June 25 1965 - Space Weather

It was in the summer of 1965 that the U.S. probe Mariner IV by-passed Mars and sent back photographs showing that the planet's surface is covered with lunar-type craters. When I presented the July programme, with K. H. Fea, the information was still coming through, but there were other topics also; both Mariner IV and the unsuccessful Soviet Mars probe Zond II had sent back information about the conditions prevailing in the space between Mars and the Earth. The term 'space weather' is by no means inappropriate, and we decided to devote the programme to it. I have left the article as it appeared at that time; up to mid-ig6y no further Mars probes were dispatched, though doubtless there will be more in the near future.

Astronomers all over the world are now busy studying the first close-range photographs of Mars, taken with the American space- probe Mariner IV. A full analysis will take some time, but at least it is now clear that the experiment has been a real success. There are grounds for suggesting that it is the greatest triumph yet in the field of space research.

The photographs are much less spectacular than those ob­tained some months ago when the Ranger craft hit the Moon after sending back highly detailed pictures of the lunar surface. However, Mars is a much more remote target; at the time of Mariner's closest approach to the planet, the distance between Mars and the Earth was in the region of 135,000,000 miles. To keep in touch with the probe across this range was in itself a major feat, and one which proved to be too difficult for the Russians, who have lost all contact with their own Mars rocket, Zond II.

It is not likely that the information obtained from Mariner will solve all the problems of Mars, but it should give us a better idea about conditions on the planet. It must, however, be remem­bered that in addition to photographing Mars, the probe’ has also told us a great deal about the space region between Mars and the Earth. We know more about the 'space weather' of this part of the Solar System than we could even have guessed a few months ago.

The term 'space weather', originally coined by an American scientist, is not so inappropriate as might be imagined. It should not be taken too literally, since there is no weather, in the accepted sense of the word, above the Earth's atmosphere. All normal clouds, for instance, are included in the bottom seven miles of atmosphere, known as the troposphere; higher up come the more rarefied layers, and the atmospheric mantle has no definite boundary, but it is a good approximation to say that the 'air: ends at about 1,000 miles above sea-level. (Incidentally, the fact that the atmosphere is of limited extent has been realized for a long time, and the Arab astronomers of many centuries ago were well aware of it.)

Beyond the atmosphere lies 'space', but we have come a long way from the time when space was supposed to be completely empty. It is now held that there is no such thing as empty space anywhere in the observable universe, though the material is very rarefied by everyday standards.

First, there are countless meteoritic particles in the Solar System, ranging from large blocks down to tiny objects smaller than specks of dust. Large meteorites, weighing more than a few pounds, are relatively rare, and seem to be close relations of the minor planets or asteroids. Meteors which produce the spectacu­lar shooting-star effects are much smaller, and are inferior in size to grains of sand, so that they are destroyed in the Earth'; upper air; there should be plenty of meteors on view during the first fortnight of August, when the Earth passes through the rich and consistent Perseids. Still smaller is the micro-meteoroid; which has too little mass to produce luminous effects when they enter the Earth's atmosphere.

The meteoroid hazard was one of the early bugbears of space­flight, and as recently as the early nineteen-fifties it was still claimed, by some authorities, that a probe venturing beyond the shield provided by the Earth's atmosphere would be promptly battered to pieces by a sort of cosmical bombardment. Luckily, this fear has proved to be ill-founded, because the larger bodies are relatively so rare, and the micro-meteoroids have too little penetrating power to be really dangerous. If interplanetary travel becomes common-place during the centuries to come, there may well be occasional disasters; but if meteoroids were the only hazard, a flight from the Earth to the Moon would be much less risky than a weekend drive from London to Margate!

Mariner IV has been recording micro-meteoroid hits through­out its journey, and there seems to be an increase in strike fre­quency near the Martian orbit, but this was not unexpected. Information has also been provided by the now-lost probe Zond II.

As well as solid particles of this sort, space also contains atoms of gas moving at more than 1,000 kilometres per second. These atoms make up what is known as the solar wind, entirely un­known before space research became a powerful tool in the hands of astronomers. Generally, the solar wind is made up of a steady flow of atomic particles streaming away from the Sun in all direc­tions; the density is so low that a space-probe would find only about ten atoms in every cubic centimetre, but when vast dis­turbances are taking place on the Sun the density increases - or, to put it more graphically, the solar wind becomes gusty. When the denser parts reach the Earth, they upset the magnetic field, as well as interfering with long-distance radio and cable com­munications, and producing spectacular displays of polar lights or auroras.

The brilliant surface of the Sun is known as the photosphere, and is at a temperature of about 6,000 degrees Centigrade. It is composed of gas, and upon it may be seen darker patches, known as sunspots. The spots are at a temperature of 4,000 degrees, and are in fact highly luminous, appearing dark only by contrast against the still more dazzling background. They are not permanent; smaller ones persist for only a few hours or days, while it is rare for even a large spot to last for as much as three months.

The Sun is in some respects a variable star, and exhibits a semi-regular cycle of activity. Every eleven years or so, activity is at its peak; spots are common, some of them showing violent, short-lived outbursts known as flares. The last maximum, which of 1957-58, was the most energetic on record, but recently the Sun's activity has been at a low ebb, and there have been long periods when no spots have been seen. It appears that minimum took place during the winter of 1964-65, and by now activity is starting to build up again; the next maximum may be expected about 1969-70.

When the Sun is active, the solar wind is 'gusty'. It originates not at the photosphere, but in the outer parts of the solar atmos­phere, known as the corona, in a region about 1,000,000 kilo­metres above the brilliant surface. Here the gas particles, mainly the nuclei of hydrogen and helium atoms, are in violent motion, and the temperature is about 1,000,000 degrees. Some areas are even hotter (for reasons not yet properly understood), and it ii thought that these areas are the sources of denser streams in the solar wind.

It is worth noting that the scientific definition of 'tempera­ture' is not the same as the everyday understanding of 'heat". Temperature depends upon the speeds at which the various atomic particles are moving about; the greater the speeds, the higher the temperature. In the solar corona, the temperatures are; colossal, but the material is rarefied, so that the actual heat is much less than in the photosphere. There is something to be gained by comparing a Guy Fawkes' hand-sparkler with are ordinary match. Each spark of the firework is white-hot, but: contains so little mass that the sparkler may be safely held in the hand - whereas to put one's finger into a match-flame is apt: be a painful process, though the temperature of the flame _ lower.

Flares, usually - though not invariably - associated with large sunspots, are sources of radiations and particles. The particles cover the 93, 000, 000-mile gap between Sun and Earth in little over one day, and cause major disturbances in the Earth's mag­netic field. They also produce auroras, known more commonly as Northern and Southern Lights because they are most common and most brilliant in the polar zones. Spectacular displays in England are infrequent, and are to be expected only when the Sun is near the peak of its cycle; from Scotland, aurora are much more usual, and a winter night in, say, Iceland or Northern Norway would seem strange without them. Occasionally, aurora reach as far south as Italy, and there is a story - almost certainly true - that the Roman Emperor Tiberius once dispatched rescue teams because he mistook a bright aurora for a disastrous fire. The Southern Lights are superb in Antarctica.

Aurora: occur in the upper atmosphere, and are caused when the atoms ejected from the Sun plunge into the higher layers. Analysis of auroral light shows that these atoms - or, more accurately, nuclei of hydrogen atoms - are moving down to the ground at velocities of as much as 5,000 kilometres per second. Aurora are mainly confined to the polar regions simply because the Earth has a magnetic field surrounding it in space, and this field channels the electrically charged atomic particles into the regions near the magnetic poles, so that the original paths of the solar-wind particles are markedly altered. The geographical North and South Poles have nothing directly to do with this; it is the magnetic poles which matter.

Some of the particles are trapped in the Earth's magnetic field, at distances up to about 100,000 kilometres from the ground. These particles are enormously accelerated, by processes not yet well understood, and 'bounce' between the Polar Regions in spiralling and drifting orbits. These regions of trapped particles were completely unknown until 1958, when they were discovered by one of the early American artificial satellites, Explorer I. They have been named the Van Allen Belts, in honour of the U.S. scientist James van Allen, who was mainly responsible for their detection. The lower layers may be the by-products of cosmic rays, but the upper layers are closely connected with the solar wind, varying according to the prevailing 'space weather'.

Some authorities believe that aurora: are caused by clouds in the solar wind which strike the outer part of the Earth's magnetic field and send down a form of shock-wave, so that some of the trapped particles are shaken out of their stable orbits and cascade into the atmosphere near the magnetic poles, rather in the man­ner of apples shaken out of a tree. On another interpretation, the particles making up the auroras are never truly trapped at all, but are deviated briefly in a complex fashion. The whole matter is now being intensively studied, but at any rate it is clear that aurora are not so simple and straightforward as used to be thought.

Little positive information about space weather could be ob­tained before the opening of the space age, in 1957, but progress since then has been amazingly rapid. The artificial satellites could provide considerable data, and many vehicles have been launched specially for the purpose. Recently, for instance, there has been another satellite of the so-called 'Imp' series; it has an eccentric orbit, so that it moves partly within the Earth's magne­tic field and partly in the relatively remote regions of undisturbed solar wind. Also important have been the Injun satellites con­structed at Iowa University, whose main function is to study the trapped and cascading particles in the Van Allen layers.

On a longer-term basis, the discovery of the Van Allen layers seems to have hammered yet another nail into the coffin of the elaborate, manned space-station proposed earlier by Wernher von Braun and other pioneers. There are sound reasons for sup­posing that a prolonged stay in the main layers would be inadvi­sable, and the whole idea of a manned base, orbiting the Earth at a height of about 1,000 miles, seems to have fallen into dis­favour, though it would be unwise to say that it has been aban­doned.

The interplanetary probes have added to our knowledge about space weather in general, and the solar wind in general. Much was gained from Mariner II, which passed within 21,000 miles of the planet Venus in December 1962. The Sun's own magnetic field was studied, and seems to vary according to the state of the solar cycle, so that there are significant differences between the findings of Mariner II and the present Mariner IV. Apparently, when the Sun is active its magnetic field is 'pushed outwards' by the solar wind. During the past few months, when Mariner IV has been on its way, the Sun has been at its least active, so that its magnetic field has reached closer in. Rather fortunately, there was a major flare on the Sun on 5 February 1965, and its effects were recorded by Mariner IV; radiation levels increased by 50 per cent, and two days later the probe detected irregular mag­netic fields in space.

One further piece of information is that Mars has no magnetic field comparable in strength with that of the Earth. This means that it can have no Van Allen-type layers either, and the whole situation there is different. It may even be that dangerous radia­tions can penetrate to the Martian surface, which will make matters much more difficult for future pioneers. However, an immense amount of work remains to be done, and every new development means that the study of space weather becomes more and more complex.

June 4 1965 - Spinning Worlds

It seems that every celestial object is rotating, even if the speeds of spin are not the same. The programme of June ig6j was prompted by the announcement that Mercury, the closest planet to the Sun, has not, after all, a 'day equal to its 'year', as had always been thought. This has been confirmed since; and neither is there any reason to question the very long rotation period of Venus, though it is possible that in neither case has the last word been said.

The planet Mercury is one of the less prominent members of the Sun's family. It is never a brilliant naked-eye object, since it can never be seen against a dark background; its distance from the Sun is only 36,000,000 miles, so that it may be seen only for a brief period after sunset or before sunrise low down over the horizon. This year the best opportunity will be during the firs: week in September, when Mercury will be a morning star.

Mercury's sidereal period (that is to say, the time taken to complete one revolution round the Sun) is eighty-eight Earth- days. It has long been supposed that this must also be the time taken for the planet to spin once on its axis, in which case 'the 'day' would be equal to the 'year'. The result would be that: Mercury would keep the same face to the Sun all the time, with part of the surface in permanent daylight and another pan plunged into never-ending night. Between these two extremes, there would be a so-called twilight zone over which the Sun would pass above and below the horizon.

Shown on the left, this is not our Moon, but a newly released mosaic (2011) of the planet Mercury as seen in images from Nasa's Messenger probe.

Behaviour of this sort is known as a 'captured1 rotation', and would mean that the ordinary day-and-night conditions as we know them on Earth would not occur. However, new method: of investigation have now been applied by scientists at Puerto Rico, using the radar equipment at Arecibo. Radar echoes were; obtained from Mercury, and have led to the surprising conclusion; that the rotation period is not, after all, the same as the planet's eighty-eight-day 'year'. The true period seems to be only 58 days.

Consequently, all our ideas about Mercury must be revised. There is no area of constant sunlight, no region of permanent darkness, and no twilight zone; the whole planet experiences a regular alternation of day and night. Yet Mercury is an unwel­coming sort of world, mainly because it has little or no atmosphere and the temperature range is very great. Unfortunately our maps of its surface features are rough, because the markings are diffi­cult to study even with large telescopes. Mercury is a mere 2,900 miles in diameter, and never comes much within 50,000,000 miles of us.

In size Mercury is comparable with our Moon (diameter 2,160 miles), while the second planet outward from the Sun, Venus, is almost the same size as the Earth. There have even been suggestions that Mercury is nothing more than an ex- satellite of Venus which broke free for some reason or other and moved off in an independent path, though such a theory will be extremely hard to prove.

The Moon provides an excellent example of a world with a captured rotation. It travels round the Earth once in just over twenty-seven days, and takes the same time to rotate on its axis, so that it keeps the same face to us all the time, and the surface markings appear fixed. Some of these markings may be seen with the naked eye, and it is possible to draw up rough charts without the use of any optical aid. In response to a request in the April Sky at Night programme, various naked-eye drawings of the Moon were sent in, two of which showed the pattern of dark, waterless seas, notably the well-marked Mare Crisium. In view of these and other similar drawings, it is rather curious that ancient astronomers of pre-telescopic days drew up lunar charts which were not even approximately accurate.

From Earth it is possible to study 59 per cent of the Moon's surface, because of effects known as liberations. Though the Moon rotates on its axis at a constant speed, its velocity in orbit is slightly variable, because its path round the Earth is an ellipse instead of a circle; it moves fastest when closest to us. Consequently, the amount of rotation becomes 'out of step' with the position of the Moon in its orbit, and we can see first some distance round the eastern side, then some distance round the western. The remaining 41 per cent of the surface is permanently averted, and remained unknown until the Russians obtained photographs of much of it with their rocket Lunik III in October 1959.

There is no mystery about this behaviour, and there can be no doubt that tidal friction over the ages is responsible for it. Some thousands of millions of years ago it may well be that the Moor rotated much faster than it does now, and we may assume that: it was then in a viscous or semi-solid condition. The Earth raise huge tides in it, and so slowed down the Moon's rate of spin until relative to Earth, the rotation had stopped altogether. Note, however, that the Moon does not keep the same face towards the Sun, so that day and night conditions on the averted hemisphere are the same as on the hemisphere facing us. A lunar day lasts for almost a month instead of only twenty-four hours.

With Venus, the situation is uncertain. There are no permanent surface markings to guide us, since Venus is concealed beneath its obscuring atmosphere, and before the era of space-probes i: was generally thought that the rotation period must be in the order of a month. Since the planet's 'year' is equal to aimer I 225 Earth-days, there would therefore be only seven or eight: I Venus days in every Venus year.

This idea was challenged in late 1963, when the American vehicle Mariner II passed within range of Venus and sent ban information to the effect that the rotation period was very lour indeed — perhaps even longer than 225 days. Radar work 21 Arecibo by R. B. Dyce and G. H. Pettengill, who have beer responsible also for the new studies of Mercury, gives the period of Venus as 247 days, so that we have a planet upon which tit day is longer than the year! This would mean that to an observer – on the surface, the Sun would rise in the west and set in the east; 10 days later. (In point of fact, seeing the Sun would be very difficult, owing to the cloudiness of the atmosphere. Mariner II gave the surface temperature as around 8oo°F., much too hot for any life to survive unprotected there.)

It would be idle to pretend that all astronomers are satisfied with this picture, but again we must await the results of further work, carried out probably by means of space-probes. When we come to Mars, the first planet beyond the Earth's orbit, we are on much firmer ground. There are visible surface markings, which are carried across the disk by virtue of the planet's rotation, and it is not hard to measure the rotation period, which proves to be twenty-four hours thirty-seven minutes. Mars, at least, has a day which is not very different from our own, and the seasons are also of the same type, since the axis is tilted at about twenty- five degrees as compared with the twenty-three and a half degrees of Earth. The main difference is that the seasons are much longer, since the Martian year is equal to 687 Earth-days.

Jupiter, first of the giant planets, is entirely different. It has a diameter of 88,700 miles as measured through the equator, but only 83,800 miles as measured through the poles, so that it is obviously flattened. This flattening may be seen with a small telescope when Jupiter is on view. Again there is no mystery; Jupiter is gaseous, at least in its outer layers, and the flattening is due to its rapid rotation. The Jovian day is less than ten hours long, so that particles at the planet's equator are being whirled round at some 28,000 miles an hour, and the effects of centri­fugal force cause the equatorial region to bulge out. There is a similar effect for the Earth, but with our world the difference between the polar and equatorial diameters is only about twenty- seven miles, as against 5,000 miles for Jupiter.

Different zones of Jupiter rotate at somewhat different speeds; the period for the equator is five minutes shorter than for the rest of the planet, while definite features, such as the famous Great Red Spot, have periods of their own, though all the values lie between nine hours fifty minutes and less than ten hours. It is also worth noting that the tilt of the axis is only three degrees from the vertical, so that there are no terrestrial-type seasons. In any case, seasons would be of minor importance on Jupiter, where the year is almost twelve times as long as ours, and where the temperature never rises above - 200°F.

Jupiter has four large satellites, again providing us with ex­amples of captured rotation, since all keep the same hemisphere turned towards their huge primary planet. This is also the case with the satellites of Saturn, and, indeed, with all major satellites in the Solar System. Saturn itself is of the same general type as Jupiter, with a gaseous surface and a relatively quick rotation of only ten and quarter hours for the equatorial zone; here, too the globe is markedly flattened.

Beyond Saturn lies Uranus, discovered by Herschel in 1781. At present it lies in the constellation Leo, and earlier in the year appeared close to Mars, but it is not easy to see without a tele­scope, and large instruments are needed to show any detail upon its pale, somewhat greenish disk. It is a giant world 29,300 miles in diameter, with a year eighty-four times as long as ours.

With regard to rotation, Uranus is a celestial oddity. The period is conventional enough for a giant planet (ten and three- quarter hours), but the axial tilt is not. The axis is inclined a: ninety-eight degrees, or more than a right angle, so that the rotation is technically retrograde, and the Uranian seasons are most peculiar. From Earth, we see sometimes the pole, sometimes the equator in the centre of the disk. The reason for this remark­able attitude is unknown, and Neptune, the last of the giants does not have a similar tilt; the angle is twenty-nine degrees, much the same as that of the Earth, while the Neptunian day amounts to about fourteen hours. Accurate data for Neptune are not easy to obtain, since there are no surface features defiant enough to be used as 'markers', and indirect methods have to be employed.

The boundary of the planetary system is the orbit of Pluto, a strange body discovered in 1930 and too faint to be seen in small telescopes. Even its diameter is uncertain, since it appears as little more than a star like point, though recent studies indicate that it may be as large or larger than the Earth instead of a smaller world such as Mercury or Mars.

At the Lowell Observatory in Arizona, where Pluto was first identified, M. F. Walker and R. Hardie have studied the slight variations in magnitude, and have found a period of six days nine hours. This, presumably, must be the rotation period of Pluto; the 'year' is known accurately, and is 2481 times as long as that of the Earth. The method of light-variations has also been applied to the minor planets, or asteroids, most of which keep to a well- defined zone between the orbits of Mars and Jupiter, and periods of a few hours seem to be the general rule. Vesta, the bright­est of the asteroids, has a rotation period of ten and a half, hours.

It is not only the planets and satellites which spin; in fact, rotation seems to be an invariable characteristic of all celestial bodies. The Sun's period may be measured easily enough by the positions of the sunspots, which are carried across the disk from day to day; here, the period amounts to about three and a half weeks, though different zones have different rates of movement. (Sunspots have been rare of late, since we have been passing through the minimum of the eleven-year solar cycle, but activity may now be expected to increase rapidly towards the next maxi­mum.) And spectroscopic work shows that the other stars also rotate, some of them with surprising speed. Even the Galaxy in which we live is in a state of rotation round its nucleus; the Sun takes about 225,000,000 million years to complete one journey, a period which has been graphically termed the 'cosmic year'.

The lesson to be learned from all this is that we must not attri­bute any real significance to a 365-day 'year' and a twenty-four hour 'day'. These periods apply only to our Earth, which is of no importance whatsoever in the universe as a whole. If we lived elsewhere, our time-standards would be quite different, since every celestial body has a rotation period peculiarly its own.

Note added in proof. In 1967 two more Venus probes were successfully sent to the planet, one of which (Venus IV. from Russia) landed there, while the other (Mariner V) by-passec Venus and repeated the Mariner II experiments with much greater precision. The results of these probes, together with radar measures from Earth, give a retrograde rotation period of 243 days and a surface temperature of at least 500 degrees F.; not perhaps so hot as Mariner II indicated, but still hot by Earth/- standards. The planet's atmosphere appears to be very dense indeed. Russian measures give the ground pressure as 15 to 2; times as great as that of the Earth's air at sea-level.

April 30 1965 - Astronomy without a Telescope

Many viewers of The Sky at Night had written in to complain that most of the objects discussed were beyond the range of all but the telescope- owner. This, of course, is a perfectly valid criticism; the astronomer would be sadly handicapped without his telescopes. On the other hand, there is a great deal to be seen with the naked eye - when one knows where to look: and, with Henry Brinton, I did my best to point some of these out.

Subsequently, I invited viewers to make drawings of the Full Moon as seen with the naked eye, and send them in. In the next programme we showed several and I was surprised how good they were. Some of them came from schoolboys, who clearly had no access to published lunar map:, and on the whole I am surprised that pre-telescopic era charts of the Moon were not better than they actually were.

An astronomer is always pictured as a man with a large telescope. This is perfectly logical; without telescopes, our knowledge of the universe would be comparatively slight, and modern research has to be undertaken with the aid of very powerful instruments, j Yet there is a great deal of interesting observation open to the enthusiast who has no optical equipment at all, and naked-eye studies are certainly not to be despised.

The first step to be taken by the beginner is to learn the various constellation patterns, which is not nearly as difficult as might be thought. One good scheme is to select a few easily-found group: and then use these as direction-finders to more obscure constella­tions as well as bright stars. The two best 'sky marks' are Orion: and the Great Bear, both of which are prominent in the evening sky during winter, though by spring Orion has more or less disappeared in the evening twilight.

Ursa Major, the Great Bear or Plough, is circumpolar in Britain; that is to say it never sets, and is always to be seen when-1 ever the sky is sufficiently dark and clear. During May evening it is fairly high up, and makes a splendid guide. For instance, the tail of the Bear (or, alternatively, the handle of the Plough) shows the way first to the brilliant orange star Arcturus, in Bootes or the Herdsman, and then to Spica in Virgo, which is less striking "than Arcturus but which is nevertheless very prominent in the south-east. Also to be found from the Bear is Polaris, the Pole Star, which lies within a degree of the north pole of the sky, and which seems to remain almost stationary with the other celestial bodies moving round it.

Polaris is not placed exactly at the pole, as a simple experiment will show. If you take an ordinary camera, point it towards the celestial pole and then give a time exposure, the result will be a series of star-trails. The bright, short trail near the centre is that of Polaris, and the position of the true pole is easy to estimate. Pictures of this sort may be taken with no trouble at all; the only point to remember in aiming the camera is that the altitude of Polaris above the horizon is approximately equal to the observer's latitude on the Earth.

The two 'Pointers' in the Great Bear are named Dubhe and Merak; Dubhe is slightly brighter than the Pole Star, Merak a little fainter. If the line from them is extended in the opposite direction, away from Polaris, it will come to the constellation of Leo (the Lion) marked by a curved line of stars shaped rather like a question-mark twisted the wrong way round. Regulus is the brightest of these stars, which make up the so-called 'Sickle'.

It is often said that a star twinkles, but a planet does not. This is not strictly true, since a planet low on the horizon may twinkle violently, but it is correct to say that a planet twinkles less than a star, because it appears as a tiny disk instead of a mere point of light. Twinkling is due entirely to the effects of the Earth's atmosphere, so that it is most marked with objects low down in the sky.

Superficially one star looks very much like another, but the careful observer will soon notice that the colours are not all the same. Of the three brightest stars in the northern hemisphere of the sky, Arcturus is orange, Capella yellow, and Vega bluish. All three are visible during spring evenings (though Vega is ad­mittedly rather near the horizon), and it is instructive to compare them. Binoculars, of course, will bring out the different colours very well, and will also show various hues among the less brilliant stars; thus Dubhe, the senior of the two Pointers, is decidedly yellower than its companion Merak.

There are also some stars which are double, since they are made up of two components very close together. The best example of a naked-eye double is Mizar, the second star in the tail of the Great Bear, which has a much fainter star, Alcor, close beside it. There is a minor mystery about Alcor, since the old Arab astro­nomers of a thousand years ago stated that it could be seen only under good conditions and by keen-sighted observers. Nowadays this is not the case, and anyone with normal vision can see it whenever the sky is reasonably dark and there is no mist or cloud about. Telescopically Mizar itself is found to consist of two com­ponents, so close together that to the naked eye they appear as one, while one of these components is itself known to be double.

Other naked-eye pairs may be found, notably Nu Draconis in the Dragon's head and Epsilon Lyrae in the small but interesting constellation of Lyra, the Harp. Epsilon Lyra lies close to Vega, and is of special interest because it is a multiple object; each component is again double, so that we have a fine example of a quadruple star. With the naked eye, the appearance is that of two rather faint stars, very close together. Another easy pair L: Theta Tauri, in the Hyades cluster close to the brilliant reddish Aldebaran.

The naked-eye observer can also interest himself in variable stars, which brighten and fade over relatively short periods of a few days or a few months. Betelgeux in Orion is one such case sometimes it is little brighter than Aldebaran, though it has ah: been known to rival the brilliant Rigel. Other variables art Scheat in the Square of Pegasus, Mira in Cetus (the Whale), an: two of the stars in the famous W of Cassiopeia. With all these stars the variations are intrinsic, but things are different with Algol, the 'Demon Star' in Perseus, which seems to shine steadily for two and a half days at a time and then exhibits a long, slow 'wink' lasting for several hours before regaining its normal brilliancy. Strictly speaking, Algol is not variable, but is made up of two stars, one much more luminous than the other, moving round their common centre of gravity; when the fainter star passes in front of the brighter, the total light shows a decrease. With Algol, the whole cycle of changes is easy to follow with the naked eye.

Of the star-clusters, much the most celebrated is that of the Pleiades or Seven Sisters, which is prominent throughout the winter but which sets soon after the Sun by spring.

Normal-sighted persons can see seven separate stars in the group, so that the familiar nick-name is very appropriate. On the other hand, Eduard Heis, a last-century German astronomer, is reputed to have been able to see nineteen Pleiades without optical aid!

The Hyades, round Aldebaran, are brighter than the Pleiades, but are more scattered, so that the effect is not so spectacular. (Incidentally, Aldebaran itself is not a true member of the cluster; it simply happens to lie in much the same line of sight, and is only half as far away from us.) Another naked-eye cluster is Praesepe in Cancer; moonlight will drown it, but on a dark night it is by no means hard to see. It is a fine sight in a telescope, and has earned its unofficial name of’ the Beehive'.

Occasionally a bright nova, or temporary star, will appear in the sky. To be precise, a nova is not a new star; what happens is that a formerly very faint star suffers an outburst which makes it flare into short-lived prominence, though after a brief period of glory it fades back to its former obscurity. The outburst affects only the star's outer layers, so that no lasting damage is done, whereas in the much rarer supernovse the star's material is blown away into space, leaving nothing more than an expanding cloud of gas. Four supernova;, the stars of 1006, 1054, 1572, and 1604, have appeared in our Galaxy since records began; the first of these has left its debris in the form of the Crab Nebula, which is of the greatest interest to both optical and radio astronomers, but which is below naked-eye visibility. On the other hand, bright novae have been seen often enough, and some of them have been discovered by amateurs. There is always a chance that the naked-eye enthusiast will be fortunate enough to detect a nova, though it must be admitted that the chances are heavily against it.

Only three of the outer galaxies are visible without optical aid and two of these, the Nubecula; or Clouds of Magellan, are too far south to be seen in Europe. The third is the Great Spiral in Andromeda, which may be glimpsed as a dim misty speck.

So far as the Solar System is concerned, the naked-eye observer has considerable scope. He can see some of the waterless 'seas' on the face of the Moon, he can make useful observations of meteors and auroras, and he can study the artificial satellites such as the American Echo balloons.

Before the war, our knowledge of meteor paths depended almost entirely upon observations made without a telescope. The procedure was to plot the apparent track of the meteor against the stars, and estimate its magnitude and duration; if the same meteor were seen by another observer some way off the true height and path could be worked out. This sort of work is still useful, though it is only fair to add that radar methods have now been brought into play and yield more accurate results. Sporadic meteors may appear from any direction at any moment, but most of the bright shooting-stars belong to definite showers, so that some periods of the year are more favourable than others. The most consistent shower, that of the Perseids, is visible in early August; anyone who stares up at a clear, dark sky for a few minutes between, say, 28 July and 15 August will be unlucky not to see at least one meteor.

Auroras, on the other hand, are phenomena of the high atmos­phere, though their cause lies in the Sun. During 1963 and 1964 there were few bright displays, because the Sun was going through the quietest phase of its eleven-year cycle, but activity has now begun to increase again, and there should be more frequent aurora: during the next few years. Since the wonderful lights are due to particles sent out from the Sun, and these particles are magnetic, displays of aurora are best seen in high latitudes; a winter night in, say, Iceland or north Norway would be dull without them. Observers in the northern and central parts of Scotland are often able to see brilliant aurorae, though in England the opportunities are much fewer.

By now there are a great many artificial satellites circling the Earth, and a few of them are really bright, appearing as slowly- moving stars creeping across the sky. Most prominent of all are Echo I and Echo II, both American-launched and both of the balloon type; since they are larger than most of the man-made moons, and were deliberately coated with reflective material, they can hardly be over-looked, and predictions for them are given in various periodicals and daily newspapers. Amateurs have done excellent work in checking on the positions of bright artificial satellites. The procedure is to time the moment when the satellite passes between two known stars or else comes to a position which may be plotted easily on the star-chart. (Gases in which a satellite passes right in front of a star are convenient, but surprisingly rare.) The only essential equipment for this sort of work is a reliable stop-watch, together with a really good knowledge of the constellations.

Though the professional astronomer would be more or less helpless without his telescope, any casual watcher can learn much even if he has no optical aid at all. There is endless variety in the night sky, and there are many fascinating objects to be seen by anyone who knows where and when to look for them. One does not need a powerful telescope in order to take a real interest in astronomy.

April 2 1965 - Astronomy and Astrology

Over the years, I have been referred to many times as 'an astrologer'. There are still some people who confuse astrology with astronomy, and after much deliberation we decided to give a programme to the subject, prompted by the fact that at that time three planets {Mars, Uranus and Pluto) were all in the constellation of Leo.

The results were, predictably, somewhat explosive. Letters from in­furiated astrologers poured in; they ranged from organizers of 'colleges' providing 'degrees' in astrology {at a suitable fee, of course) to one earnest viewer who wrote saying that as I appeared on the television screen, my aura was dull yellow and speckled. All I could do in the latter case was to write back assuring the correspondent that I would do my best to have my aura dry-cleaned before the next programme. There were also letters from flying saucer enthusiasts and followers of the Atlantis cult. I spent many hours battling with the deluge of mail, though with regard to the 'colleges' I admit that I merely put them in touch with each other and took no further action.

The most interesting point about it all was that not one of the astrologers offered any answer to the objections I had put forward. And there were, of course, many letters too from people who were glad that I had pointed out the difference between astronomical science and 'what the stars foretell'.

At the present moment Mars is still a prominent feature of the evening sky. Though it is receding from the Earth, it is still much brighter than any of the stars near it, and its strong red colour marks it out at once. During early April binoculars or low-power telescopes show another planet close beside it; this is Uranus, the third of the remote gas-giants, much larger than either Earth or Mars, but so distant that it is never easy to see with the naked eye. A moderate telescope is enough to show its rather dim, greenish disk, though larger instruments are needed to reveal any of its five satellites.

Uranus lies far beyond Mars, and is in fact very much farther from Mars-than we are. The two planets appear close together simply because they happen to lie in more or less the same line of sight. Both are in the constellation of Leo the Lion - and yet this, too, is merely a conventional means of expression, since even the nearest star is immensely more distant than a planet.

The diagram shows what is meant. The main stars of Leo are shown, together with their distances in light-years. (A light-year, or the distance travelled by light in a period of one year, is equal to rather less than six million million miles.) The curved line of stars which includes Regulus, Gamma, and Epsilon is generally nicknamed 'the Sickle'. Mars, at the moment, is only about 75,000,000 miles away - that is to say, less than seven light-minutes!

To say that Mars is 'in' Leo is therefore decidedly misleading. .After all, a sparrow flying at rooftop-height against a background of clouds is not 'in' the clouds.

There is the further point that the stars of Leo are themselves totally unconnected with each other. Epsilon Leonis is over 250 light-years away from Regulus, which is considerably more than the eighty-four light-years separating Regulus and the Sun. The pattern of the constellation is nothing more than a line of sight effect, and if the Solar System lay in a different direction the stars of Leo might well be spread out all over the sky. In fact, a 'con­stellation' is not truly a constellation at all.

Yet these constellation groups, and the apparent positions of the planets in the sky, form the whole basis of the pseudo-science of astrology, which was widely studied in mediaeval times and which is still taken quite seriously in a few countries, notably India. It was claimed that a person's whole character and destiny was influenced by the positions of the Sun, Moon, and planets against the stars at the moment of birth, and an astrological horoscope was regarded as a most important document. Famous scientists of past times were believers in astrology; even Johannes Kepler, who laid down the famous Laws of Planetary Motion, cast horoscopes (though whether he believed in them is another matter), while the great Sir Isaac Newton was most decidedly a mystic. Still earlier, astrology was thought to be just as important as true astronomy.

Originally the Earth was thought to be flat, and to lie at the centre of the universe. The old Greek philosophers realized that the Earth is a globe, but only a few of them were bold enough to suggest that our world might move round the Sun; indeed, it was not until the sixteenth and seventeenth centuries that the idea of a Sun-centred system became firmly established. Astrology, then, is related entirely to the Earth as a centre.

The mediaeval astrologer was a most influential person. He cast horoscopes for kings and princes, he made weighty pro­nouncements, and sometimes he even predicted the approaching end of the world. Comets were regarded as particularly unlucky, but, according to the astrologers, conjunctions of several planets were even worse.

It is natural that the planets - or some of them - should at times appear close together in the sky; as we have seen, Mars and Uranus are at present only a few degrees apart. But when several bright planets met in the same constellation, in 1524, a famous German astrologer named Stoeffler took the opportunity to fore­cast the end of the world, and caused widespread panic; people even went so far as to build boats and arks so as to escape the expected flood. Much more recently, in 1962, five planets were together in the constellation of Capricorns (the Sea-Goat), and once again the astrologers were much alarmed. In India, parti­cularly, there was great relief when the planets spread out among the constellations once more and the world still survived.

The Sun, Moon, and planets are confined to a certain region of the sky, known as the Zodiac. This is because the orbits of the planets, including the Earth, lie in much the same plane; the inclination is seven degrees for Mercury and less for the remaining planets. There is, however, one exception: Pluto, which is a relatively faint telescopic object, and was not discovered until 1930. (At present it, like Mars and Uranus, will be found in Leo.) The inclination of Pluto's orbit amounts to seventeen degrees, so that it can leave the Zodiac. This is unlikely to worry the astrolo­gers, and in any case the 'signs' of the Zodiac no longer correspond to the actual constellations, since the effects of precession - that is to say, the slight wobbling of the direction of the Earth's axis - have become quite appreciable since classical times. The vernal equinox, or point where the ecliptic cuts the celestial equator, is still known as the First Point of Aries, but by now it has moved out of Aries (the Ram) into the neighbouring constellation of Pisces (the Fishes).

Two of the other planets, Uranus and Neptune, were also unknown to the old astrologers; Uranus was discovered in 1781. Neptune in 1846. If these planets do in fact exert an influence upon human destinies, it would be interesting to learn why the astrologers did not track them down long before the astronomers could do so with their telescopes! There are also the numerous minor planets, or asteroids, which move round the Sun between the orbits of Mars and Jupiter. It is true that they are small in size, but quite a number of them are brighter in our skies than remote Pluto. One of the asteroids, Vesta, is even visible with the naked eye when best placed, whereas Neptune and Pluto, among die 'proper' planets, are always far below naked-eye visibility.

One of the biggest absurdities of astrology lies in the names of die Zodiacal constellations themselves. The familiar groups, such as Leo, Taurus (the Bull) and Gemini (the Twins) are of ancient origin, though it is worth noting that the Chinese and the Egyp­tians used a completely different system. Nobody is quite sure in which country our own constellations were first described. The old Ghaldcean star-gazers may have been responsible; astronomers in the island of Crete have also been suggested. In any case, Ptolemy, last of the great scientists of ancient times, listed forty- eight constellations in his catalogue of the stars. Ptolemy died about a.d. 180; even then, the patterns were very old indeed.

Yet few of the constellations bear the slightest resemblance in outline to the objects after which they are named. It requires considerable imagination to make a bull out of Taurus, a lion out of Leo, or a crab out of Cancer. Moreover, many of the names are mythological; Leo commemorates the Nemaean lion killed by the hero Hercules during his twelve labours. (Hercules is also in the sky, but he is not in the Zodiac, and is much less brilliant than his leonine victim.)

What was evidently done was to draw up arbitrary figures bearing little or no relation to the star-patterns concerned, and then allot names. When this had been done, the astrologers assigned 'characteristics' to the constellations according to the names that had been given. Cancer, the Crab, is said to be a watery sign. Leo, of course, is virile and positive; it is said that the Sun is at its greatest astrological strength when in Leo.

Altogether, the whole procedure seems to be an excellent case of reasoning round in a circle, and it is hard to understand how any thinking person can take it seriously. It is hardly rational to take a collection of totally unrelated stars, make some sort of a figure out of the pattern, give it a name and then claim real significance for it. One can only echo the words of the Duke of Wellington when greeted in the street by a stranger with 'Mr Smith, I believe?' 'Sir - if you believe that, you will believe any­thing.'

Some astrological predictions come true. This is only to be expected; it would be most surprising if they did not, since they cover all sorts of subjects and are usually wrapped up in suitably nebulous language. Now and again some astrologer will achieve a lucky hit, which will be well publicized. The same is true of personal horoscopes, though for every correct statement there are always several which are very wide of the mark.

One typical case may be cited. Not long ago, an astrological magazine forecast the sudden death of President Kennedy, and gave the correct month of the assassination. This prediction was regarded as a convincing justification of astrology - but it must also be related that during the previous three years the same magazine had foretold the death of President de Gaulle, the deposition of General Franco, and the removal of Dr Salazar of Portugal. It is worth noting, too, that in 1938 and 1939 British astrologers were as emphatic as they were unanimous: there would be no war against Nazi Germany.

Only the credulous will believe that line-of-sight effects 0: planets and stars will have any effect upon a man's character or life, but it is nevertheless illuminating to ask a serious astrologer just how these alleged influences occur. I did ask precisely this question of an astrologer a few months ago. His reply was: hasn’t the slightest idea.' This was, at least, a straightforward admission, and differed from the usual attitude. Most astrologer; faced with such a query, would have started talking about mysticism, ancient teachings, and, of course, vibrations. The latter word is a favourite of all devotees of what may be termed the 'fringe' of science; it is also very convenient, because in such a context it may be taken to mean practically anything.

Arguments which have no basis of common sense are always hard to refute. It is so with astrology, which lacks any scientific or logical foundation, and which is a relic of the past, when super­stition was rife and concrete knowledge was very limited. In ancient times, when the nature of the universe was not under­stood, it was natural enough to regard the Earth as of supreme importance, with the remaining bodies set in the sky merely for the sake of Earthmen; in such a climate, astrology could be ex­pected to flourish. By now it has, of course, been completely dis­credited in Europe, though in the East it lingers on.

It is, after all, virtually harmless, and many people are amused to read the 'What the stars foretell' columns in the popular press. It must also be emphasized that professional and amateur astrologers are, in general, completely honest and sincere. They take themselves most seriously; they give each other 'degrees', they put impressive-looking letters after their names, and they offer instruction to the unenlighted, all with the best of intentions. The same can be said of other equally sincere bodies, such as the International Flat Earth Society, which still exists, and the Ger­man Society for Geophysical Research, whose members be­lieve the world to be the inside of a hollow globe, with the Sun in the centre of the hollow and Australia situated somewhere above our heads.

There seems no need to say more. Astrology is not a science, and no person with any scientific background will take it seriously, but its name still leads to a certain amount of confusion. Suffice it to say that astrology and true astronomy are entirely different, and entirely unassociated. Yet the sincere astrologer does little damage, and, like the flat earther and the hollow-globe believer, he means well.

February 12 February 1965 - Balloon Astronomy

In this era of rocket probes and artificial satellites, it may seem a retrograde step to turn back to the humble balloon. Yet as an astronomical vehicle, the balloon has much to recommend it, and it seemed well worth while to devote a programme to balloon astronomy in general.

To an astronomer, the Earth's atmosphere is a source of conti­nual annoyance. It is dirty and unsteady, so that images of celes­tial bodies are always to some extent blurred; to take really long- exposure photographs, of say, the surface of Mars is pointless, since the fine detail will not be registered. From this point of view the Solar System observer is in worse case than the astro­physicist, who is concerned with what are to all intents and pur­poses point sources of light; no star, apart from the Sun, shows a measurable disk even in the largest telescope yet built. However, there are more serious troubles to be faced. The atmosphere is not transparent to radiations of all wavelengths; in a large part of the electromagnetic spectrum it is depressingly opaque, so that astronomical information obtainable at ground-based observa­tories is very incomplete. From this point of view, the practice of setting up large telescopes on high mountains is of little help, though it does improve the clarity of the optical images.

The atmosphere of our world is quite extensive, and traces of it linger on up to at least 2,000 miles above sea-level, but its density falls off rapidly with height. More than 90 per cent of the total air-mass lies below thirteen miles, and more than 99 per cent of the mass below twenty miles; the upper reaches are very tenuous indeed. Unfortunately, the chief blocking-out of radia­tions is due to layers in the ionosphere, that part of the atmosphere which begins at an altitude of forty miles or so. The so-called D. E, Fi and F2 layers range up to more than 100 miles; these are regions where free electrons are found in great numbers, making ±.e levels electrically conducting (and, incidentally, making I ring-range radio communication possible, by bouncing wireless raves of certain frequencies back to earth). The layers are pro­duced by X-rays and ultra-violet rays from the Sun, which act : n the thin air; the X- and many ultra-violet rays are themselves absorbed in the process, so that they do not penetrate to the ground.

Lower down comes the ozone layer, which absorbs most of the remaining solar ultra-violet. If, therefore, we are to study these important radiations, we must send our recording instruments up to great altitudes.

Of course, rockets and artificial satellites can go far above the ionosphere, and even escape from the Earth. Yet they are com­plex and expensive, and in their present stage of development they cannot lift massive telescopes, keep them steady while the observations are being carried out, and then return them safely. Balloons are much easier to handle, and are also vastly cheaper. Their main limitation is that they are incapable of rising to the ionosphere. A height of between 80,000 and 90,000 feet is as much as can reasonably be expected, and so balloon-borne instruments can contribute little to either ultra-violet astronomy :r X-ray astronomy. All the same, the balloon has much to be said in its favour, since it can at least carry heavy equipment above most of the atmospheric mass - thus eliminating blurring and unsteadiness of the images. Moreover, water-vapour and carbon dioxide in the lower air absorb most of the infra-red radiations sent to us from the planets. Balloon ascents overcome mis hazard with ease.

Hot-air balloons date back to the year 1783, and within a few months of the first flight a French scientist, Charles, went up two miles in a free balloon. Yet there is little resemblance between less crude vehicles and a modern scientific balloon, which has : y now become an important research tool.

The main development has been carried out by M. Schwarzschild and his team at Princeton University in the United States, in collaboration with the United States Navy, the National Science Foundation, and the National Aeronautics and Space Administration. The 'Statoscope' flights of 1959, concerned mainly with studies of the Sun, were remarkably successful, and the project has now been extended. With Statoscope II, the over­all height from the telescope to the top of the launch balloon is 660 feet; the balloons together weigh over two tons, and another two tons of ballast are carried for later release if height has to be maintained during the night. The telescope, plus its controls, weighs three and a half tons. Two large parachutes are also carried; in case of emergency, the instruments and their records can be separated from the main balloon system, and brought down gently. Many of the radio and electronic devices used are similar to those of artificial satellites.

The launch of such an enormous, fragile system must be under­taken on a calm day, with a wind speed of less than fifteen miles an hour or so, and takes some time. The inflated launch balloon lifts the long, folded main balloon, the twin parachutes, and the telescopes with its associated instruments and controls. Two large lorries are used to cancel out any ground-wind; finally, when the long train of items is extended vertically, the tethering stay is dropped and the whole assembly rises into the upper air at a rate of 800 feet per minute.

A model of Stratosphere 2 shown is on the left, a view of the actual flight is shown on the right

During the ascent, helium gas from the launch balloon vents into the main balloon, so that at the chosen 'cruising' altitude (about fifteen miles) the main balloon will be fully inflated. At this point, radio commands direct the telescope towards the objects under study. The telescope locks on to each object in turn, while the observations are made. The procedure is far from simple, but is much more straightforward than with an earth satellite or space-probe; the balloon is what may be termed a 'local' vehicle, and is much more under control. The vital ad­vantage is that the actual instruments and records are recovered, to be examined at leisure and in comfort.

The new telescope for Statoscope II would be regarded as very fine even by ground-observatory standards. It is a reflector, with a 36-inch mirror made of fused quartz. The entire telescope unit will be able to point to selected objects in the sky with re­markable precision, and will remain pointing steadily to an accuracy of 1 /50 of arc second of arc. At peak altitude, where the atmosphere is so thin, the telescope will be capable of detecting fine details on the Sun, Moon, and planets only 1/10 of a second of arc across - which is comparable to using a telescope based in London to see a man who is as far away as New York.

Important discoveries have already been made by balloon- telescope techniques. For instance, detailed photographs of the Sun's surface have been taken, showing features beyond the range of ground telescopes. It has long been known that the solar surface is 'granulated', but the individual granules are compara­tively small, and are in rapid motion, so that they are not easy to study. Balloon photographs show them with amazing clarity, and show that the granules are not of uniform size. The structure of sunspots has also been shown in more detail than ever before.

Stellar astronomy has also benefited, but in view of the present emphasis upon rocket probes it may be worth saying something about the balloon results with regard to Mars and Venus.

Mars, where there is an atmosphere - even though a thin one - has been specially studied, particularly with regard to atmos­pheric water-vapour. There is so little moisture that ground-based instruments were unable to detect it at all; its traces were masked by the water-vapour in our own air. Balloon telescopes have been able to observe from heights where terrestrial water-vapour can b e neglected, and for the first time definite traces of moisture have been found in the Martian atmosphere.

The first serious balloon studies of the second of our 'neighbour' planets, Venus, were made in November 1959, by Ross and Moore in the United States. Until then it had been tacitly assumed that the atmosphere of Venus contained no appreciable water- vapour, and the clouds, whatever they might be, were not made t: H20. The balloon results showed otherwise. Instead of being bone-dry, the atmosphere contained more water-vapour than that of the Earth at an equivalent level. These early results have been substantially confirmed by more recent balloon flights, and it also appears that the clouds are composed of ice crystals. (This does not mean that the planet's surface is likely to be refreshingly cool; Venus is almost certainly very hot indeed.)

So far as balloon astronomy as a whole is concerned, we must admit that it will always be limited in scope; a balloon, by its very nature, has a definite 'ceiling'. For studies which involve going up above the shielding layers in the ionosphere the rocket must remain our only answer. Yet there can be little doubt that balloon techniques have proved their worth, and will be widely used in the future. By space-research standards they are very cheap; the main cost lies in the telescopes and other instruments which are to be lifted - but it is not essential to use giant telescopes. Reflectors of, say, forty to fifty inches aperture probably represent the practicable maximum.

Manned ascents for scientific research have often been made, but generally speaking there is no need to risk human life. The records are obtained photographically, and a visual observer on the spot could not add a great deal, interesting though the ex­perience would be. It is the unmanned, instrument-carrying balloon, operating from altitudes of fifteen miles or so, which will prove of most value in the long run.

Almost all modern work in balloon astronomy has been carried out by the Americans, but since the results are so encouraging it is presumably only a matter of time before the Russians join in ; we may hope, too, for British participation. It is too early to say just what the next developments will be, but we may be sure that the balloon, as an astronomical tool, has come to stay.

December 11 1964 - How old is the Earth?

The question of the Earth's age has long fascinated both scientists and non-scientists. For the Christmas programme of 1964, in which I was again joined by Henry Brinton, this problem was discussed, but I have had to make revisions to the article as then printed, because new information is now at hand. We gave the value for the Earth's age as 4,500,000,000 years, which was then the official figure. Subsequently, studies have been made of the Rocks of St Paul, which lie between Africa and South America. They contain the element rubidium, which decays spontaneously, by radio­active processes, into strontium; the half-life is 46,000 million years, so that the decay is hardly rapid. From these researches, it is evident that the Rocks of St Paul are about 4,500,000,000 years old, so that the Earth itself must be still older; in the article below, I have revised the figures and given the currently accepted age. It may still be wrong to some extent, but, as I said originally, it is certainly of the right order.

The last months of 1964 were interesting to the astronomer and the space-research worker; in particular there were three attempts to send probes toward the planet Mars.*

One of the objects of the space experiments is to find out more about the Martian atmosphere, which is much thinner than that of the Earth. Mars is also short of water, and it has often been described as a planet older than the Earth. This is not strictly accurate. It may well be in a more advanced state of evolution, mainly because of its smaller size, but there seems little doubt that its absolute age is very similar to that of our own world. Most authorities agree that the planets in the Solar System were formed in the same manner, and at the same epoch.

We are by no means certain how the planets came into being.

The tidal theory made popular by Sir James Jeans, according to which the planets were pulled off the Sun by the action of a passing star, was widely favoured for some time, but has now been rejected because the mathematical objections to it are fatal. More recently, C. von Weizsacker has suggested that the Sun once passed through a cloud of gas and dust in space, collecting the material out of which the planets were gradually built up. This idea seems promising, but the whole question is still wide open. So far as the age of the Earth is concerned we are on rather firmer ground, and the figure now generally accepted is 4,750,000,000 years.

Authorities of a few hundred years ago would have found this impossible to believe. The account of the creation given in the Bible was taken literally, and it was thought that the world could not possibly go back more than a few thousand years. By adding up the ages of the patriarchs, and making similar calcu­lations which seem naive today, the seventeenth-century Arch­bishop Ussher of Armagh claimed that the Earth came into existence in the year 4004 B.C.; the final figure given was nine o'clock in the morning of 23 October. Many churchmen were satisfied with this, and the matter was generally regarded as settled.

The first serious doubts arose with the development of geolo­gical science. Toward the end of the eighteenth century James Hutton, the first great British geologist, went so far as to declare that 'in the economy of the world I can find no traces of a begin­ning, no prospect of an end'. Hutton belonged to the uniformtarian school of thought - that is to say, he explained the charac­teristics of the Earth's crust in terms of processes that were going on unceasingly, albeit very slowly. It did not take many more years for geologists to gain a fair idea of the vast periods of time required for the development of geological strata, and Ussher's date began to look painfully inadequate.

Orthodox churchmen tried to escape from this dilemma by supposing that although the Earth itself might be very old, the simultaneous creation of species would allow life to be of recent origin. However, difficulties soon arose here, too. Palaeontologists, headed by the French scientist Cuvier, showed that fossils - the remains or traces of long-dead creatures - were to be found in rocks of immense age; since the fossils could not be younger than the rocks which contained them, living things must also be dated back a long way.

A last-ditch stand was made on the assumption that Man might be a recent creation. Of course, it is true that the history of man and his ancestors is brief compared with the age of the Earth, but even so it is to be measured in hundreds of thousands of years, just as the history of mammals has to be measured in tens of millions of years. The decisive evidence was provided by a Frenchman who rejoiced in the name of Jacques Boucher de Crevecoeur de Perthes, who undertook some researches in grave pits and quarries near his home town of Abbeville and discovered flints which had unquestionably been fashioned into shape by human hands. De Perthes continued his work, and by 1838 he had become convinced that men had lived in France at a period much earlier than had been generally thought. In fact, the artefacts dated back far before the period assigned to the Biblical Flood.

It took De Perthes many years to persuade others even to look at his artefacts, but eventually he won his point, and in 1859 he had the satisfaction of having his theories vindicated and acknow­ledged before the British Association and the Royal Society.* The work of Darwin completed the change-over in thought, and the way was at last clear for a proper scientific inquiry into the Earth's age. Geology showed that the time-scale must be very long. Quite apart from strata and fossils, there were several means of proving the Earth's antiquity. For instance, a fair estimate can be made of the rate at which sodium finds its way from the land into the sea; the oceans must originally have been of fresh water, and al­though there are many complicating factors it was clear that to make the sea as salty as it is now would take millions of years. Calculations were also made of the rate at which large rivers, such as the Mississippi, were washing sediment into the ocean, which provided an estimate of the age of sedimentary rocks.

At this stage there was a conflict between the geological and the physical evidence, and Lord Kelvin tackled the problem from three directions. First, there was the question of the rate at which the Earth had cooled down (assuming, as most people then did. that it had originally formed part of the Sun). Given that it started at a certain temperature and cooled at a calculated rate, it was possible to work out the time needed for the Earth to reach its present condition.

Kelvin's second yardstick was the rate of the Earth's rotation. It was known that the length of the 'day' is slowly increasing, because of tidal forces, and for various reasons Kelvin calculated that going back 1,000,000,000 years, the Earth must have been spinning 15 per cent quicker than it is at the moment. If the crust were formed about this time, he reasoned, the Earth's globe would be much more flattened than it actually is, whereas if the Earth solidified only 100,000,000 years ago then the shape of the Earth would be approximately as measured now, with a difference between the polar and equatorial diameters of less than thirty miles. Finally, Kelvin believed that the Sun could not have existed in its present form for as much as 500,000,000 years. Originally he had attributed the Sun's radiation to the kinetic energy of meteors falling on to its surface, but when this theory had been disproved he adopted Helmholtz's suggestion that the source of solar energy lay in gravitational contraction. Kelvin concluded that the Earth must be somewhere between 20,000,000 and 100,000,000 years old. He favoured the lower limit, even though it was far too short to satisfy the geologists.

The argument between physicists and geologists was not settled until more became known about the nature of matter. With the discovery of radioactivity, for instance, the 'cooling Earth' idea ceased to be valid, and the much later investigations into the source of stellar energy showed that the Sun is much older than 500,000,000 years. The Sun radiates not because of meteoric in fall or gravitational contraction, but because of nuclear processes taking place deep inside it; hydrogen is being converted into helium, admittedly by a somewhat roundabout method, so that mass is being lost and energy produced. The mass-loss is 4,000,000 tons per second, but this does not amount to much relative to the total mass of the Sun's vast globe.

It was radioactivity, too, which gave a really reliable means of working out the age of the oldest rocks. Many rocks contain uranium, the heaviest element known to occur naturally, and it is known that uranium decays spontaneously, ending up as an isotope of lead. The rate of decay, so far as is known, is quite unaffected by environment, and is assumed to be constant. The 'half-life' of uranium-238 (that is to say, the time taken for half the original quantity to decay) is more than 4,000,000,000 years. If, therefore, uranium and uranium-lead are found together, the ratio of the amounts is a pointer to the time during which the decaying has been in progress.

On this basis, the oldest rocks seem to date back around 4,000,000,000 years. This agrees well with the geological evidence, since a figure of 1,500,000,000 years has been suggested for some sedimentary rocks while life is thought to have appeared upward of 3,000,000,000 years ago. Taking all the facts into account, it seems that the Earth began its separate existence about 4.750,000,000 years ago. This estimate is not precise, but there can be little doubt that it is of the right order.

Nobody can possibly visualize what this figure really means, but some idea may be gained by means of a scale in which the age of the Earth is taken as one day (twenty-four hours). A table may then be drawn up, as follows:

So if we take the Earth's age as being scaled down to one day, men have existed only during the last couple of seconds. In a way, the old churchmen were right in supposing mankind to be a very recent addition to the scene, but we have certainly learned a great deal since Archbishop Ussher fixed the date of the Creation so exactly as October, 4004 b.c.

Old though it is, the Earth is still only about half-way through its expectation of life. Its eventual fate will, of course, be linked with that of the Sun. Opinions have changed here, too. It used to be thought that the Sun was gradually cooling down, so that it would eventually turn into a dim red dwarf star - in which case all living things on Earth would be frozen to death. This whole theory is wrong. As the Sun ages, and uses up its hydrogen 'fuel', it will become more luminous, and in from 8,000,000,000 to 10,000,000,000 years' time it may well become a red giant, with a cooler surface but a much greater size and luminosity than the Sun we know. If so, the atmosphere of the Earth will escape, the oceans will boil, and all life here will perish - it is even pos­sible that our world will be destroyed.

The crisis lies so far ahead that speculation is rather pointless. Modern theories about stellar energy are subject to constant revision, and we cannot claim to know more than a very small part of the truth. All we can say is that if we are on the right track, then the eventual fate of all Earth creatures is likely to be a heat death rather than a cold death. If intelligent beings still survive here at that remote epoch, the only solution will be to migrate elsewhere. However this may be, we may be sure that there is no fear of any-imminent disaster. The Earth has existed for more than 4,000,000,000 years, and it will remain habitable for at least an equal period of the future. It will not last for ever, but nothing in the universe is eternal - perhaps not even the universe itself.

October 30 1964 - The Earth seen from space

In October 1964 the Soviet vehicle Voskhod went into orbit round the Earth. Its captain was Colonel Vladimir Komarov, who later lost his life when his space-craft came down out of control, so that there is bound to be a certain sadness when reading of his earlier, successful flight.

It was in late 1964 that the space programmes seemed to be gathering momentum, and there was speculation as to what the Earth would look like if it could be seen from a great distance. I discussed this with K. H. Fea, of the Space Research Group at University College, London, and we showed some of the current photographs. Since then, of course, the Earth has been photographed by the U.S. Orbiter vehicle, which was then in the region of the Moon ; and the results are very much as had been anticipated.

The first manned space-flight took place on 12 April 1961, when Yuri Gagarin made a single circuit of the Earth in the original ' Vostok' craft, landing after a flight lasting for less than two hours. Since then other men and one woman have been into space.

The flight of the Soviet vehicle 'Voskhod' (Sunrise) was made on 12-13 October 1964. In many ways it represented a distinct ad­vance, even though it was of comparatively short duration. The ship carried a crew of three men, of whom only one (Vladimir Komarov) was a true 'astronaut'; of the others, Konstantin Feoktistov was a scientist, while Boris Yegorov was a doctor. In view of the potential hazard due to radiations in space, there was every reason to send up a medical man, and Yegorov's findings will be of particular interest and importance when they are made known in full.*

The 'Voskhod' made sixteen orbits and landed safely, ap­parently in the position intended. It made a 'soft' descent; on this occasion the crew members were not separated in ejector- seats, so that the procedure seems to have been similar to that which will have to be adopted for a landing on the Moon. It has been stated that the ship could well be used for a further trip, and that it is totally undamaged. There have been suggestions that the flight was terminated prematurely, and that the astronauts received a heavier "dose of radiation than had been expected; however, the Russians say that this is not the case, and that the 'Voskhod' was not intended to stay aloft for more than the actual period of twenty-four hours seventeen minutes.

It is worth noting, too, that on this occasion the astronauts wore normal clothes instead of space-suits. Food included roast beef, which was sliced and vacuum-packed in polymer film; slices of filleted Caspian roach, and even caviar sandwiches. Communications with the space-craft were excellent throughout, and the Russians claim, no doubt justifiably, that the whole flight was an unqualified success.

The altitude of the 'Voskhod' ranged between 110 and 255 miles, which meant that the crew members could see over large portions of the Earth. During the first orbit they have related how they saw the forests of Siberia; when passing over darkened Australia they looked at the lights of the great cities, while Antarctica was distinguished by its white ice-cap. Of particular interest was their description of what may be called 'cosmic dawn'. Since the revolution period of the space-ship was only just over ninety minutes, a total of fifteen 'dawns' was observed during the trip, and Yegorov's own description is worth repeating:

The atmosphere brightens. It grows yellow, then darkens again, after which appears the blue halo. Then an orange strip appears, which becomes brighter and brighter. At last the edge of the Sun comes into sight, and suddenly it is clear, like a brilliant red oval. Its brightness increases quickly, since sunrise in outer space lasts for only a few seconds . . . The Moon looks the same as it does from the Earth, the only difference being that the sky around is absolutely black.

A display of aurora australis was also observed.

All this is in agreement with the descriptions given by earlier Soviet and United States astronauts. Even before Gagarin's pioneer flight, photographs of the Earth had been obtained from unmanned rockets, and were clearly of great importance to geophysicists, geologists, and meteorologists, as well as to those engaged in pure space research. No photograph of the Earth as a complete globe has yet been obtained, since this would mean travelling out a distance much greater than a couple of hundred miles; but at least the curvature of the Earth's surface has been clearly shown on many pictures, together with the completely dark sky.

Cloud coverage is, of course, variable - but over wide areas it often obscures the surface detail completely. The absence of cloud over the Tibet region during the flight of Gordon Cooper, on 15-17 May 1963, led to some really spectacular pictures, taken with a hand-held camera through the transparent window of the space-craft, the fourth manned Mercury capsule. When the photographs were taken Cooper was moving at an altitude of over 100 miles above the Tibetan plateau.

Lakes show up with particular clarity, as dark patches - apart from the shallower ones, which were partly covered with ice. Some of the hills were also capped with ice and snow, and identi­fication presents no difficulty. Other photographs taken by Cooper showed the glaciers and snow-masses of the Himalayas, and have been of considerable value to map-makers and geologists. Ice floes drifting near the coast of north Russia have been recorded on photographs taken from the various Soviet space-craft, pre­sumably also with hand-held cameras; and it has been suggested that useful duties could be performed in ice patrolling. In any case, there can be no doubt about the value of the pictures sent back by the unmanned meteorological satellites of the American Tiros series, most of which are taken from 500 miles or so. On one of them, obtained from 400 miles, Great Britain is well shown; England and Wales are covered by 'fair-weather' cumu­lus clouds, and appear bright, since clouds are more reflective than clear ground. On this particular photograph Ireland seems to be covered with high cirrus cloud; a low-pressure system is approaching from the Atlantic, indicating the probability of bad weather in the near future.

Hurricanes and cyclones have also been photographed from the Tiros satellites, and the circulations of these systems - clock­wise in the southern hemisphere, anti-clockwise in the northern - have been evident. Violent storms of this sort have often been photographed in their early stages, so that people living in the danger zones have been given ample warning. There seems little doubt that a full-scale 'storm patrol' by means of satellites will be in operation within the next decade.

However, all the Earth photographs so far obtained have been taken from comparatively close range, and it is interesting to speculate about what our world would look like when seen from much greater distances. Seen from the Moon, the Earth would be magnificent indeed. Its light would be much more brilliant than full moonlight at home, partly because the Earth would appear considerably larger in the lunar sky and partly because it is a better reflector of sunlight. Its albedo, or reflecting power, has been estimated as almost 40 per cent, which compares very favourably with the 7 per cent of the Moon. Indeed, the Moon has an albedo lower than that of any other sizable body in the Solar System, with the possible exception of the remote planet Pluto.

Of course the Earth would show phases to a lunar observer. It would be full at midnight, and for protracted periods it would be a glorious object; the Moon rotates much more slowly than the Earth, so that a lunar night (that is to say, the time during which the Sun would be below the horizon) amounts to almost two of our weeks. At lunar noon the Earth would be new, and its dark side would be turned moonward. In point of fact, the Moon's rotation period is equal to its period of revolution around the Earth. This means that the Moon keeps virtually the same face turned towards the Earth, and the averted side remained unknown until a large part of it was photographed by the Russian rocket 'Lunik III' in October 1959. It also means that the Earth would seem to stay almost still in the lunar sky; it would not rise or set, and would always appear in virtually the same position, though it would admittedly oscillate slightly. From the averted hemisphere of the Moon, the Earth would never be seen at all, and the nights would be consequently much darker.

The Moon is only about a quarter of a million miles away, and the Earth would therefore appear as a relatively large object in the sky, but things would be very different for Mars, which never comes much within 35,000,000 miles of us. To a Martian observer, the Earth would show up as nothing more than a very brilliant 'star', probably bluish in hue. It would appear as an inferior planet, best seen in the west near sunset or in the east near sun­rise. This is how Venus actually appears to us, but there would be one important difference. Venus has no satellite; from Mars, our Moon would be a conspicuous object when seen with the naked eye. Telescopically, our hypothetical Martian would be able to see considerable detail on the Earth. The outlines of con­tinents and oceans would be obvious, and the variable cloud cover would be followed. Maps of Earth could be drawn up, though small details would naturally be lost.

Venus may approach us to within 25,000,000 miles, so that it comes considerably closer than Mars, but so far as we can tell the planet's cloudy atmosphere would preclude any visual obser­vations; even the Sun would not be seen, though its position would be indicated by the brightening of the sky. But for this, the Earth seen from Venus would be extremely brilliant, particularly as it would be a superior planet and would be visible against a really dark background.

From greater distances the Earth would fade rapidly into inconspicuousness. From Jupiter it would be hard to see at all, partly because of its smallness but mainly because it would keep very close to the Sun in the sky. And from Pluto, the outermost member of the Sun's family of planets, the Earth would be hope-lessly lost. Pluto moves at a mean distance of over 3,600,000,000 miles from the Sun, and would be a very poor site for carrying out observations of the other planets; only Neptune would be at all conspicuous. A hypothetical Plutonian astronomer would never be able to detect the Earth, and could have no knowledge of its existence. If this is so, then it would clearly be impossible to observe the Earth from the distance of even the nearest star, which lies at roughly 25 million million miles - over four light- years. No telescope of the sort that we ourselves could construct would be powerful enough, as a moment's consideration will show.

The Sun is a star, but by no means a particularly luminous one. It is average in size and luminosity, and only to its nearest stellar neighbours would it be at all conspicuous. Even Sirius, the brilliant star which is so prominent in our winter skies, is officially ranked as a dwarf even though it has twenty-six times the Sun's luminosity. It lies at a distance of eight and a half light-years, or roughly 50 million million miles; to a Sirian observer, the Sun would be invisible with the naked eye, and to detect the Earth, which is not only very small but is also inherently non-luminous, would be an obvious impossibility.

There is every reason to suppose that planetary systems are common in space, and there are a few cases of relatively nearby stars which seem to be attended by planetary bodies. The most interesting example is that of Barnard's Star, which is a red dwarf much less massive than the Sun. Here the companion seems to have only two and a half times the mass of Jupiter: this is much too low for a star, and so the body is presumably a planet, though we have no definite proof. Yet it is invisible, and it has been tracked down only because it produces minute irregularities in the motion of Barnard's Star itself. Were its mass as low as that of the Earth, it would cause no observable effects, and so it would remain unknown. In fact, we can never hope to detect Earth-like planets of other stars unless we can develop instruments very much more sensitive than those available to us at present.

Space research has made amazing progress during the last few years, and further spectacular developments may be expected in the near future, but we must always keep our sense of propor­tion. So far as rocketry is concerned, our range is limited; our Solar System is a very small part of the universe, and it would be misleading to suppose that it is of any real importance. To astro­nomers living in other planetary systems - if they exist, as surely they must - even the Sun will be obscure, and from a distance of only a few light-years the Earth will fade into total insignificance.

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.