October 26 1959 - The hidden face of the Moon

Ever since astronomy began, there has been curiosity about 'the other side of the Moon' - that part of the lunar surface which we cannot see from Earth, since it is always turned away from us. At last, in October 1959, the Russians dispatched Lunik III, which passed beyond the Moon and sent back photographs of the averted side. This, remember, was only two years after the ascent of the first artificial satellite. Sputnik I was launched on 4 October 1957; Lunik III on 4 October 1959.

Live Photos from Lunik 3

The photographs became available on 26 October. I had actually started the television programme when the first copies arrived, rushed down from London at a speed which would not have pleased the police {at that time traffic wardens lay, mercifully, in the future). Colin Ronan was in the studio with me, and we were able to show the pictures immediately, so giving people in Britain a first view of them. By good luck I was able to identify the grey plain known as the Mare Crisium, which was clearly recognizable, and so give what I hope was an intelligible commentary.

The article for The Listener was written that night, and is printed here as published three days later. Actually, the conclusions have been borne out by subsequent investigations. The hidden area is indeed crowded with craters, which do not show up well because they were photographed under the equivalent of full-moon lighting; 'seas' are fewer than on the visible face; Wilkins' estimates of the positions of ray-craters were not very wide of the mark. As for Lunik III itself, signals from it ceased abruptly some time later, and contact was never regained, so that its final fate will never be known.

On 26 October 1959 news was received of one of the greatest triumphs in scientific history. The Russians released pictures taken by their rocket vehicle Lunik III, which had passed beyond the Moon and photographed regions of the lunar surface which are always turned away from the Earth, and which therefore had never before been examined.

The photographs were sent to London in the evening, and became available a few seconds before the start of transmission of the BBC programme The Sky at Night. It was possible to show them during this programme, and people in Britain saw the pictures for the first time. Various features were clearly identifiable on the section of the picture which showed regions visible from Earth, and previously mapped; details on the uncharted zone were also to be seen. The quality of the photographs was remarkable in view of the circumstances under which they were taken, and demonstrated once again how far Soviet science has advanced.

Up to now we have seen only part of the Moon. This is because the period of revolution round the Earth is precisely the same as the period of axial rotation - 27 days 7 hours 43 minutes 11-47 seconds in each case - so that the same hemisphere is permanently presented to us. The situation is, however, rather less simple than is often believed. For instance, it is not strictly correct to say that the Moon revolves round the Earth; more accurately, the two bodies revolve round their common centre of gravity, or barycentre - though since the Earth is much more massive than the Moon (the ratio is 81 : 1) the barycentre actually lies within the terrestrial globe. Secondly, though the Moon's rate of axial rotation is con­stant its orbital speed varies, since its path is elliptical with respect to the Earth, instead of being circular. This means that, each month, the axial spin and the position in orbit become slightly 'out of step', and we can peer a little way beyond the mean limb first in one direction, then in another. Moreover, the lunar orbit is appre­ciably inclined, and we can similarly see a little way beyond the mean limb in the polar regions. These effects are known as librations, and altogether it is possible to see a total of 59 per cent of the Moon's surface - though, needless to say, only 50 per cent can be seen at any one moment. The remaining 41 per cent remained unknown until the Lunik sent back photographs of it.

It should be emphasized that 'day' and 'night' conditions on the hidden part of the Moon are identical with those which prevail on the side visible to us. The Moon keeps the same hemisphere turned toward the Earth, it is true, but not to the Sun, and it is quite wrong to suppose that the averted region is always in darkness. From this part of the Moon, the Sun would appear to rise and set, though, of course, the Earth could never be seen.

It may seem strange that the revolution period and the axial rotation period are of the same value. This is not pure coincidence, however. Tidal friction in past ages has been responsible: long ago the Moon probably rotated comparatively quickly, but the power­ful tidal pull of the Earth slowed it down to its present value. Other planetary satellites, such as the four Galilean moons of Jupiter, behave in the same way with respect to their primaries; and so does the planet Mercury with respect to the Sun.

The Russian achievement with Lunik III marks the culmination - so far - of a series of experiments which began in January 1959 with the launching of the first lunar probe. Lunik I passed within 4,000 miles of the Moon and then entered an orbit round the Sun, so that it became a tiny artificial planet; unless some disaster over­takes it, such as a collision with a meteor of appreciable size, it will remain in orbit indefinitely, so that in a million years from now it will still look precisely the same as it did immediately after its launching. Admittedly, there is little chance that it will ever be re-observed from Earth.

Lunik II actually landed on the Moon on 13 September 1959. The impact point is still uncertain; within the last few days, reports from Russia indicate that astronomers there have discovered a 'new craterlet' which may be due to the landing of the rocket, but these reports are still unconfirmed. *

Lunik III had a different mission. It was designed not to hit the Moon, but to photograph the hidden 41 per cent. Despite the tremendous skill shown by the Russians in their space programme, it is probably fair to say that few people expected the pictures to be good enough for interpretation, even if they could be received at all. Once again the world has been surprised; once again the Russians have earned the highest congratulations.

* They remain unconfirmed, and it now seems that no traces of the impact of Lunik II are detectable.

Strange theories about the averted region were current in past years. Hansen, a Danish astronomer, once supposed that the Moon's centre of gravity lay some way from its centre of figure, so that all the lunar air and water had been drawn round to the far side. This is sheer fantasy, and can easily be shown to be unsound, but it has also been suggested that the features on the hidden 41 per cent may be arranged rather differently from those on the familiar side, and this in turn is bound up with the question of how the craters were formed. Two main theories are current here. According to one, the craters were produced by meteoric impact; on the second, the main formations are volcanic, though the term is to be used in its broadest sense and without implying that there is much resemblance between a lunar crater and a terrestrial volcano such as Vesuvius. (It is true that certain minor features, visible only with telescopes of some power, do resemble volcanoes. I have listed over forty, yet the main craters are quite different in form.)

There must be many small meteor craters on the Moon, but my own inclination is to believe the major features to be of igneous origin, since their distribution appears not to be random. There is a strong tendency toward lining-up, and this may be due to lines of weakness in the lunar crust. The Earth's pull may have played a part in producing these lines of weakness, and in 1953 I suggested that in such a case the averted hemisphere, which is always turned away from the Earth, and has been so for a very appreciable part of the Moon's history, is probably devoid of large 'seas'. (It follows that a true sea, or Mare, is basically similar in origin to a crater.) My suggestion seems to have been borne out by the pictures taken by Lunik III.

According to the Tass statement of 26 October, the pictures were taken on 7 October; the photographic equipment in the Lunik was switched on at 04.30 G.M.T. The statement gave the first information about how the camera operated, and said that 'the Lunik was supplied with a system of orientation and photographic television apparatus with special equipment which automatically processed the film'. This meant that some form of control in the vehicle kept the camera pointed toward the Moon, and that the pictures were automatically processed on board.

The transmission from Lunik to Earth was accomplished on 18 October, when the vehicle had returned to the neighbourhood of our world; much other information was also received, and will no doubt be released in the very near future as soon as the Soviet team has had time to analyse it fully. It was also stated that the Lunik will last for another six months, after which it will re-enter the Earth's atmosphere and be destroyed in the same way as a meteor.

It seems that about 70 per cent of the hitherto unknown portion has been covered by the photograph, and it is likely that several more pictures will be released in the near future, some of which will be more detailed. * (The Tass statement points out that the space camera was equipped with two different lenses, to give different magnifications.) This, then, represents our first positive knowledge, and confirms what modern astronomers have always believed: the hidden region is of the same basic nature as the part of the Moon which we can see - it is equally barren; it contains mountains and craters; and it is equally lifeless.

The photograph shown for the first time in the programme The Sky at Night is remarkably informative. Certain familiar features are clearly shown, including the Mare Crisium, the Mare Marginis, and the Mare Smythii - all at first sight appearing rather strange to lunar observers, since they are of course being viewed from a new angle. Among the interesting new features is a small, well- defined dark plain which has already been named 'the Moscow Sea', an object which is possibly a crater with a central mountain ('Tsiolkovskii') and a long mountain range ('the Soviet Moun­tains'). Large maria are not present, apparently, and much of the [1] The photographs were in fact released, and when I was in the USSR, in 1960, I was able to study the originals, which are naturally much clearer than the reproductions. Y. N. Lipski and his colleagues at Moscow have used them to draw up a detailed chart of the averted region of the Moon.

averted hemisphere now photographed for the first time appears to be more or less featureless. Yet it would be most misleading to suggest that this is actually the case. Though the photograph is of amazing quality, its resolution is naturally not to be compared with that of the detailed pictures familiar to us which show the part of the Moon turned in our direction; moreover, the lighting when the photograph was taken means that there are few shadows. It is likely that the newly studied portions are decidedly rough, and contain numerous craters - as is only to be expected. It is too early to say much about the arrangement of the craters, since as yet we are in a position to discuss only the main features. My own suggestion that the lining-up tendency of large craters may be less marked than on the familiar face may or may not prove to be correct.

There are also indications of ray-craters. We already know of many such features; near full moon, as most people know, the rays centred on craters such as Tycho and Copernicus dominate the scene, and in past years various observers have studied faint rays coming from over the edge of the visible disk which seem to indicate ray-centres on the averted side. H. P. Wilkins, author of the largest lunar map in existence, has constructed a chart, based on observa­tions by himself and others, which shows over half a dozen possible ray-centres which can never be observed from Earth. It will be most interesting to see how correct these estimated positions are.

One minor point concerns the naming of the new features; it is understood that the Russians are now holding a conference to dis­cuss this matter. In general, the craters are named after eminent scientists and other personalities; Newton, Ptolemy, and Julius Caesar are among those who have had craters named in their honour. Up to now these names have been allotted mainly by Italian and German astronomers, beginning with Riccioli - a Jesuit priest - in 1651; many Russians have been commemorated. The present situation is, of course, entirely novel, and nobody will deny that the Soviet workers have well earned the right to name the new features as they think fit. One can only hope that their suggested names will be accepted without demur, even though we must resign ourselves to the fact that many of them will be rather tongue-twisting from the Western point of view!

In general, then, the Lunik has shown that the averted face resembles the general picture which scientists had expected, but there can be no doubt that the photographs will enormously in­crease our understanding of the Moon as a world. It is now logical to consider what the next steps are likely to be.

The Russians have shown that they are able to obtain pictures which can be processed inside the rocket and then sent back sufficiently clearly to be interpreted; this is in itself a great technical feat. During the next few months further experiments on the same lines will be carried out, certainly by the Soviet team and possibly by the Americans, so that in the foreseeable future we will have a really adequate map of the whole Moon instead of only 59 per cent of it. But this is only one aspect of the series of experiments. As well as taking pictures, the Lunik has certainly sent information as to other equally important matters - such as that of the Moon's magnetic field; it seems that such a field is extremely weak, and possibly too weak to be detected at all, which again is by no means unexpected.

Before long, we may expect that an instrument-carrying rocket will be landed on the Moon sufficiently gently to avoid breaking ' its instruments, so that we will have what may be termed a laboratory on the lunar surface itself. And, of course, the age of manned space-flight draws steadily nearer. Just when the first men will reach the Moon remains to be seen, but the moment is cer­tainly far closer than could have been expected only a year or so ago.

Full analysis of the results from Lunik III will take some time, but at least it is clear that the experiment has been an outstanding success. No praise can be too high for the Soviet scientists who made it possible.