that will be made and given serious consideration. The question is often raised as to whether developments in space, especially such a major step as the Space Telescope, will render ground-based astronomy redundant. This question has been raised at each major advance in astronomical research and always proved to be false. Major advances always create additional problems which do not justify time on the more advanced instruments. It is likely that this will again happen so that ground-based observations will be directed towards new problems.

February will again see all the major planets in the morning sky. Mars and Saturn will rise about an hour before midnight by the end of the month but still not in the most convenient viewing position. These two planets will be close together in the sky, especially on the morning of February 16 when they will be less than a degree apart. Again on the morning of February 23 Saturn will be just north of the Moon and Mars immediately south of the Moon.

Venus will be conspicuous, rising just before dawn but it will be difficult to see Mercury. That planet will rise just before dawn on February 1 but move rapidly towards the Sun and will be hard to see against the bright dawn sky after the first few days. Jupiter will rise in close proximity to the Moon on February 27 but will still rise too late for viewing in a satisfacatory position.