September 18 1964 - Vesta

Very little is heard about the minor planets, apart from the few whose exceptional orbits bring them relatively close to the Earth. Of these minia­ture worlds, the brightest, though not the largest, is Vesta; and this was the subject of our programme for September 1964.

One of the less-familiar members of the Solar System is now ex­cellently placed for observation in the evening sky. This is Vesta, the brightest of the swarm of asteroids or minor planets. It is far from spectacular, and even in large telescopes it appears as nothing more than a star-like point, but keen-sighted observers may be able to glimpse it without optical aid provided that they know exactly where to look for it.

Vesta is of the sixth magnitude, which means that from the viewpoint of the naked-eye observer it is hidden by the slightest trace of mist in the Earth's atmosphere; a dark, transparent sky is essential. Telescopically it looks exactly like a star, so that its nature is not evident at a glance. The only way to identify it with certainty is to observe the whole area from night to night; the stars will remain in the same relative positions, but the minor planet will move. This is brought out by Acfield's photograph. The date of the photograph itself was 8 September 1964, at 0.30 hours; the cross well to the right of Vesta shows the position of the minor planet in the early morning of 13 September. The five-day interval has been quite enough to reveal a decided shift.

Since Vesta is on the fringe of naked-eye visibility, even a very keen-sighted observer will be hard pressed to identify it without optical aid; binoculars, however, should suffice. Its faintness is by no means surprising in view of its small diameter and its con­siderable distance from Earth. Vesta is a mere 241 miles across, and moves round the Sun in a period of 3.6 years at a mean distance of 219,300,000 miles. A drawing showing Vesta and Eng­land to the same scale would demonstrate that we are indeed dealing with a dwarf world; yet only two of the minor planets - Ceres and Pallas - are larger.

The existence of at least one body moving in the region be­tween Mars and Jupiter was suspected long before the first asteroid was discovered. The Solar System is divided into two distinct parts. First come the four terrestrial planets (Mercury, Venus, the Earth, and Mars), after which there is a wide gap. Then follow the four giants (Jupiter, Saturn, Uranus, and Nep­tune) together with Pluto, a curious little world which may not be a true planet; according to some authorities, it is merely an ex-satellite of Neptune which has moved off in an independent path. A striking mathematical relationship, known as Bode's Law, led astronomers of the late eighteenth century to suppose that an extra planet might exist between Mars and Jupiter, and in 1801 the missing world war duly found. The Italian observer Piazzi, who discovered it, named it Ceres. It never attains naked-eye visibility, but with its diameter of 427 miles it remains the senior member of the asteroid swarm.

Further discoveries followed: Pallas in 1802, Juno in 1804, and Vesta in 1807. Number 5, Astrsea, came to light almost forty years later, due to the systematic labours of a German amateur named Hencke, and since 1847 no year has passed without the discovery of several new minor planets. Several thousands have now had their orbits worked out. On the other hand, very few are of appreciable size; most have diameters of under fifty miles, and the smaller members are probably not even approximately spherical.

Most asteroid discoveries have been made by means of photo­graphy. With a time-exposure, a star will appear as a sharp point, provided of course that the telescope is clock-driven to compensate for the east-to-west movement due to the rotation of the Earth; a minor planet will crawl across the sky, and will appear as a short trail on the photographic plate. Astronomers are not always glad to see these trails. Plates exposed for quite different reasons are often found to be crowded with asteroids, all of which have to be checked and eliminated. To make matters worse, some of the minor planets have high orbital inclinations - over 34 degrees in the case of Pallas, for example - so that they do not keep within the bounds of the Zodiac in the same way as the true planets. One infuriated observer referred to the asteroids as 'vermin of the skies'.

Most of the asteroids keep to the main zone between the paths of Mars and Jupiter, but some have eccentric orbits which carry them into other parts of the Solar System. No. 433, Eros - dis­covered in 1898 - may pass within 15,000,000 miles of the Earth ; No. 1566, Icarus, has a curious path which carries it closer to the Sun than Mercury, the innermost of the main planets, while No. 944, Hidalgo, swings out almost as far as the orbit of remote Saturn. In 1937 a dwarf body now known as Hermes approached the Earth to within 500,000 miles, and there were suggestions that it might even collide with our world. Such an event is most unlikely, but in any case Hermes can be little more than a mile in diameter, so that it could cause only local damage; it could certainly destroy a city, but it could not produce world-wide devastation.

Vesta appears the brightest of the minor planets because it is appreciably closer to the Sun and Earth than its two seniors, Ceres and Pallas. Even so, its distance from us is never as little as 100,000,000 miles, and to study details upon its surface is quite out of the question, even with the most powerful telescopes in existence. We can only speculate as to what it may be like, but we have at least a few concrete facts to guide us.

First, its small size means that it must have a very low escape velocity. The most important result of this is that it can retain no vestige of atmosphere, while its surface temperature must be very low indeed. No precise value for the escape velocity can be given, since we have no information about its density, but its gravita­tional pull is certainly very feeble. An astronaut who managed to land there would find that he would be able to jump an im­mense distance above the ground, and his subsequent descent would be extremely gradual. Yet it is not correct to say, as some writers have done, that a man could jump clear of Vesta by the power of his leg-muscles alone. This would not be possible except with an asteroid less than about two miles in diameter, assuming normal density; and Vesta, though small, is very much larger than that.

Probably, too, Vesta is more or less spherical, which means that its surface must be sharply curved; the 'horizon' would be strangely close. Minor variations in its light have led to the con­clusion that its rotation period is about 10 3/4 hours - slightly longer than the periods of either Ceres or Pallas, and three hours longer than that of Juno. This may indicate either that its shape is not entirely regular, or that some parts of its surface are more reflec­tive than others. _

When we come to consider the nature of the surface, we have to confess our complete ignorance, but Vesta seems to be a better reflector of sunlight than its companions. Its albedo, or reflecting power, has been estimated at 25 per cent as against 3 per cent for Ceres, 5 per cent for Pallas, and 11 per cent for Juno; the value for the Moon is considerably less than 10 per cent. Vesta, therefore, may have a smoother surface, but one cannot be sure, and moreover the albedo estimates - due to the German astronomer N. Richter - are bound to be somewhat arbitrary.

The origin of Vesta is similarly uncertain, but it was certainly formed in the same manner as the other members of the asteroid swarm. There are two main theories. It may be that the asteroids resulted from the break-up of an old planet or planets which used to move round the Sun between the orbits of Mars and Jupiter, in which case they are the visible remains of a tremendous out­burst which took place thousands of millions of years ago. Alter­natively, it has been supposed that the asteroids, together with meteoroids, were produced from material which was 'left over', so to speak, when the major planets were born.

The first of these theories is attractive, and has met with wide support, but it is not easy to see how a former planet could have met with disaster; a direct collision between two smaller bodies seems to be the only real possibility, and it is worth noting that the total mass of the asteroids is relatively small. There is no definite information as to the total number of members; R. S. Richardson, in the United States, has suggested 44,000, while Russian astronomers tend to believe that the true number may be as large as 100,000. Yet even if all the asteroids could be lumped together, the prove difficult to solve, and for the moment, at least, the nature of Vesta and its companions is not known.

It is worth noting, incidentally, that some of the smaller satellites _of the main planets may be captured asteroids. This applies to /Photos and Demos, the dwarf attendants of Mars, both of which appear to be less than a dozen miles in diameter, while the same may be true of the seven outer satellites of Jupiter and the smallest satellites of Saturn and Neptune. There is also a possibility that a second asteroid ring exists in the outer regions of the Solar System, though to observe small bodies at such a distance would be impossible with our present-day telescopes.

Science-fiction writers have put forward the idea that in the far future, when interplanetary travel has become commonplace, navigational beacons may be set up on Vesta and other asteroids. It would be rash to say that this will never be done, but at least it is not likely to be attempted for many centuries, even if it proves to be desirable! It is pointless to speculate about the pos­sibility of finding valuable minerals upon worlds such as Vesta, and we need spend no time upon another science-fiction idea, that of wrenching small asteroids free from their present orbits and steering them into more convenient paths to serve as natural space-stations. This remarkable scheme has been seriously dis­cussed by American scientists during the past few years, but it is quite impracticable. The energy needed to alter the path of an asteroid would be so great that there is no chance of anything of the sort being attempted.

If an astronaut could in fact go to Vesta, he would find him­self on a strange world. The temperature would be very low; there would be no atmosphere, and the Sun would appear relatively small and pale in the black sky. The constellation-patterns would of course seem the same as those we know, but there would be many additions to the sky, since Vesta lies within the main minor planet belt, and many of the other members of the swarm would appear as conspicuous naked-eye objects. From time to time, passing asteroids might even appear as distinct disks, and they would not add up to a body as massive as our Moon. The problem of the formation of the asteroids is likely to chances of collision certainly could not be ruled out. The pull of

gravity would be slight, though persistent; an Earthman would have very little weight.

There is one fact about which we may be quite positive: Vesta, like its companions, is utterly without life; no living organism of the kind known to us could survive under such conditions. In its way, Vesta is just as hostile as the Moon. Whether it will ever be reached seems extremely doubtful - and yet it is certainly not devoid of interest. If we knew how Vesta came into being, we should be well on the way toward solving the problem of the origin of the Solar System.