Galileo and the Dolphins

by Adrian Berry

  There remains the possibility of life. Donahue speculates: ‘With water trickling to the surface, there is always the possibility that microbial creatures could exist at the bottom of some crater.’

  Although the experiments on Mars of the Viking lander in 1976 found no trace of life - only of chemical reactions -there are clues in other Martian meteorites that have landed on Earth, in the form of amino acids, the chemical building blocks which comprise the most basic proteins that form living tissue. Whilst this does not ‘prove’ that there is life on Mars - as amino acids are only the ingredients of life, not life itself - conversely, neither does it disprove the life-on-Mars theory. All we can say is that life may be present on Mars now, or it may have been there three billion years ago - but we don’t know if either or both of these beliefs are correct.

  The trouble is that nobody knows how long the amino acid meteorite had been flying through space before it fell to Earth. Was it ejected from the Red Planet when our ape-like ancestors were making their first tools, or did it first fly at a far earlier epoch when life was only beginning to appear here?

  Was it, in short, the product of today’s frigid Mars, or of a lush, comparatively warmer planet? These speculations aside, within a half-century from now there is likely to be advanced life on Mars - our own. Whether we find microbes or not, easily accessible water will enable future settlers to turn it into a permanent second world for mankind.

  The Great Comets

  n the summer of 1995 there appeared over the Andes an unusually bright comet never hitherto recorded. Although easily visible to the naked eye, this seventeenth such discovery - found by the Australian William Bradfield, the world’s most successful comet-hunter - was not even reported in the northern hemisphere press because it could only be seen below the equator.

  New comets are not especially remarkable. They appear on average three times a year. Yet the science of comets is fascinating, since the comets surrounding the Sun almost outnumber the stars in the Milky Way.

  About a light-year away lies a vast belt of at least 200,000 million comets known as the Oort Cloud, debris left over from the formation of the solar system five billion years ago. The nucleus of each is no longer than a few kilometres, but their combined mass would create a planet 100 times heavier than the Earth.

  Periodically they fly inwards towards the Sun at the prompting of unknown gravitational disturbances. To such events we owe Comet Shoemaker Levy 9 which crashed into Jupiter, and the approaching Hale-Bopp, which promises to be a tremendous sight. It could rival the great comet of 1811, mentioned in ’s War and Peace, and famous because in Russian folklore it presaged the invasion of Napoleon. The chances of this particular comet hitting Earth were always very low. In fact it missed us by more than a million kilometres.

  The comet of 1811 was also the only comet to be associated with port wine. In that year, for some unrelated reason, the vintage in Portugal was unusually good and wine merchants displayed it as ‘Comet Port’.

  The only brighter comet was that of 1843, perhaps less famous because it had no link with military or political legend. It was also the biggest, with a tail-length estimated at 300 million kilometres, greater than the distance between the Sun and the orbit of Mars.

  , in the 1995 fifth edition of his Guinness Book of Astronomy, lists some of history’s famous comets. The closest known approach to Earth - always a subject of alarm since it may have been a comet’s impact that killed the dinosaurs 65 million years ago - was that of Lexell’s Comet of 1770 which passed within two million kilometres of us, only five times the distance to the Moon. But its orbit was changed nine years later by a close approach to Jupiter, and it may never be seen again.

  The most frequently seen comet is Encke, which returns to Earth’s vicinity every 3.3 years. In 1979 it suffered a ‘change of sex’ - it was found to have lost its snow and gaseous tail and turned into a stony asteroid. Many asteroids are likely to be comets which have ‘burned out’. And the comet least often seen is Finsler’s of 1937. Its period - the time between its appearances - has been calculated at 13.6 million years, so that it last appeared long before there were people to observe it.

  The ‘tail’ of a comet is a misleading term. When the comet travels away from the Sun, it is preceded by its tail which consists of gas and dust blown off it by sunlight. Some rare comets have more than one tail.

  There is a remarkable history of cometary superstitions. Before it was known that they appeared so often, it was easy to associate them with the adventures of great leaders. One came at the time of ’s assassination. In ’s play of that title, the dictator’s wife remarks on it with the alarmed observation:

  When beggars die, there are no comets seen

  The heavens themselves blaze forth the death of princes.

  Another was seen by the emperor in ad 79. He dismissed it, calling it ‘hairy’ (as all tailed comets seem to be), remarking that it ‘menaced rather the King of the Parthians, for he is hairy while I am bald’. But died in the same year. Another emperor, Macrinus, died in 218, the year of a visit by Halley’s Comet whose later appearance in 1066 was used with great political effect by the Conqueror.

  Some advanced thinkers have proposed that comets could one day be used as spaceships. They have many advantages for such a purpose: speed, free propulsion, and storage space for people and cargo. The only disadvantage is that they would be difficult to steer.

  The Most Accurate Clock

  The most accurate clock in the universe is so precise that it will lose or gain only one second in three million years.

  It is no work of man, but one of a class of extraordinary celestial objects called ‘millisecond pulsars’. These are collapsed stars that spin faster than aircraft propellers and are so densely packed with matter that a tennis ball made of the same stuff would weigh more than 20,000 million tons.

  The density and speed of rotation are directly connected. As a general rule, the more densely packed an object is in space the more rapidly it will rotate. This ‘super-clock’ rotates six thousand times a second with almost absolute reliability, emitting radio beeps as it does so, making it more than ten times more accurate than the atomic clocks that regulate Earth time.

  The fastest of the millisecond pulsars is fascinating astronomers: it is known somewhat prosaically as PSR 1937 + 21, a name that describes its position in the sky 10,000 light-years away in space. Says one of Britain’s leading cosmologists, , of the Institute of Astronomy at Cambridge: ‘If the Earth were to rotate much faster than it does, then centrifugal force would soon cause it to break up and fly apart in all directions. The fact that these pulsars do not fly apart, despite their speeds of rotation, proves that they must be tremendously dense with almost unimaginably strong gravitational fields holding them together.’

  Such pulsars were once stars like the Sun but very much larger. Being so huge they used up their nuclear fuel at a profligate rate and exploded as cataclysmically violent ‘supernovae’, blasting their outer layers out into space at speeds of more than 50,000 kilometres per second - nearly a fifth of the speed of light. Their inner cores then collapsed under their own weight, reducing a star that might once have been three million kilometres wide to a super-dense object perhaps 15 kilometres in diameter - the estimated size of PSR 1937 + 21.

  All sorts of practical uses are being proposed for this cosmic clock. ‘We could use it to make much more accurate measurements of the solar system,’ says , of Manchester University. ‘All the clocks on Earth, even the atomic clocks are biased by various gravitational fields. The tidal pull of the Sun, for example, slows down and speeds up clocks on Earth by very small amounts. But PSR 1937 + 21 is subjected to no gravitational influences other than its own. It is alone in space, not in orbit around another star. Its measurement of time is therefore absolutely without bias.

  ‘As the Earth moves around the Sun it is sometimes approaching this remote pulsar and at other times receding from it. Because of the way radi
o waves behave the pulsar’s beeps will differ accordingly. From this, we should be able to calculate the Earth’s position - and the orbits of all the other planets with an accuracy never before achieved.’

  of Princeton University outlines even more practical future uses for the pulsar.

  ‘We could use it for the navigational guidance of interplanetary spacecraft, where even the tiniest error can put a craft hundreds of kilometres off course,’ he says. ‘And I believe it may eventually replace the atomic clocks we now use.’

  It is fortunate that this pulsar is so far away. Pulsars are ‘cannibal’ stars, with magnetic fields more than a million times stronger than the Earth’s, whose gravity devours any other star with which it is in orbit. A pulsar that came too close to the Sun would doom humanity.

  Another recently discovered millisecond pulsar, PSR 1957 + 20, is ripping apart the star it is orbiting. The unfortunate star’s outer layers are being ‘eaten away’, says . This star, which might once have been as massive as the Sun, will vanish in a few million years. The pulsar, having entirely devoured it, will continue to spin as before, keeping absolutely accurate time.

  There’s Nobody Here

  Is there life on Earth? Or anywhere else for that matter? Even if the Galaxy were filled with Star Trek-like alien ships, they might be unable to recognize us.

  An experiment using photographs of the Earth taken by Nasa’s Galileo spacecraft from a mere 1,000 kilometres found not a single indication of the existence of intelligent life - a result that seems amazing in the light of our ever growing technology.

  ‘If that spacecraft had been sent by a group of alien scientists from another planet, nothing in these pictures would prove to them that this world was the abode of intelligent creatures,’ said one of them, W. Reid Thompson, of Cornell University, New York, co-author of a paper in Nature.

  ‘All they show is white clouds, the blue of the oceans, and the brown outlines of South America and Australia. Neither cities nor agricultural fields can be identified as artificial objects, since their rectangular shapes are undetectable. The spacecraft might see the glint of sunlight from satellites and large aircraft, and it might observe aircraft contrails. But the conservatives among the alien scientists - in any scientific debate there are always conservatives and radicals -could all too plausibly insist that these phenomena were merely cosmic rays striking their own detectors.’

  But what if the spacecraft photographed the planet at night (which Galileo did not)? ‘I think they could explain the lights of cities as natural fires or lowly phosphorescent organisms with whom it would not be worth communicating.

  ‘Since our languages would be unintelligible to them, and perhaps not even recognizable as languages, they might even mistake our radio and television signals for natural radio activity. Similarly, the oxygen in our atmosphere might be considered, not as supportive of life but as poisonous to it.’

  The moral is that we should not be too hasty to conclude that other planets and moons in the solar system, of which more than 60 have been surveyed by our spacecraft, were necessarily lifeless. ‘When looking at other planets close-up, one should always examine that extra bit of data, even if the exercise might at first sight seem pointless. For example, Saturn’s giant moon Titan, which has a rich methane atmosphere, may well contain the building blocks of life,’ said .

  The Nature team were ‘radicals’ who believe that the Galaxy contains perhaps millions of advanced civilizations. But a scientist of the rival ‘conservative’ school, , of the University of Michigan at Ann Arbor, argued in the Quarterly Journal of the Royal Astronomical Society that these must be extremely rare in the Galaxy.

  Teske said: ‘The Earth is the only planet in our solar system that is highly active geologically. Having valuable metals close to the surface and easily minable gives us our high technology. If our precious metals were buried deeper, as is the case with the planet Mercury, then we would not have advanced much beyond the eighteenth century.’

  This would explain the surprising negative findings of of , who failed to detect any intelligent radio signals from any planet within 25 light-years of the Earth. Ominously for the radicals, this region of 65,000 cubic light-years around the Earth contains no fewer than 12 stars that are similar to the Sun. If Sun-like stars cannot produce advanced civilizations in their orbits, what other stars will?

  So what makes the Earth unique? Scientists at the Bureau des Longitudes in have produced a surprising answer. They found that the Moon’s gravitational field keeps the Earth’s climate stable by moderating its axial tilt. It must be highly unusual in the Galaxy for an Earth-sized planet to have so large a moon, and be at the right distance from its parent star.

  A Corridor Both Curved and Straight

  , transported from Wonderland to a black hole in space, would be at home to find that a corridor can be straight and curved at the same time.

  ‘Imagine a manned space station in the form of a ring that encircles a black hole,’ says of Goteborg University, Sweden, in the Scientific American. ‘The corridor can curve inwards, outwards or be absolutely straight. It depends on the distance of the ring from the hole.’

  From the outside, it will look like a ring, but to the Astronaut the corridor will adopt different shapes. If it is at a critical distance from the hole - one and a half times the radius of the black hole itself - would see its corridor as absolutely straight ahead of her and (if she dared look) behind her.

  ‘Imagine that she attaches a lamp to the roof of the corridor and then walks away from it,’ writes Abramowicz. ‘As she looks behind her, she sees the lamp become progressively dimmer, but never obscured from view by any bend in the corridor. Finally, she peers forward and sees the lamp become progressively brighter. In fact, the image from the lamp circulates round the tube many times, so she sees multiple images of it.

  ‘Although she might have difficulty explaining why the lamp appears both behind and in front of her, she must conclude that the curved tube is straight because its walls never obscure the lamp.’

  The explanation is that space itself around the black hole is warped - and so is the path taken by light. A light-ray, trying in vain to escape from the hole, would orbit the hole in a perfect circle at this critical distance. And because humans are equipped to perceive light as travelling in straight lines, the ray’s curved path would appear to as perfectly straight.

  In a larger ring, the warping would be negligible, and the corridor would curve inwards. But if it was closer, then the hole’s stronger gravity would pull it inwards so that centrifugal force would make it curve outwards. ‘ Inward and outward are not absolute concepts,’ Abramowicz adds. ‘They become relative when space is warped.’

  The Multi-coloured Cosmos

  The universe, when properly seen, is a riot of colour. Yet it does not always appear so. Amateurs are often disappointed that when they look into the night sky with binoculars, they see only a uniform black and white.

  Happily, this is only an illusion caused by the fact that they are using their eyes instead of a camera lens. As a series of articles in Astronomy Now reminded us, every celestial object, whether star, planet or moon, has its own particular colour. The result is a blaze of multi-coloured light that makes the view of the most garish city from an aircraft window look dull.

  The colour of a star depends on its surface temperature. We can visualize this by heating a poker in a fire. At first it glows orange. A little hotter and it becomes red. And if the fire was hot enough, the poker would turn white. At the highest possible temperatures, impossible in an ordinary fire, it would be blue-hot.

  Temperature is in turn is related to the star’s age. A star will start its life at superheat and will gradually cool through time. The hottest stars tend to be the youngest and the coolest are usually the oldest. The hottest stars, like Rigel, with surface temperatures of up to 160,000F, are bright blue. Then come the brilliant white stars like Sirius (with a tinge of blue) at about 20,000F
.

  Cooler still are Sun-like stars with temperatures of 12,000F. Then we are down to red giants, such as Aldebaran, 30 times bigger than the Sun, and a mirror of what the Sun will become in about five billion years, when the exhaustion of its nuclear fuel makes it swell up. Aldebaran is no more than 6,000F. One star of this class has been described by an astronomer as ‘gleaming like a crimson jewel’.

  But after the red giant stage, the star can get hotter again. Gravity takes over and it collapses, until most of its original mass is compressed into a body the size of the Earth. This is a white dwarf, a thimble-full of whose material would weigh about ten tons. It is literally white-hot, since the pressure of its density has driven up its temperature to some 10,000F.

  The colours of the planets are just as striking, although for chemical rather than nuclear reasons. The skies of Mars, for example, are orange-red because of the ochre-red iron dust from the surface driven into the atmosphere by dust storms.

  The Moon has two distinct shades, dark and light, created by its differing rock compositions. The dark regions, or ‘seas’, are basalt, the result of ancient lava floes, and made of iron, magnesium and titanium. The lighter parts -which future tourists will find easier to walk on - are rich in calcium and aluminium.

  The four gas giants, Jupiter, Saturn, Uranus and , are all similar in general composition, yet they have different colours. Jupiter has brown strips across a greenish background and the enigmatic Great Red Spot. Yet Saturn is much whiter. This may be because it has less mass than Jupiter, with weaker surface gravity. Its clouds are therefore proportionately deeper than Jupiter’s, making them reflect sunlight more evenly. Both Uranus and are of a startling blue. Their atmospheres are so cold, since they are so far from the Sun, that they consist largely of clouds of freezing methane which absorb the red part of the Sun’s light.