Friday, 28 December 2007

Target Earth!

The name "Project Spaceguard" was deliberately borrowed from the 1973 science-fiction novel Rendezvous with Rama, by Sir Arthur C. Clarke, which describes a catastrophic meteorite impact in northern Italy in the year 2077, as a result of which a global early-warning system is set up to ensure that such a tragedy is never repeated.

Ironically, the real Project Spaceguard was given impetus by a cataclysmic event occurring not on Earth, but millions of miles away, on the planet Jupiter.

However, for the real beginnings of Project Spaceguard, we must go back just over a century, to 1905, when the mining engineer D. M. Barringer and physicist Benjamin C. Tilghman claimed that the Coon Butte Crater in Arizona was caused by a meteorite impact. The suggestion met with considerable scepticism, though it must be remembered that at the time, the idea that meteorites could cause cratering was highly controversial and the majority of scientists believed the craters on the Moon (the only example of large-scale cratering then known to science) had a volcanic origin, a view that did indeed have some support until quite recently. Not until the 1920s was Coon Butte Crater accepted as being meteoritic in origin, since when it has been known, not entirely correctly, as the Meteor Crater.

One of the reasons the impact theory of cratering was slow to win acceptance was because it was rather at odds with the then prevalent doctrine of Uniformitarianism, under which change, both geological and biological, occurs gradually over million years. The idea that large meteorites could wreak enormous changes was reminiscent of Catastrophism, which states that events on Earth such as geological change, the evolution of life and even human history have been shaped by upheavals of a violent or unusual nature.

Catastrophism was not a new idea and until the early 19th Century, it was generally accepted that stories like the Biblical Flood related to actual events. It should be remembered that at this time the Bible was interpreted more literally that is usual now and it was widely believed that all life on this planet had been created exactly as described in the Book of Genesis. Such beliefs had largely died out after Charles Darwin put forward his Theory of Evolution and fossil evidence confirmed that life had evolved gradually over millions of years.

It was easy to be complacent at that time. The Arizona crater and others that came to light later were thousands and in some cases millions of years old; and the face of the Moon has not changed throughout recorded human history. Not for many decades would it be discovered that cratering exists on many other bodies in the Solar System. Yet the danger signs were there for those who cared to look.

On the morning of 30 June, 1908, a fragment of cosmic debris entered Earth's atmosphere somewhere over western China. Travelling eastwards on a shallow slanting trajectory, it trailed a series of loud explosions in its wake until at 14 minutes and 28 seconds after seven o'clock local time, by which time it was around 5-10 kilometres above the ground, it exploded near the Tunguska River in Siberia releasing energy that has been conservatively estimated at 10 megatons and could have been as much as 20 megatons. The airburst felled around 80 million trees over an area of 2150 square kilometres and even now, a century after the blast, satellite imaging shows reduced tree cover in the area around ground zero. Curiously no trace of the object has ever been found, which has lead to speculation that it was a small comet rather than an asteroid (inevitably there have been suggestions of a more speculative nature such as black holes, antimatter and of course crashing UFOs). Fortunately, the region was largely uninhabited, and casualties were restricted to a dozen or so nomadic tribespeople who were slightly injured. But had the meteorite arrived 4 hours 52 minutes later, the city of St. Petersburg would have been totally destroyed, together with most of its inhabitants who at that time included one Vladimir Illich Ulyanov, later known as Lenin. Just a few hours, and the history of the 20th Century might have been very different!

In October 1937, Earth experienced an event we now know to be commonplace, but at the time caused some consternation when the news became public. The asteroid 1937 UB, later named Hermes, passed by at less than twice the distance of the Moon. Hermes was large enough to have caused a global disaster had it hit Earth. Hermes caused such a stir that it was given a name despite being lost after its close passage; it was not relocated until 2003 and is now known to comprise two 300-metre objects separated by just 1200 metres. It has also been calculated that it came even closer to Earth in 1942, but it was missed. At the time, of course, many astronomers would have been otherwise engaged.

In February 1947, a meteorite exploded just over two hundred miles from Vladivostok, detonating above the ground like the Tunguska object though in this case specimens from the fall were recovered. Although the explosive yield in this case was much less, it was still at least five times more powerful than the nuclear bombs dropped on Hiroshima and Nagasaki eighteen months earlier.

Both the Siberian events took place over sparsely populated regions, but then on 10 August 1972 came a decidedly close call, when an asteroid approximately 10 metres in diameter entered Earth's atmosphere above southern Utah. Travelling due north, it passed over Salt Lake City, before making its closest approach to Earth at an altitude of around 53 kilometres above Montana, where sonic booms were heard. Still travelling above escape velocity, the body then drew away, exiting the atmosphere over Canada.

Had it come a tiny fraction closer, it would have impacted with multi-megaton effect in the densely populated region between Provo, Utah and Idaho Falls. In the prevalent tensions of that era, and before the threat of meteorite strikes was fully appreciated, such an explosion could easily have been mistaken for a nuclear attack and triggered World War III.

Scientist had in fact been warning of the danger since the war, but they have been largely forgotten - few, for example, will have read the 1953 work Target Earth, by Allan Kelly and Frank Dachille, who advocated the use of rocket powered "tug boats" to deflect incoming meteorites. However in the 1980s the first concrete evidence of a meteoritic catastrophe on this planet began to emerge – albeit concerning events occurring millions of years before the dawn of mankind.

The sudden demise of the dinosaurs, 65 million years ago, had long been a mystery to science. Incidentally, it is worth pointing out a few common misconceptions about the dinosaurs. They were never contemporaries of man, despite suggestions to the contrary by one or two rather silly Hollywood motion pictures: they were not all large and ferocious, the majority being much smaller than a man: and finally, they certainly were not the stupid, blundering creatures of popular belief. They were the unchallenged rulers of the Earth for 150 million years. If mankind hopes to emulate the feat there is a long way to go.

In June 1980, the physicist Luis Alvarez, his son Walter and a number of other collaborators published a paper which claimed that a large meteorite had struck the Earth 65 million years ago, resulting in the extinction of the dinosaurs. The evidence for this claim was based on studies of a layer of clay, half an inch thick, laid down between two layers of limestone that had been seen in rocks near the town of Gubbio, in northern Italy. The clay was clearly located at the so-called K-T Boundary that delimits the Cretaceous and Tertiary geological time periods. There was no element of doubt; the limestone below the clay contained Cretaceous fossils; that above contained fossils from the Tertiary. It was at this point in time that the dinosaurs had become extinct.

The Alvarez team has originally intended to find out how long it had taken the clay to be deposited, because sudden though the transition from Cretaceous to Tertiary was, nobody believed it could have literally happened overnight. The method chosen was to measure the amount of iridium in the clay.

Iridium is one of the so-called "Splendid Six" group of metals, which also includes platinum, and is considerably rarer than gold. It is very rare in the Earth's crust, but relatively abundant in meteorites. There is nothing mysterious about this. Because of the great density of these metals, much of the terrestrial supply has sunk right down to the Earth's core. However, meteorites are formed from much smaller parent bodies in which the iridium is more evenly distributed.

A constant trickle of iridium reaches Earth all the time among the dust grains which constantly rain down from outer space as a result of micrometeorites entering the atmosphere. The rate of fall has been constant throughout geological time, the amount falling over, say, 100 years being the same now as it was 65 million years ago. This effect could be used to provide a "clock" to time how long it had taken the clay layer to form.

The Alvarez team had expected to find a small amount of iridium, consistent with, at most a time scale of 10,000 years to deposit the clay, which is what one would normally expect for a layer of its thickness. Instead, they found iridium concentrations were so high that if the "iridium clock" model were correct, it would have taken four million years for the clay to form.

This result was clearly nonsensical. Something else must have produced the anomalously high iridium levels and the only possible causative agent was a large meteorite, which had smashed into the Earth, spreading billions of tons of fine dust around the world comprising pulverised rock and debris from the meteorite itself. This fine dust eventually settled out of the atmosphere to produce a uniform layer of iridium-enriched clay all over the world. The shroud of dust, encircling the globe, would have cut off the light of the Sun and plunged the Earth into darkness. Plant life would have died off, unable to photosynthesise, and temperatures would have fallen. It was this "cosmic winter" that had killed off the dinosaurs.

One consequence of this discovery was the realisation that a "nuclear winter" would follow even a limited nuclear exchange. This led to the signing of several major arms treaties by the end of the 1980s and for a time there were genuine hopes that the madness of nuclear weapons might finally be eliminated. Sadly, these hopes seem to have been misplaced, with many states scrambling to acquire weapons of mass destruction, including nuclear weapons.

Like all radical scientific theories, the impact theory of the dinosaur extinction was slow to gain acceptance, especially from the palaeontologists who felt their territory had been invaded by a bunch of physicists and geologists. Some geologists put forward a rival theory, claiming that a series of volcanic eruptions occurring in India at the same point in time had been responsible. They argued that the volcanoes, spewing forth material from the bowels of the Earth, could have produced the iridium anomaly.

Meanwhile, the search was on for the "smoking gun", the crater left by the impact. In 1990, a huge buried crater at Chicxulub, in the Yucatan Peninsula in Mexico came to the attention of scientists. The crater had been formed in what had then been shallow water and consequently was covered in a limestone layer. It had been discovered by industrial geologists in the 1970s, but they had kept quiet about their discovery because of the possibility of oil in the region. Using radioactive argon dating techniques, the scientists determined that the crater had been formed 65 million years ago – the time at which the dinosaurs disappear from the fossil record.

In the light of this evidence, the impact theory has now become widely accepted. A giant meteorite impact killed the dinosaurs, and indeed 70 percent of all species then existing on Earth. But what was disaster for the 70 percent was good news for the rest, including the tiny shrew-like mammals, which were able to fill all the vacant niches and diversify into the vast range of modern mammals, including ourselves, that now inhabit the Earth.

Interesting though all this was, it had all happened rather too long ago for the threat of a recurrence to be taken seriously by the general public, and what finally moved the danger from cosmic impacts onto the public agenda was a timely demonstration of just what such an impact could do. Fortunately, Nature was kind enough to arrange for the demonstration to take place at a safe distance from the Earth!

In 1993, a team of comet watchers comprising Carolyn Shoemaker, her husband Eugene Shoemaker and David Levy, observing at the Mount Palomar Observatory, discovered a peculiar cometary object subsequently named Shoemaker-Levy 9 (it was in fact the team's ninth discovery). Shoemaker-Levy 9 resembled a string of beads strung out on the same orbit and once its orbit had been calculated, it was determined that it was the remains of a single comet that had passed so close to Jupiter the previous year it had not only been captured by the giant planet, it been torn apart by tidal forces during its close approach. Furthermore, it soon became clear that the cometary fragments were now on a collision course for Jupiter, with a series of impacts due to begin on 16 July 1994.

What followed is of course well-known. Although all the impacts occurred on the side of Jupiter not facing the Earth, the fireballs produced as the fragments rained down upon the giant planet billowed up to 2000 miles above the cloud-tops and were clearly observed by the Hubble Space Telescope. The impact sites, carried into view by Jupiter’s rapid rotation, showed that great dark scars had been produced. The dramatic photographs of the scars, stretched out across the face of the wounded planet convinced even the politicians the threat posed to Earth by such objects was very real, and even while the bombardment of Jupiter continued, the US House of Representatives passed a bill requiring NASA to submit to the Congress a costed proposal to chart all objects in Earth-crossing orbits larger than one kilometre in diameter. There were similar political initiatives in Europe, Russia and Australia.

Meanwhile, Hollywood wasted no time in leaping on the bandwagon, and in the late 1990s "meteorite movies" almost become a genre in their own right, though for the most part they were little better than the dire (and misnamed) Meteor, which was released in 1979. A scene from one such movie shows the inundation of New York by an impact-induced tsunami; ironically (in the light of later events) only the twin towers of the World Trade Center survive.

To implement a scheme to monitor space for hazardous objects, it was proposed to set up a global network of eight purpose-built telescopes. Six would have an aperture of 100 inches, and would be used to search for near-Earth objects. The other two, one in each hemisphere, would have an aperture of 200 inches and would be used to search for faint comets beyond the orbit of Jupiter.

By the end of the century a hazard scale for potentially threatening near-Earth objects had been devised. Known as the Torino Scale (named for Torino [Turin], Italy, where it was first proposed), it accesses a threat from 0 (no possibility of collision) through to 10 (the end is nigh). Threat level is based on both probability of a collision, and the consequences of that collision. The latter is obviously a function of the size of the threatening object.

What action could be taken if an asteroid was determined to be on a collision course? Simply blowing it up with a nuclear device would do no good – the fragments would still hit Earth with disastrous consequences, but given sufficient lead time, danger could still be averted by deflecting the threatening object. Two methods have been proposed. The first is to use the enormous velocity of such objects relative to the Earth to deflect them. By firing a large interceptor rocket into the path of one, the change of momentum so imparted would be sufficient to nudge it onto a new, harmless trajectory. With ten years warning, objects a mile in diameter could be so diverted. However, with only the same warning, much larger objects up to 20 miles in diameter could also be diverted by exploding one or more nuclear devices a short distance away, so as to vaporise the surface of one hemisphere. In this case, the expanding gases would act as a rocket motor and push the object away from the direction of the blast. With a century to react, even objects the size of a small moon could be turned aside.

Unfortunately, politicians being politicians, the initial enthusiasm for Project Spaceguard seems to have been lost: global warming seems to be the hot (pun intended) topic now. One sincerely hopes that the project does eventually come to fruition, because it is a matter of pure luck that no impact having global consequences has occurred during recorded history; and the respite can hardly be expected to continue forever. Inevitably, there has been speculation that such an impact did occur in late prehistoric time.

The idea isn’t new and the suggestion that astronomical events have caused global catastrophe within the last ten thousand years have been the subject of speculation for decades, though most of which is firmly in the realms of pseudo-science. The best known proponent of this view was Belarus-born Emmanuel Velikovsky, who had a number of very strange ideas, including the belief that Venus was a comet until just a few thousand years ago. In 1950, he published a book called Worlds in Collision in which he meticulously catalogued graphic accounts of global catastrophe as described in the Bible; the records of the Mayan, Aztec and Inca civilisations; and Greek and Nordic mythology. Velikovsky’s approach was to assume that these myths and legends were literally true, and he sought to interpret them as being references to catastrophes caused by close encounters first with Venus and then Mars, occurring around 2000 BC and 800 BC. Worlds in Collision unleashed a storm of Salman Rushdie proportions in the scientific community, and there were even threats by some universities to boycott Velikovsky's publishers unless the book was withdrawn. This ridiculous over-reaction only served to lend the book spurious credibility, with the result that it continues to find its way into pseudo-scientific speculations to the present day.

Velikovsky’s book is very readable and well-researched, and one wonders how many of his critics have ever actually taken the trouble to read it. That said there is no doubt that the theory he puts forward is the purest nonsense and suggests a near-total disregard for the laws of physics. Had Venus genuinely been on a collision course for Earth it would have taken rather more than a lightning bolt (as Velikovsky asserts) to avoid total annihilation; and had its influence halted the Earth’s spin the effects would not have been confined to the walls of Jericho.

Worlds in Collision didn’t even represent the full extent of Velikovsky’s bizarre theories and among the speculations that never made it into print was the idea that Earth had once been a satellite of Saturn, but a nova-like disturbance there had shifted our planet into its current orbit, producing Noah’s flood in the process; Jupiter was to blame for the destruction of Sodom and Gomorrah; and Mercury was somehow mixed up in the Tower of Babel.

Stephen Jay Gould makes what is probably the fairest comment on Velikovsky. In an essay entitled Velikovsky in Collision, he stated Velikovsky is neither crank nor charlatan — although to state my opinion and to quote one of my colleagues, he is at least gloriously wrong. But is the idea of a global catastrophe in Neolithic or Bronze Age times something we can completely dismiss?

In 1982 the British astronomers Victor Clube and William Napier proposed that large comets can from time to time end up in short-period orbits and wreak havoc in the inner Solar System. Over time a large object would break up and Earth would experience not just impact events but global cooling arising from meteoroidal dust building up in the atmosphere. The theory met with considerable scepticism at first but was widely quoted in the spate of books on the “meteorite menace” that appeared in the second half of the 1990s. Speculations in these books included the suggestion that the collapse of Mycenaean civilization towards the end the Bronze Age had been caused by an impact, an idea that certainly does deserve to be taken very seriously. However attempts to link more or less every myth and legend (including of course Atlantis) to meteorite impacts; and the suggestion by one author that Stonehenge was a Neolithic early-warning system, intended to look out for incoming meteorites; should serve as a reminder that there is a fine dividing line between informed speculation and pure hokum.

One possible reference to a prehistoric catastrophe is to be found in the Norse legend of Ragnarok, the end of the world, which tells of a world fire followed by the Fimbulvetr, a great winter lasting three years. This is startlingly suggestive of a meteoritic impact followed by a "cosmic winter" of the kind associated with the death of the dinosaurs.

But are the Norse legends, as compiled by Snorri Sturluson in the Prose Edda, based on original material? Many have commented on the similarity between the opening of the Sixth Seal in the Book of Revelation and the Norse account of Fenris-Wolf devouring the sun. It has been pointed out that the Prose Edda was compiled from a Christian standpoint. Could the Ragnarok legend not simply be a rehash of the Biblical account of St. John the Divine?

In the early Thirteenth Century, the ancient Norse legends were becoming rather frowned upon because of the rise of Christianity in the Scandinavian countries and Snorri Sturluson decided to record these wonderful and now all too often neglected tales for posterity. It is true that he was a Christian, but the considered opinion is that he did not embellish the Ragnarok legend with Biblical material and that it is largely unaltered from its original form, and of independent origin. So could the Ragnarok legend be an account of real events?

The last Ice Age ended around 10,000 years ago, but the thaw set in some millennia previously. This was interrupted by an event known as the Younger Dryas occurring 12,700 years ago and lasting for 1,300 years. It has recently been suggested that this might have been the result of a meteorite impact near the North American Great Lakes. Of course the Norse legends are far more recent in origin – but it is possible that some elements have their origins in these events millennia early, just as the universal Flood mythology probably has its origins in the rise in sea levels that accompanied the end of the last Ice Age. However I have to say I am somewhat sceptical – I do feel that it is far more likely that the Younger Dryas was caused by freshwater running off from the melting North American ice sheets. This could have caused the Gulf Stream to cut off, bringing a temporary halt to the warming. If the Ragnarok legend does relate to actual events, possibly it is a description of a major volcanic eruption. This could also precipitate temporary global cooling, such as happened in 1815 after the eruption of Mount Tambora. One eruption that undoubtedly made its mark on prehistory is that of Thera in the Mediterranean around 1600 BC. This seems to have brought about the downfall of the original Minoan civilization on Crete and its absorption by the mainland-based Mycenaeans.

While it is possible that a major meteorite impact gave rise to some of the Norse legends, the evidence is sketchy and the idea of a late prehistoric impact still largely speculative, unlike the K-T Boundary event, which few now dispute was caused by a meteorite. What is not speculation is that the next major impact will occur one day. It may be a hundred millennia away or just a few days. Let us hope that the latter is not the case, for it may be many years before a proper early warning system is in place.

It seems to be human nature to react to tragedy rather than try to prevent it from happening in the first place. Less than a century ago, major transatlantic liners sailed quite legally with sufficient lifeboats to save only a fraction of those aboard should disaster strike. Today it is quite unthinkable that a ship should sail without lifeboats for all. The story of the Titanic is indelibly etched on mankind’s collective psyche. Unfortunately, the needless loss of human life has continued unabated. For example, it took the deaths of nearly two hundred football fans in three separate disasters in the 1980s before something was done about the medieval conditions in which enthusiasts were expected to follow their teams.

Even a small meteorite strike on a major city would pale all this into insignificance. The death toll would far exceed that of the terrorist attacks of 11 September 2001, possibly running to tens or even hundreds of thousands. Even if a warning was given and there was sufficient time for the affected region to be evacuated, it is also worth considering the cultural heritage that would be lost should a meteorite land in the middle of Paris, Rome or any one of a dozen other European cities. The annihilation of the Louvre, the British Museum or the Hermitage would be a disaster comparable to the destruction of the Alexandria Library in antiquity.

Let us hope it does not come to that….

© Christopher Seddon 2007

Thursday, 27 December 2007

Everything you wanted to know about the Moon

Most people will think nothing of the Moon should they happen to see it in the sky. This is hardly surprising, the Moon is after all one of only two distinct, instantly recognisable objects (the other being the Sun) that we are guaranteed to see (even here in Britain!) during our lifetime; there can be nobody alive who has not known of it from their earliest childhood. The Moon is a ubiquitous part of our culture and almost certainly has been since earliest times. Its beauty in the night skies has inspired writers, poets and artists for centuries. Reaching for the Moon was once synonymous with desiring the impossible - until man reached it. Even now, almost four decades on from Armstrong's momentous giant leap for mankind, the Moon remains the only astronomical body other than Earth to be visited by humans.

What’s in a name?

Many people think the “official” name of the moon is the Latin form Luna, but in common with Terra (Earth) and Sol (the Sun) the term Luna has no official standing and is rarely encountered outside of science-fiction novels, though the adjectival forms “lunar”, “terrestrial” and “solar” are in common usage. The “official” name for the Moon is – the Moon (capitalised)! The uncapitalised form – “moon” – is a generic term for any natural satellite of any planet, including our own. Some prefer this term over “satellite” thinking the latter implies something manmade. Strictly speaking a manmade satellite should be referred to as an “artificial satellite” but this usage is now very rare.

The phases of the Moon

The most obvious thing about the Moon is that its appearance changes from night to night. The Moon is not the only body visible from Earth to exhibit phases – Venus and Mercury do also – but without a telescope those of Venus are very difficult to see and those of Mercury are way beyond human perception. The explanation for the phase is straightforward; only one hemisphere of the Moon is illuminated by the Sun at any one time (in common with all other non-luminous solar system objects) and the portion of the illuminated hemisphere visible from Earth changes as the Moon travels round the Earth on its orbit. When the Moon and Sun are on opposite sides of the Earth a full moon is seen; when they are on the same side the Moon disappears altogether. When they are 90 degrees apart a half moon is seen.

The time taken for the Moon to cycle through its phases (the synodic month, defined as the time taken for the Moon to return to the same position relative to both Earth and Sun) is actually longer than the time taken for it to complete a single orbit (the sidereal month) – 29.53 days on average, as opposed to 27.32 days. The reason for this is while the Moon is completing an orbit of the Earth, the latter is moving on its own orbit around the Sun, and the Moon has to move slightly further before it can return to the same position relative to both Earth and Sun.

The wrong time of the month

In 1972 the American researcher Alexander Marshack claimed that people were making records of the phases of the Moon 30,000 years ago. After extensive research that entailed examining just about every prehistoric artefact he could lay his hands on for calendrical notches, he published his findings in a book entitled The Roots of Civilization. Marshack claimed that the tallies corresponded to lunar months. On the face of it, this seems highly plausible. It is now generally accepted that the people of that era were every bit as mentally capable as we are today, and there is little doubt that they would have been aware that the phase of the Moon changes from night to night in a predictable manner. But there are two problems – firstly, it seems unnecessary to record, say, the days since the last full moon when one can simply look at the Moon, note the current phase, and work forward to when the next full moon will occur. The second problem is the tallies vary in numbers of days by more than can be explained by the small seasonal variations in the length of the lunar cycle, or by observational error. However there is another cycle with an average length almost identical to the lunar cycle that does show a certain amount of variation – the human menstrual cycle. It is my guess that this is what was being recorded, since the advantages of knowing when that time of the month is approaching are fairly obvious, and this was probably also the case 30,000 years ago!

That the menstrual cycle is almost exactly one lunar month in duration is now thought to be pure co-incidence, but it is one that was noticed many thousand years ago. The words “moon”, “month”, “menstruate” and “measure” (time) all have the same Proto-Indo-European root. The proto-Indo-European language is the hypothetical common ancestor of the Indo-European languages, which include Latin, Greek, Sanskrit and the modern languages derived from them. According to one popular theory, the Proto-Indo-Europeans were warlike nomads who originally expanded from the Eurasian steppes at around 4000 BC, taking their language with them. A rival theory, proposed in the mid-1980s, claims that Proto-Indo-European origins go even further back, and that they were originally farmers living in Asia Minor, shortly after the end of the last Ice Age. Regardless of which theory is correct (I personally favour the farming theory), the origin of the word “moon” is very ancient indeed.

The Moon from an astronomical viewpoint

The Moon is ranked as a satellite of the Earth. Most of us will be aware that the Earth is in astronomical terms quite undistinguished, and that the same goes for the Sun. Even though the Milky Way, of which the Sun is a part, is classed as a large galaxy one doesn’t have to look far (in fact a mere two million light years) to find a larger galaxy (the Andromeda Galaxy). In a way this is exactly what we should expect from the Copernican Principle or Principle of Mediocrity, an important principle in the philosophy of science which states that Earth holds no special place in the universe and that humans are not privileged observers. Right, so this presumably means that the Moon is equally average? Well, actually, no.

The Moon is a remarkable object and as far as the Solar System is concerned, it is unique. The Moon is a fully paid-up member of the Solar System's "Big Seven" group of satellites, all of which are larger than Pluto and Eris (the two smallest planets(or largest “dwarf planets” if you insist)). The Moon is by no means the largest member of this group, but all the other six are satellites of giant planets: the Earth is at best only medium-sized. Indeed many astronomers take the view that the Moon is too large in relation to the Earth to be considered a mere satellite and elevate it to the rank of a sister world, classifying the Earth-Moon system as a binary planet. However this view is not really valid. Large though the Moon is, it is still only 1/81 the mass of the Earth; the centre of mass for the Earth/Moon system lies below the surface of the Earth and the Moon cannot be classed as anything other than a satellite of Earth.

Lunar geography – or Selenography

Though most will know what it means, terms like “lunar geography” , “lunar geology”, etc, are oxymorons as the prefix “geo-“ means pertaining to the Earth. The correct terms are “selenography”, “selenology”, etc; the prefix “seleno-“comes from Selene, the Greek goddess of the Moon.

It is not true, as is often believed, that Galileo was the first to map the Moon using a telescope. That distinction must go to Thomas Harriot in 1609, a year before Galileo. However, both men clearly observed mountains, valleys, craters and comparatively smooth areas known as maria or seas. It was at one time believed that these latter features actually were seas, or at least dried-up sea beds, but we now know from samples brought back from the Moon that they have never contained any water.

However, in 1998, it was widely reported that NASA's Lunar Prospector probe had found water on the Moon, allegedly from comets that had landed in polar regions permanently hidden from the Sun and thus remained frozen. In fact, the probe had only detected evidence of hydrogen on the Moon's surface. While this could be due to water, I have to say I am highly dubious. Any comet impacting the Moon would almost certainly do so at a relative velocity high enough to vaporise it instantly as its kinetic energy is transformed into heat.

Another theory, popular before the Space Age, was that the maria were great dust-bowls, and any spacecraft landing there would be swallowed up. The idea was featured in two vintage novels by Sir Arthur C. Clarke, Earthlight and A Fall of Moondust, the latter telling the story of a "dust cruiser" designed to "sail" the lunar "seas".

Today we know that the maria are large dark plains of basalt, formed by volcanic activity billions of years ago.

The origin of the lunar craters has been the subject of considerable controversy over the years. It was once believed that they were volcanic in origin, similar to calderas, but it is now generally accepted that they are the result of meteoric impacts. It was however quite a long time before the volcanic theory was abandoned and a number of astronomers, including Sir Patrick Moore, continued to argue for it until as late as the 1990s.

A Canterbury Tale

Assuming that the impact theory is correct could any new craters have appeared in historic times? In theory, there is no reason why not, though in practice it seems unlikely with impacts forming craters visible from Earth being fairly rare events. However in the 1970s an American astronomer named Jack Hartung claimed that a report made on 18 June 1178 by a Canterbury monk named Gervase could be interpreted as an eye-witness account of the formation of the crater Giordano Bruno.

... after sunset when the moon had first become visible a marvellous phenomenon was witnessed by some five or more men who were sitting there facing the moon. Now there was a bright new moon, and as usual in that phase its horns were tilted toward the east; and suddenly the upper horn split in two. From the midpoint of this division a flaming torch sprang up, spewing out, over a considerable distance, fire, hot coals, and sparks. Meanwhile the body of the moon which was below writhed, as it were, in anxiety, and, to put it in the words of those who reported it to me and saw it with their own eyes, the moon throbbed like a wounded snake. Afterwards, it returned to its proper state. This phenomenon was repeated a dozen times or more, the flame assuming various twisting shapes at random and then returning to normal. Then after these transformations the moon from horn to horn, that is along its whole length, took on a blackish appearance.

It has been suggested that Gervaise saw a meteorite impact, and that the crater Giordano Bruno (named for the Italian philosopher who was burned at the stake for heresy in 1600) was formed as a result. Proponents of this idea point out that the time of the year is consistent with an impact from the so-called Taurid Complex, associated with Enke's Comet, but the whole thing really has to be taken with a king-sized pinch of salt. Surely a small group of men in Canterbury would not have been the only people in the whole world to see and note such a major disturbance in the natural order of things? A more recent mathematical treatment of the theory showed that Earth would have been bombarded with ejecta from the impact. This would have resulted in spectacular meteor showers of roughly 50,000 meteors an hour being visible all over the world for a week – yet there is absolutely no record of anything of the sort being seen.

Crucially, the Moon was close to the horizon at the time and what Gervaise reported was almost certainly an unusual cloud phenomenon or atmospheric disturbance.

The Moon in fiction

What is arguably the world’s first ever work of science-fiction, entitled A True Story, was written by the Greek satirist Lucian of Samosata in the 2nd Century A.D. and dealt with imaginary voyages to the Moon, but the topic did not become popular until the invention of the telescope in the 17th Century. Authors who wrote about journeys to the Moon included Johannes Kepler, Francis Godwin and Cyrano de Bergerac though the heroes tended to travel by unlikely means such as harnessing a flock of geese.

About a hundred years before Project Apollo, Jules Verne described an American moon program in which a projectile is launched from a space gun in Florida and splashes down in the Pacific, just as Apollo would later do. Some 35 years later, H.G. Wells sent his characters to the Moon in a vehicle utilising anti-gravity – much to the disgust of the by then elderly Verne. This criticism evidently affected Wells, who much later used a space gun himself in the moon shot sequence at the end of the movie Things to Come.

The Moon featured in innumerable works by the 20th Century’s “Holy Trinity” of Sir Arthur C. Clarke, Isaac Asimov and Robert Heinlein.

Inevitably the Moon has featured in many science fiction movies and television series, with manned moonbases being a popular theme for the latter. Gerry Anderson, best known for his classic puppet shows such as Thunderbirds, made two live-action series featuring moonbases. In the first, UFO, interceptors were launched from a moonbase to destroy hostile alien spacecraft. The second, Space 1999, was an altogether more ambitious affair. It was billed as a British answer to Star Trek but despite a huge budget, excellent special effects and a star cast that included Martin Landau, Barbara Baines, Barry Morse, Catherine Schell, Joan Collins, Brian Blessed and Judy Geeson, the series was not a success and was cancelled mid-way through its second run. The main problem was an utterly implausible plot device in which a nuclear explosion sent the Moon careering off into outer space at what one must presume was many times the speed of light (a physical impossibility in itself), given that most weeks would find it hurtling towards a new planetary system. Hopes would rise among those marooned on Moonbase Alpha that the new system would contain an inhabitable world on which they could settle, but on the occasions that it did something would always prevent colonisation, be it paranoid aliens fearing cultural contamination by “primitive” humans (this one cropped up on several occasions); an interplanetary battle of the sexes (a group of rather butch-looking women hijacked Alpha and used it as a platform to lob nuclear missiles at the men, who had already been banished to another planet for being “unreasonable”); a time-warp that reverted the crew to cavemen (this provided an excuse to put the lovely Zienia Merton in a leopard-skin), or the putative new home turning out to be rather inconveniently composed of antimatter. Even when the Moon was in interstellar space things were rarely quiet: black holes and time warps were as frequent as tailbacks on the M25; other menaces included a space brain, a monster dwelling in a Sargasso Sea of abandoned spaceships and miscellaneous aliens in suspended animation, who invariably turned out to be bad guys sent into exile by their peace-loving compatriots.

Is the Moon Earth’s only natural satellite?

Could the Earth have a second, undetected satellite? On the face of it, there is absolutely no reason why not. Jupiter is now known to have at least 63 satellites; Saturn has about the same number; and even Pluto has three. However if a second Earth satellite were to exist, it would have to be very small indeed to avoid detection. It is not often appreciated that were the Moon only two miles in diameter, it would still be visible to the naked eye.

Nevertheless, the idea that the Moon might not be our planet's sole attendant has intrigued astronomers for the better part of two hundred years. In 1846 Frederic Petit, Director of the Toulouse Observatory, claimed that a second Earth satellite had indeed been discovered. Petit's claim was soon refuted, but he became obsessed with the idea of a second satellite. Fifteen years later, he published an abstract in which he proposed the existence of a second satellite to account for then-unexplained anomalies in the Moon's orbit. The theory attracted little interest among astronomers, and doubtless would have been entirely forgotten by now had a young French writer by the name of Jules Verne not read the abstract and immortalised Petit and his satellite in the novel From the Earth to the Moon, in which the Petit object passes close to the space travellers projectile, pulling it off course and swinging it into an orbit around the Moon.

The idea of a second moon was revived several times during the last century, and shortly after the end of the Second World War, Clyde Tombaugh, discoverer of Pluto, carried out a most comprehensive search. He used equipment so sensitive that it would have shown a lump of coal the size of a football a thousand miles away. He failed to find anything.

It is now believed that the combined gravitational effects of the Earth, Moon and Sun would rapidly eject any small satellite from Earth’s orbit, ruling out the existence of a second moon. Nevertheless in 2002 an object known as J002E3 was discovered in Earth orbit – but it was soon discovered to be almost certainly the discarded third-stage booster from the Apollo XII mission in November 1969. It is believed that the object left orbit in June 2003 and may return around 2032.

It is sometimes claimed that the asteroid 3753 Cruithne ranks as a second Earth satellite. Discovered in 1986, Cruithne has an unusual orbit, known as a “horse-shoe” orbit, due to the influence of Earth. However it is in orbit around the Sun, not the Earth and therefore it is not an Earth satellite.

A Cosmic Coincidence

One of the most singular features of the Moon is the fact that it appears almost exactly the same size as the Sun in the sky. The reason for this is that while the Sun is 400 times the diameter of the moon, it is also 400 times further away, so both objects appear the same size when viewed from Earth. This is a pure co-incidence, but it is responsible for what is surely one of the most spectacular phenomena to be seen anywhere in the Solar System - a total eclipse of the Sun. A solar eclipse is, of course, due to the Moon passing directly between the Sun and the Earth, casting its shadow upon the latter (strictly speaking, the phenomenon is an occultation, not an eclipse). Because the Moon's disc is just sufficient to hide that of the Sun, the latter's atmosphere, the so-called corona can be seen in all its splendour. In fact it is a close call and for a total eclipse to occur, the Moon must be close to perigee (i.e. its minimum distance from Earth). Otherwise, a thin ring of the Sun's disc is left showing, quite enough to drown out the glorious corona, and the eclipse is said to be annular. Because the Moon's orbit is inclined at five degrees, an eclipse does not occur every month, though at least two must occur in a given year. However this figure includes partial eclipses, when the Moon does not pass directly in front of the Sun. Even when a total eclipse does occur, the area experiencing totality is only a small corridor, though it may extend for thousands of miles as the Moon's shadow races across the Earth's surface.

I have only witnessed one total eclipse of the Sun, that being the one in Cornwall in August 1999. Although cloudy skies prevented me from seeing totality, it was still an awesome experience as day became night in a matter of seconds. Sea birds, believing night really had fallen, hooted in great excitement. On the horizon was seen a band of orange light, marking the limits of totality. The scene was one of great beauty and although it was disappointing to have missed something I had been waiting to see since my childhood, it was still a worthwhile experience.

We will not always be able to enjoy the spectacle of a solar eclipse, because tidal effects are causing the Moon to recede from the Earth by 3.8 centimetres per year. That might not seem like a lot, but it adds up. When the first maps of the Moon were being drawn up, three centuries ago, the Moon was 11.4 metres (just under 40 feet) closer to the Earth. When modern humans first reached Australia, 50,000 years ago, the Moon was 1900 metres (rather more than a mile) closer; when the dinosaurs became extinct 65 million years ago, it was 2470 kilometres closer.

Eventually it will be too far away for its disc to fully block out the Sun, even at perigee. These effects are also causing the Earth's spin to slow and the day is gradually lengthening. Again, these effects are small but they add up over time and account for discrepancies amounting to several hours in the timing of eclipses observed in antiquity.

The Dark Side of the Moon

As is correctly pointed out in the eponymous Pink Floyd album, there is no "dark" side of the Moon: each part of the Moon experiences as much daylight as it does night time. So where does the idea that the Moon has a “dark” side come from? In common with almost all bodies circling a larger primary, the Moon exhibits so-called "captured rotation", meaning that it turns on its axis exactly once in each circuit of its primary. In other words, a lunar day is exactly a month long. It is often said that this results in half the Moon's surface being permanently hidden from view on Earth, leading to the misconception that the hidden side is in permanent darkness. If this were true, we’d see a full moon all the month round! The phase is of course due the part of the Earth-facing side being in darkness. In fact it is not strictly true that only half of the Moon’s surface can be seen from Earth. Because the Moon (in common with all other objects in the Solar System) does not move in a perfectly circular orbit, its orbital velocity varies slightly during the course of a month in accordance with Kepler's Laws of Planetary Motion. This means that the orbital motion and axial spin are at times slightly out of step, and in consequence we can see portions of the "hidden" side. Because the Moon's orbit is inclined at five degrees to that of Earth, we can also see alternately beyond the north and south lunar poles. Finally, parallax effects result in observers being presented with slightly different portions of the Moon's surface at different times of the day and in total, about 59 percent of the Moon's surface may be observed from Earth at various times.

Origin of the Moon

As one might expect, the origin of the Moon has been the subject of many theories over the years. The first theory to gain widespread acceptance was put forward by Sir George Darwin (son of Charles). Darwin suggested that the Earth and Moon had originally formed a single rapidly rotating, molten mass. The tidal forces raised by the Sun and the centripetal forces of its own motion caused it to become pear-shaped and eventually split into two objects of unequal size. A strong supporter of the fission theory was the American astronomer W.H. Pickering, who suggested that the scar left by the Moon's breakaway was now the basin of the Pacific Ocean.
Unfortunately the theory was pear-shaped in more ways than one. A mathematical treatment of the dynamics involved showed that it was unsound and it had to be abandoned. This did not prevent it from being used as the basis of an ingenious science fiction movie, Crack in the World, in which an attempt to tap energy from the Earth's molten core goes disastrously awry and triggers a series of earthquakes. A growing rupture in the Earth's crust threatens to tear the planet apart and rival scientists Stephen Sorenson (Dana Andrews) and Ted Rampion (Kieron Moore) are forced to put aside their differences and try to come up with a solution. An attempt to avert disaster by exploding a hydrogen bomb in the shaft of an active volcano is only partially successful, and a whole portion of the Earth is blasted away into space, where it forms a new satellite. The movie's closing reel shows the Moon and its new sibling in the sky together, the whole process having been observed from no more than a few hundred yards by Rampion - accompanied, of course, by the movie's love-interest (Janette Scott).

The next theory to be put forward suggested that the Moon was originally an independent body, but it wandered too close to the Earth and was captured. There is little doubt that this has happened elsewhere in the Solar System, Mars's dwarf attendants and several satellites of the giant outer worlds, including Neptune's major satellite Triton – only slightly smaller than the Moon - were almost certainly captured from independent orbits. The theory was popular for a time and in the middle part of the last century an Austrian researcher named H.S. Bellamy even suggested that it might have happened fairly recently (needless to say, this accounts for the destruction of Atlantis). But captures that are believed to have occurred all involve objects that are very small in relation to their captors, and as we have observed, the Moon is fairly large in relation to the Earth.

Another theory states that the Moon simply formed in Earth’s orbit from the same primordial material, but this model fails to explain why the Moon is less dense and deficient in iron in comparison to Earth.

The currently popular theory, put forward by American scientists W.K. Hartmann and D.R. Davis in 1974, proposes that an object about the size of Mars collided with Earth, and while the bulk of its mass including its iron core merged with the Earth, enough debris was ejected into space from Earth's mantle to form the Moon. The theory explains why the Moon is rather less dense than the Earth, as denser materials were not blasted into space by the impact. The theory is not without its problems, but seems to be the most plausible explanation put forward to date.

From the Earth to the Moon

As we have seen, some 59 percent of the Moon’s surface can be seen under various conditions from Earth. Not until the dawn of the space age was anything definite learned about the remaining 41 percent. In October 1959, the Soviet probe Lunik III made a fly-by of the far side of the Moon. Because the probe was out of radio contact with Earth as it passed behind the Moon's far side, the pictures it took could not be simply beamed back to Earth. Accordingly, film was automatically exposed and developed. As the probe emerged from behind the Moon, the developed film was imaged by a TV camera and the first blurry images of the Moon's hidden side were transmitted back to Earth. It sounds crude, and by today's standards it was, but it was a tremendous technical feat for the time.

As the Cold War ratcheted up tensions between East and West, so the Soviets continued to score an impressive succession of “firsts” in space, but the US was galvanised into a response and on 25 May 1961 President John F. Kennedy threw down his historic challenge:

I believe that this nation should commit itself to achieving the goal, before this decade is out, of landing a man on the Moon and returning him safely to Earth. No single space project in this period will be more impressive to mankind, or more important for the long-range exploration of space; and none will be so difficult or expensive to accomplish.

In May 1961, Alan Shepard had only just become the first American to fly in space, yet little over nine years later after the flight of Lunik III, men saw the Moon's hidden side with their own eyes as Apollo VIII made its historic circumnavigation of the Moon at Christmas 1968. Seven months later Armstrong and Aldrin became the first men to actually land there, realising Kennedy’s goal with less than six months to spare. The technological leap that made this possible might sound incredible, but it must be remembered that even the technology of Project Apollo was quite primitive by today's standards. It is a fact that the Eagle's on-board computer was actually far less powerful than that of a modern-day mobile phone! (I refuse to comment on conspiracy theories that the Moon landings were faked because it is patently obvious that the idea is absurd.)

At all events, the US won the race to the Moon. Not until much later did it emerge that early Soviet successes owed more to the genius of Chief Designer Sergei Korolev than to any superiority of communism over capitalism. But Korolev’s health had been ruined by a spell in the gulag during Stalin’s reign of terror and he died in 1966 during a botched operation to remove a tumour. With his death ended any hopes of perfecting the N1 booster with which he had hoped to put a man on the Moon. The race to the Moon lost, the Soviets turned their attention to establishing a near-permanent human presence in Earth orbit – which in the long run was of far more benefit than simply duplicating the efforts of the US.

When will people go back to the Moon? In 1972, when Cernan and Schmitt blasted off from the Moon's surface, it was said that nobody would be going back in the 20th Century. I did not believe this (I assumed that men would be on Mars before the century was out), but the public’s attention-span is short and after the Moon landing had been made, only the astonishing drama of Apollo XIII made the headlines (and, a quarter of a century later, an excellent if not entirely accurate Hollywood movie). NASA turned its attention to the Space Shuttle, setting back the manned exploration of space by decades. As an experimental proof-of-concept spaceship, there is no doubt that the Shuttle was a technological triumph. As a practical manned reusable heavy-lift system however it has been an unmitigated disaster that cost the lives of the crews of Challenger and Columbia. It was the latter tragedy that prompted President George W. Bush, in one of the very few highlights of his presidency, to announce what has since become known as Project Constellation, which will return humans to the Moon, and on to Mars – using designs that draw heavily from Project Apollo, albeit using hardware developed originally for the Shuttle.

A permanently inhabited base on the Moon should be established no later than the middle part of this century. When it is, one of science fiction’s oldest and most central themes will be a reality at last.

© Christopher Seddon 2007

Saturday, 1 December 2007