October 30 1961 - Has the Earth Three Moons?

The idea that the Earth may have a minor satellite is not new, but the matter was reopened in 1961 when the Polish astronomer K. Kordylewski announced that he had detected two nebulous objects which might be in the nature of meteoric debris moving in the same path as the Moon. When the broadcast of October 1961 was made, Kordylewski's results were uncon­firmed; three years later they were still unconfirmed, and so far there is nothing to be added to what I originally said during the programme.

Some theoretical investigations were made by Colin Ronan, Director of the Historical Section of the British Astronomical Association, who appeared with me.

Almost a century ago, the great French writer Jules Verne published his book From the Earth to the Moon. With its sequel, Round the Moon, it may be regarded as the first important story about a Moon-voyage - 'important', that is to say, in the sense that Verne did his best to keep to scientific facts.

Naturally the story appears old-fashioned now, even though it is still worth reading. Verne's suggested lunar vehicle was fired from the mouth of a vast cannon 900 feet long with an internal diameter of 9 feet; even though many of his calculations were correct, it is now known that the whole idea of such a space-gun must be discarded, partly because of the violent initial shock, but mainly because of air-resistance, which would produce enough frictional heat to vaporize the projectile almost at once. Moreover, Verne was seriously in error with regard to zero gravity or weight­lessness. In his story, the three voyagers - Barbicane, Captain Nicholl, and Michel Ardan - became weightless only when they reached the so-called 'neutral point' where the Earth's gravita­tional pull balances that of the Moon. Actually, the occupants of the projectile would have been in free fall, and therefore weightless, from the moment of firing - and in any case there are many com­plications concerning the 'neutral point'.

On the other hand, Verne made some remarkably shrewd fore­casts. To mention only a few: the site of his fictional 'Columbiad' cannon was not far from the modern launching ground at Cape Canaveral; * the projectile went round the Moon just as Lunik III actually did in 1959; and the adventurers landed in the sea close to the point where one of the two American astronauts, Grissom, was picked up by helicopter a few months ago.

In the story, the projectile was meant to land on the Moon, so that Barbicane, Nicholl, and Michel Ardan would have been stranded there with no hope of return. Verne avoided this difficulty in a most ingenious manner. He told how the projectile met a minor satellite of the Earth, with the result that its orbit was perturbed, so that it swung round the Moon and came back to the Earth. It may be worth while to quote the actual description from Verne's book:

As Barbicane was about to leave the window... his attention was attracted by the approach of a brilliant object. It was an enormous disk, whose colossal dimensions could not be esti­mated. Its face, which was turned earthwards, was very bright; it might have been taken for a small moon reflecting the light of the larger one. It advanced very rapidly, and seemed to be following an orbit round the Earth which would intersect the path of the projectile. Revolving on its axis, it showed all the characteristics of a body floating freely in space.... The object grew enormously, and the projectile seemed to be rushing into its path. . . . The travellers instinctively recoiled. Their alarm was great, but it did not last long. The object passed within a few hundred yards and vanished, merging into the absolute blackness of space.

Verne's information was drawn from a paper by a French astronomer, Petit, who believed in the existence of a minor satellite moving round the Earth at a distance of 4,650 miles from the ground, in a period of 3 hours 20 minutes.

We may be sure that no appreciable body of this kind exists so close to the Earth, but the whole question of possible minor satellites is interesting enough to warrant further investigation, particularly in view of a recent announcement by the Polish astronomer K. Kordylewski. First, then, let us say something about the sizes and distances of known planetary satellites.

If we begin with satellites of considerable size - say over 150 miles in diameter - we find that Jupiter has four, Saturn nine, Uranus four, and Neptune and the Earth one each. We might also include the fifth attendant of Uranus (Miranda) and the second of Neptune (Nereid), whose diameters are probably in the region of 150 to 200 miles. Of all these, our own Moon is much the largest in relation to its primary, and there is strong support for the idea that the Earth-Moon system should be regarded as a. double planet instead of as a planet and a satellite. \

Jupiter has eight small satellites as well as its four large ones; their diameters range from about 100 miles down to perhaps 15 miles, and they are excessively faint, so that in some cases photo­graphs taken with large telescopes are necessary to show them. Mars, too, has dwarf attendants - Phobos and Deimos, neither of which is as much as a dozen miles across. According to one theory, minor satellites of this sort are really in the nature of captured asteroids, and this may also be true of Saturn's ninth moon, Phoebe, which - like the four outermost members of Jupiter's family - travels in a retrograde or east-west direction. If so, there is no valid reason why the Earth should not have similarly captured one or more small bodies.

Mention should also be made of Saturn's ring-system, which is certainly made up of large numbers of small independent bodies moving round the planet in the manner of tiny satellites. It may be that the rings were formed from the debris of a former large body which approached within the Roche limit for Saturn and was broken up, or it may be that the particles were never collected together in a single mass. In either case, the ring-particles exist, so that it is permissible to say that Saturn now has many thousands of minor satellites.

The next step is to see what may be deduced about the brightness of a possible minor satellite of the Earth. Even bodies with low albedoes (low power to reflect light falling on them) are visible over surprisingly great distances; for instance, I have glimpsed Phobos over a distance of 40,000,000 miles with the 12 J inch reflector in my own observatory. Assuming normal albedo, it is found that a 25-mile satellite at the distance of the Moon (roughly a quarter of a million miles) would have a magnitude of I-o, so that it would be as bright as Aldebaran or Betelgeux, and would have been known from the very earliest days of human history. Even at a distance of 2,000,000 miles, it would still be easily visible to the naked eye. Similar calculations made by C. A. Ronan may be summed up as follows:

On the whole, it seems unlikely that the Earth would have a natural satellite at a distance of much greater than 2,000,000 miles, and it may be assumed that even a I2th-magnitude body would have been detected long ago. Therefore, our hypothetical satellite must be a real dwarf, considerably less than a mile across, and probably not even approximately spherical.

Searches have been made from time to time. The latest and most exhaustive was carried out by C. Tombaugh, the American astronomer who discovered the planet Pluto in 1930. The method is, of course, photographic; a relatively nearby satellite would shift appreciably against the starry background even in a few minutes, and so would betray its nature. This is also the way in which new asteroids (minor planets) are detected, though since the asteroids are much farther away the time interval needed is longer. Tombaugh met with no success, but a recent announcement from Poland has brought the whole matter into prominence again.

According to the report, K. Kordylewski, of Krakow Observa­tory, has been conducting a search ever since 1951, mainly at Kasprowy Wierch and Lomnica in the Polish mountains. On 6 March and 6 April 1961 two photographs taken from Kasprowy Wierch revealed two faint 'clouds' moving in the same orbit as the Moon, and presumably consisting of meteoric debris. The objects should again be observable from Poland in January of next year; mean­while Kordylewski has gone to South Africa to undertake further studies of them.

The most significant part of this report is that Kordylewski states that his 'clouds' are moving in the same orbit as the Moon. There is an analogy here with the Trojan asteroids which move in the same orbit as Jupiter. In fact there are two groups of Trojans, one of them 60 degrees ahead of Jupiter and the other 60 degrees behind. They do not, of course, keep together in a clump, and may be spread out by many millions of miles, but the 60-degree points represent their average positions, and there is no fear of their suffering a close encounter with Jupiter. They represent an inter­esting example of the so-called 'three-body problem'. .

The same is true of Kordylewski's reported objects. If they occupied positions A or B in the diagram, they would behave in the same way as the Trojans, and would be in no danger of colliding with the Moon. A body at A would keep approximately 60 degrees ahead of the Moon,' while a body at B would keep 60 degrees behind. (This is very much of an over-simplification, since all sorts of perturbations have to be taken into account, but the general principle is clear enough.)

So far, Kordylewski's observations are completely unconfirmed, and it would be unwise to come to any definite conclusions; there may well be serious errors of interpretation. In any case, it is not likely that the objects, even if they exist in the same orbit as the Moon, are proper satellites, and the idea of a loose collection of meteoric debris is much more plausible. Further comment at the moment is rather pointless, and must be kept until the objects are confirmed - which may well be difficult.

In passing, it may be worth referring to an old theory according to which the Earth has a second satellite lying behind the Moon, and so permanently concealed by the lunar disk. This is ingenious, but is completely unsound. For one thing, perturbations produced by other bodies in the Solar System would soon destroy the align­ment of Earth, Moon, and second satellite; for another, a body moving at a distance greater than that of the Moon would neces­sarily have a longer period of revolution, and could not possibly keep behind the Moon for long. (Perturbations also dispose of another old idea - that of a 'counter-Earth', a planet moving in the same orbit as our own, but on the far side of the Sun.)

All things considered, then, it seems that a minor satellite with a diameter of as much as 25 miles is almost out of the question; since it has not been detected, it would have to be so remote that it could probably not be permanently retained by the Earth. Even a satellite with half .this diameter is most unlikely. If there are any undiscovered attendants, they must be so small that they cannot be regarded as true satellites, but rather as tiny meteoric particles. Discussion of Kordylewski's observations must be deferred until further information is to hand, but in any case his 'clouds' cannot be actual satellites, though aggregations of meteoric debris moving in the lunar orbit cannot be entirely ruled out.

Yet it can no longer be said that the Earth has only one Moon. During the last few years many artificial satellites have been launched, and once in orbit they behave in precisely the same way as natural bodies. This is a new development in the story of the Solar System, and where it will lead us we do not yet know. By now our world has an impressive retinue of attendants, even though all except one have been created by man.