Gobekli Tepe's Pillars

Chapter 6: Comets vs Asteroids (from Prehistory Decoded, uploaded for Asteroid Day 2019)

Did a huge cosmic catastrophe occur around 10,900 BC that wiped out many large animal species, brought the world to its knees, and triggered a mini ice age? Regardless of myths and legends, this question can only be definitively answered by science, based on evidence and logical deduction.

We saw in Chapter 5 that geochemical evidence points very strongly towards this view, although precise details of the event are elusive. It is not clear what kind of object, or objects, caused the destruction, and the impact it had on climate, megafauna, and human cultures continues to be debated. Furthermore, the impact hypothesis has been challenged, mainly by Mark Boslough, on the grounds that an event involving multiple impacts spread across several continents even defies the laws of physics. This view contends that comets and asteroids orbit the inner solar system alone, without any accompanying debris. They cannot team up in pairs or swarms of fragments. This is an odd position to take, since we all know that the Shoemaker-Levy 9 impacts in 1994 on Jupiter were caused by a long train of fragments resulting from the splitting of the parent body. But Boslough denies this can happen within the inner solar system, and therefore to Earth.

       If he is correct on this point, then the implication is that the Younger Dryas impact event was likely caused by a single impactor – a single comet or asteroid. But to have caused the massive destruction implied by all the geochemical evidence, it must have been rather large – so large that we should expect to find an obvious crater along with shocked quartz. To date, none have been found. 

Then, there is the more general view that even one large impact event is unlikely to have occurred on Earth over the course of, say, the last 10 to 20 thousand years, because there are not currently enough objects of the required size in near-Earth space. The next most recent extinction event thought to have been caused by a cosmic event is the dinosaur-killing impact 66 million years ago. So, how could such an apparently rare event as the Younger Dryas impact event have occurred so relatively recently? And in any case, this proposition contradicts the uniformitarian view that is so popular in academia, even today. 

But, notice the assumption here: that near-Earth space has not changed appreciably over the course of human civilisation. Is it true that the number of large objects, i.e. asteroids and chunks of comet, that we observe right now near Earth has been the same for tens of thousands of years? Is it possible there were many more comet fragments, for example, 10 or 20 thousand years ago? What evidence is there for this? How quickly can Earth’s cosmic environment change? 

     The area of research that deals with these questions appears to be rather unpopular. Few are working on these problems. But one group of determined British scientists has made considerable progress, despite a relative lack of interest from research funding agencies. They are led by Bill Napier, an astronomer and cometary scientist, and Victor Clube, an astrophysicist, both formerly professors at the universities of Edinburgh and Oxford. Over the last forty years, their small group of British neo-catastrophists has pioneered a new view of Earth’s place in the solar system. Their vision is startling, and a wake-up call for the rest of us. But before I outline their theory of terrestrial catastrophism, which commonly goes by the name of ‘coherent catastrophism’, we should review a few basic principles first, namely the nature of minor bodies of the solar system, and their orbits.

Orbits and their Precession

The orbits of solar system planets are not circular. They are all very slightly elliptical, or squashed. Because of this, they all have a specific direction in which their orbits point. If we imagine a line connecting the two most widely separated points of an orbit, and call this the orbital axis, then for any given orbit this line could be pointing east-west or north-south, or any direction in between.

Neither do the planets all orbit in the same plane – the solar system is not flat as a pancake. If we use Jupiter’s orbit to define the plane of the solar system, as it is the most massive body (other than the Sun, of course), then all the other planets’ orbits are slightly inclined with respect to it. Imagine someone trying to hula several hoops at the same time – planetary orbits are a little bit out of kilter like this.

Nor is any orbit fixed for all time, because every orbiting body in the solar system is influenced by the gravity of every other orbiting body. As there are no exact mathematical solutions to this problem (for three interacting bodies or more) and as there are an unknown number of such bodies in the solar system, we cannot know the precise orbit of any solar system object indefinitely far into the future, just as we cannot forecast our weather on Earth indefinitely far into the future. Orbits are chaotic on very long timescales.

      The most stable orbits in the solar system belong to the most massive objects: the major planets, especially Jupiter. These are, therefore, the main gravitational perturbers of the orbits of smaller bodies, and usually it is sufficient to consider only their influence on other solar system objects. By taking the gravitational effect of the sun and these eight planets into account, the position of any other object orbiting the sun can then be forecast, either forwards or backwards in time, with computer simulation methods, just like making a weather forecast.

If only things were this simple. In addition to the gravitational effects of the sun and the major planets, an orbiting body is also subject to non-gravitational forces, such as viscous drag due to dust in the solar system. That’s right, space is not entirely empty – it is filled by very low-concentration dust and gas, which drags on any object that moves through it. We do not need to go into the physical details of each type of non-gravitational force – they are many and complex. It is sufficient to know that the magnitude of all these non-gravitational effects combined depends on the body’s surface area, whereas gravity depends on the body’s mass and therefore its volume. So, for very large bodies (planets and large asteroids) we can generally ignore these surface forces, because gravity wins out. Gravity dominates for large bodies. But, as an object’s size decreases, its surface area increases relative to its volume, which means these surface forces come to dominate the smallest bodies, especially microscopic dust.

We therefore know that the orbit a body follows depends largely on its size. A large dense body, such as a huge asteroid, will follow an orbit that is almost perfectly elliptical. But, the gravitational effects of the planets, especially Jupiter, will cause this elliptical orbit to evolve – it will not follow the same elliptical orbit for all time – it will precess. This means the direction in which its elliptical orbit points, defined by its orbital axis, changes very slowly.

      Two types of orbital precession are important for our story (see Figure 15). The first is apsidal precession, also called precession of the perihelion. Imagine a flower head at the top of a stalk with one elliptical petal that points in a particular direction. The outline of the petal represents an elliptical orbit. Now imagine this petal slowly rotating around the flower head, even though the flower head is held fixed. This is like an elliptical orbit slowly undergoing apsidal precession, even though the plane in which the orbit resides is fixed. Now imagine twirling the stalk between your fingertips so that the whole flower head, which is inclined, rotates. This is like another kind of orbital precession known as nodal precession, or precession of the longitude. Here, the plane in which the elliptical orbit resides slowly rotates.

Figure 15. Earth’s orbital plane is shaded dark grey. The orbit of an asteroid or comet is represented by the thick black line. Apsidal precession (upper arrow) causes the direction of its elliptical orbit, or axis, to rotate around the Sun within the same fixed plane, shaded light grey. Nodal precession (lower arrow) causes the entire orbital plane, in which the asteroid or comet’s orbit resides, to rotate around the sun.

The orbits of smaller bodies influenced by non-gravitational forces are even more complex. They too follow orbits that precess, but due to the non-gravitational forces acting on them they gradually become less elliptical, and more circular. The smallest particles, microscopic dust grains too small to see with the naked eye, are dominated by non-gravitational forces, and so their orbits are quite unstable. Depending on their size, they are either drawn in towards the Sun, where they are consumed, or are driven outward by the solar wind, generated by the sun, towards interstellar space.

Comets

Apart from the sun and its eight planets and their moons, there are a multitude of smaller bodies in the solar system. The most well-known and studied (other than Pluto and its moons) are the asteroids of the main belt that take nearly circular low inclination orbits between Mars and Jupiter. The main asteroid belt, a diffuse orbiting ring of asteroids, defines the outer limit of the inner solar system, comprising the terrestrial planets, Mercury to Mars. There are many thousands of large asteroids in the main belt, yet its total mass is only around half that of Charon, Pluto’s largest moon, and nearly one-third of that is in Ceres, the largest main belt asteroid with a diameter of around 950 kilometres. The asteroids are generally distinguished from comets by their composition, with asteroids being mostly dense rock and metal, with some organic compounds and a little ice. They are found mainly within the inner solar system.

       Comets, on the other hand, are generally composed of ices (a mixture of many volatiles, including water, carbon dioxide, carbon monoxide, methane, ammonia, etc.) and organic compounds, similar to oil and soot, with smaller amounts of rock and metals. They are like huge fluffy, dirty snowballs found mainly in the outer solar system or beyond.

       Comets form in the outer solar system beyond Jupiter where it is cold enough for these ices to condense from the very low concentration of vapour in space, like frost condenses on cold car windscreens overnight. The very outermost comets are thought to form a very diffuse spherical cloud, known as the Oort cloud, which surrounds the solar system out to vast distances, perhaps halfway to the nearest star. Although there could be billions or even trillions of large comets out there, we cannot see any of them from Earth even with our most powerful telescopes – they are just too small and far away. But, we know they must be there, because occasionally they are knocked into the inner solar system, where they can be seen.

If they are only slightly nudged, for example by the weak gravitational pull of a distant passing star or by the viscous drag of an interstellar dust cloud, comets in the Oort cloud can fall into the outer and inner solar systems. Those that, after many thousands of years of drifting inwards, eventually reach semi-stable orbits in the outer solar system, between Jupiter and Neptune, are called Centaurs. Those that enter the inner solar system are called period comets – with short (less than 200 years) or long (longer than 200 years) period orbits. Additionally, some are called sun-grazing comets if they approach the sun very closely. But these comets don’t tend to last very long – perhaps only a few thousand years before they decay completely to dust.

      The Jupiter family of comets are short-period comets whose aphelion (largest distance from the sun) is not far short of Jupiter’s. The aptly named Apollo and Aten objects are those short-period comets that cross Earth’s orbit, so that their perihelion (closest distance to the sun) is inside Earth’s orbit, while their aphelion (furthest distance from the sun) is outside Earth’s orbit. Because of apsidal precession, these orbits will eventually intersect Earth’s orbit, by definition. This will happen four times every complete cycle of apsidal precession – twice as the orbit precesses below Earth’s orbit and twice more as it precesses above (see Figure 15). Encke-type comets are those specific Apollo objects that have orbits similar to comet Encke, a comet of particular importance to our story.

Not all comets in the inner solar system have orbits that cross Earth’s; some remain entirely within Earth’s orbit while others remain entirely outside it, although eventually strong gravitational interactions with the terrestrial planets can cause them to become Earth-crossers.

Very rarely, both comets and asteroids can collide with each other, to produce fragments of, generally, a smaller size. The orbits of these fragments will depend on the details of the collision – they can be quite different to those of the parent bodies. Comets can also decay via outgassing if they are close enough to the sun. Typically, comets within the inner solar system, where it is warmer, will outgas, while those in the outer solar system, where it is much colder, will not. Outgassing is just the reverse of the process by which they formed. The volatile ices, warmed by the sun in the inner solar system, evaporate (or more precisely, sublimate) away into space.

       The closer a comet approaches the sun, the warmer it becomes and the more gas it releases. As a comet is held together by these volatile ices, when the ice evaporates other particles are also released, mainly dust and larger pebbles. Due to the dominance of non-gravitational forces, the smallest dust particles and gas released do not follow the same orbit as the comet; instead they appear to form the comet’s tail which points away from the sun due to being blown outwards by the solar wind. However, larger pebble-sized fragments released by a comet, which are not so strongly influenced by non-gravitational forces, can follow similar trajectories as the comet, only slowly diverging from it over many orbits. In this way, comets can form trails of larger stones and boulders. Observations of short-period Jupiter-family comets show that most of them have trails86. To be clear, comet ‘tails’ and ‘trails’ are quite different. Their tails are composed of gas and tiny dust particles and point away from the sun, while their trails are composed of pebbles and larger boulders and are spread out along the comet’s orbit.

Asteroids, being composed mainly of solid rock and metal, normally have neither tails nor trails, and they reflect sunlight reasonably well. They therefore have a fairly high albedo (reflectivity). Pristine comets arriving from the Oort cloud that have not yet come close enough to the sun to begin outgassing have surfaces mainly composed of ices and organic compounds. They too have a reasonable albedo. As the possibility of detecting a comet or asteroid directly via optical astronomy depends simply on its apparent size and albedo, which determines how much sunlight it reflects into our telescopes, the largest asteroids and pristine comets can be located with careful observations and not a little luck. The smaller or more distant the object, or the lower its albedo, the less chance of spotting it.

        Of course, when comets first enter the inner solar system and begin to form a tail as they are warmed, they become much easier to spot, as the tail can be immensely long with a high albedo. In fact, there are even historical reports of large comets passing close to Earth being visible in the daytime87. But, if a comet orbits within the inner solar system for a very long time, such as an Encke-type comet, or if one approaches the sun very closely, such as the sun-grazer comets, it can release much of its volatile surface ices and eventually lose its tail. It is thought the remaining organic compounds, along with remaining dust, at the comet’s surface can then form a dark and thick layer, like a coating of tarmac or pitch. This surface can have a very low albedo, making these particular ‘dormant’ comets unreflective and hard to pick out against the night sky, even if they are large. This is not to say the comet is completely de-gassed, and therefore dead or extinct. Rather, the surface layers only are de-gassed and quite unreflective, while the comet’s interior can remain in a reasonably pristine state. Quite strangely, dormant and extinct comets can be some of the darkest objects in the solar system88,89.

As observations of comets have steadily improved it has become clear they can also decay through more sudden and disruptive processes. Outbursts appear to be relatively common for Jupiter-family comets, perhaps occurring nearly every orbit near perihelion (when the comet is closest to the sun). It is generally thought these outbursts are caused by a build-up of gas pressure under the surface of the comet, which can form a consolidated crust, or skin, as it is warmed. Rather like a volcano, or tyre blowout, the pressure can build sufficiently to cause a sudden rupture in the comet’s surface, which later heals as its surface cools again when it moves past perihelion. A particularly spectacular outburst from comet Holmes occurred in 2007 near perihelion, causing the comet to brighten massively for many days90. Millions of tonnes of gas, dust and larger fragments were ejected at a wide range of velocities (up to 1,000 kilometres per hour for the smaller particles) in the shape of a conical explosion in the general direction of the sun.

       It is now known that comets can also undergo splitting, where they disintegrate completely into multiple large fragments91. Around twenty individual splitting events have been recorded for Jupiter-family comets within the inner solar system in the last few decades. Although the cause of these splitting events remains unknown, in most cases it cannot have been caused by tidal forces (where a small orbiting body is gravitationally stretched due to one side being closer than the other to a large body, like the sun) since the comets concerned were not close enough to the sun to be disrupted. Although it was Jupiter’s tidal force that split comet Shoemaker-Levy 9 before it crashed into Jupiter in 1994, a different mechanism must account for the splitting of Jupiter-family comets within the inner solar system.

For example, Figure 16 shows comet Schwassmann–Wachmann 3 after splitting into several dozen fragments in 2006, and a close-up of one of the fragments taken by the Hubble Space Telescope. The fragment upper left is trailed by a series of smaller fragments. Debris from outbursts and splitting events, like the debris from outgassing, will follow an orbital path depending on its size. The tiniest dust and gas particles form tails, while larger particles will add to, and thicken, the trail.

Very old debris streams that have been decaying for thousands of years can be very broad. Eventually, once the whole stream has completed an entire cycle of apsidal precession, a roughly circular ring, or doughnut, of debris is created which contains within it denser elliptical trails92, which themselves contain dense nodes or cores that contain the largest comet fragments. Often, it is difficult to distinguish between the large fragments and the ‘haze’ or coma of smaller debris and dust that surrounds them.

Figure 16. Left: The splitting of comet Schwassmann-Wachmann 3 in 2006, observed with the Spitzer Space Telescope (image courtesy of NASA/JPL-Caltech/W. Reach (SSC/Caltech). Right: Close-up of one of Schwassmann-Wachmann’s larger fragments, observed with the Hubble Space Telescope (image courtesy of NASA, ESA, H. Weaver (APL/JHU), M. Mutchler and Z. Levay (STScI)).

When Earth intersects a cometary debris stream a meteor shower is observed to radiate from a specific direction in the sky, depending on the apparent velocity of the stream relative to the Earth at the point of intersection. For a broad debris stream the radiant (i.e. the point in the sky from which a meteor shower appears to radiate) can appear to move across the sky each night as the Earth continues on its orbit. Very many meteor showers are known, including the Piscids, Taurids, Leonids, Orionids, Geminids, and so on. They are observed at different times of year, with a range of intensities, according to when the Earth intersects their particular debris stream.

Each Apollo-type comet can create a meteor stream observable from Earth, because all Apollo-type comets are Earth-crossers. Two notable examples include the Taurid meteor stream, which is thought to have arisen from disintegration of comet Encke93,94, or from a progenitor comet which ejected Encke’s comet as a large fragment thousands of years ago, and the Leonid meteor stream, which is thought to be caused by disintegration of comet Tempel-Tuttle. As denser regions of a comet’s debris trail precess, the intensity of a meteor stream observed from Earth can be seen to wax and wane over many centuries.

       The gas and smaller particles of dust to which the entire comet will eventually decay, unless it is flung out of the inner solar system via gravitational slingshot around a planet or it collides directly with a planet or large asteroid, will diffuse throughout the inner solar system. Each grain of dust reflects a little light, just like that in a dusty sunlit room, and altogether this dust reflects enough sunlight to be just visible to the naked eye at dawn or dusk under favourable conditions. This spectacle is known as the Zodiacal Dust Cloud, which forms a hazy triangle of light distinct from the Milky Way (see Figure 17). As the dust lies in the plane of the inner solar system, it appears to envelop the planets, which, as seen from Earth, move along a line in the night sky that projects from Earth’s surface, known as the ecliptic.

Figure 17. The Zodiacal dust cloud observed with ESO’s La Silla Observatory in Chile (image courtesy of ESO/Y.Beletsky).

Until about ten years ago, it was generally thought that the zodiacal dust cloud resulted mainly from collisions between asteroids in the main belt. Because the main asteroid belt is considered a very old structure – as old as the solar system – the zodiacal cloud was thought to be more or less unchanged over the same period. But recently it has become clear that this is wrong. Measurements of many kinds now show it is mainly composed of cometary dust, and that most of this dust must be produced by the decay of comets within the inner solar system. If the dust were mainly asteroidal we would expect to find a higher concentration of dust near the main asteroid belt. But space missions that have traversed the asteroid belt show this is not the case. Also, the dust swept up by Earth is found to be most similar to the dust collected directly from cometary comas by several space missions. Overall, around 90% of zodiacal dust in the inner solar system is thought to originate from Jupiter-family comets that decay entirely within the inner solar system.

With this understanding of how comets orbit and decay within the inner solar system it is easy to counter Boslough’s main point. He is quite wrong to suggest small bodies of the solar system always orbit alone. Perhaps he was thinking only of asteroids. Comets, on the other hand, and their trains, orbit the inner solar system in broad bands that generate the meteor streams we see on Earth. Within these broad bands are denser regions, which themselves contain clumps, or swarms of debris. Being an expert in impact physics, knowledge of cometary science is fundamental to his own research, so his position on this is surprising. Quite possibly, his Fellowship of the Committee for Skeptical Enquiry (CSE) had caused him to overreact to the Younger Dryas impact hypothesis and ignore the latest cometary science. The CSE, a private organisation, is dedicated to debunking unscientific claims of the paranormal, UFO kind, and includes among its fellows many famous scientists, such as Richard Dawkins and Steven Weinberg. In general, I support its aims, as I am a strong advocate of evidence-based scientific enquiry. But, for whatever reason, it seems to have become involved in the debate surrounding the Younger Dryas impact hypothesis, which is a long way outside its remit as this debate is proper science. But this just shows how contentious this matter is – any proposal that threatens the uniformitarian paradigm is considered by some to be so ridiculous that it is placed in the same ballpark as ghosts and fairies.

An encounter by Earth with a swarm of small comet fragments can be just as damaging as an encounter with a single large fragment, if similar amounts of energy are released by the collision. Consider, for example, the damage caused by a fierce sandstorm. As the energy released by a cosmic impact depends on the total mass of all the material encountered, it doesn’t matter whether the swarm consists of one large fragment or many small fragments. But the details of the encounter will differ, including any geochemical signals, because that energy is released in different ways at different heights in the atmosphere. Very large fragments, over a few hundred metres in diameter, will likely punch through the atmosphere and impact the ground, creating a crater and shocked quartz, while smaller fragments will generally explode as airbursts, leaving a geochemical calling card that depends on their composition and the height at which they explode. Provided these smaller fragments are spread out in a swarm broader than a single continent, then the kind of geochemical evidence so far detected at the base of the Younger Dryas black mat can be expected. Figure 16 suggests this scenario of collision with a fragmented comet train is entirely reasonable.

But what about Boslough’s other argument that we must address – are there sufficient large bodies in near-Earth orbits to have threatened Earth over the course of civilisation? How likely is an impact of the size suggested for the Younger Dryas event?

By the 1980s it had become clear near-Earth space was teeming with asteroids that posed a threat to Earth, and some of them were large enough to cause a global extinction event. Also, Alvarez et al.’s 1980 proposal of the dinosaur-killing asteroid had caught the public’s attention; the threat from space had become more than just an academic issue. Several astronomy projects across the world began focusing their telescopes on near-Earth space to try and catalogue the threat.

      By the early 1990s politics had got involved, with the US administration setting up the Spaceguard committees, tasked to report on the threat of near-Earth objects and what to do about them. David Morrison, formerly a Professor of Astronomy at Hawaii observatory and then a career scientist at NASA, chaired the ‘detection’ committee. His committee’s 1992 report to US Congress was very influential, and set the tone for future debates. It recommended significant funding for a coordinated search effort using new telescopes across the world. The focus was squarely on spotting near-Earth objects in Earth-crossing orbits which, at the time, were thought to mainly be asteroidal.

The 1994 Shoemaker-Levy 9 impact on Jupiter provided much-needed funding impetus, and within the last decade the Spaceguard committee’s initial aims have been realised. Their target of cataloguing 90% of near-Earth objects larger than 1 kilometre in diameter in Earth-crossing orbits, considered large enough to bring an end to civilisation if we encounter any of them, has been achieved. More recent plans aim to catalogue much smaller bodies in near-Earth space.

According to the best data we have from Spaceguard-linked searches95 using advanced telescopes, including some with thermal imaging cameras that can spot even very dark objects by detecting their weak heat signature, it appears an event of the size proposed for the Younger Dryas impact hypothesis is unlikely. From the relatively small number of objects detected larger than 1 kilometre in diameter (there are around 1,000 of them) it seems that the chance of a collision with an object of this size over the last 20,000 years or so is quite low. Not impossible, of course, just fairly unlikely – perhaps just a few per cent.

      But this conclusion only considers collisions with single large objects. It neglects entirely the possibility of collisions with comet swarms composed of smaller objects that might not yet have been spotted. And, moreover, it assumes near-Earth space has not changed much over this time. Crucially, we now know that this last assumption is almost certainly wrong. It appears that 20 to 30 thousand years ago, or perhaps more, a giant comet around 100 kilometres across entered the inner solar system, and by now has largely decayed to dust, leaving behind the Taurid meteor complex and comet Encke, as well as many other large bodies in Earth-crossing orbits.

The Taurids

The Taurids are a broad and diffuse meteor stream that, as seen from Earth, occur around early November, radiating from the direction of Aries and then Taurus. It is not currently the most intense or spectacular – there are many other meteor streams that can lay claim to that title. But the latest observations show that it is by far the most massive. It only appears weak because it is spread out so thinly in space, because it is so old. In fact, it has spread so much that the Taurids are thought to be only the main sub-stream of a much broader meteor complex that also includes several other meteor streams, seen from Earth at different times of the year and radiating from different points in the sky.

Although consisting mainly of small grains that create the commonly seen shooting star, the Taurids occasionally throw larger boulders at us that create significant fireballs96. These are sometimes called ‘Halloween fireballs’, and indeed, this association might be very ancient. The main night-time streams consist of two broad branches, the Northern and Southern Taurids, which appear respectively just above and just below the ecliptic, which describes the plane of the solar system in which the main planets orbit.

     Although the Taurids peak in the direction of Aries/Taurus in November, weaker influx from this broad trail can been seen for several weeks before and after. The Taurids also occur during the summer months, peaking around mid-June, in the daytime from the direction of Taurus and Perseus. These showers, the beta-Taurids and zeta-Perseids, are not usually visible to the naked eye, as they occur during daytime, but they can be detected by radar. They are actually the same Taurid meteor stream as the night-time ones – the different timing and apparent direction being due to Earth intersecting the Taurid stream twice at different points in its orbit88.

It has long been thought that these main Taurid meteor streams are associated with comet Encke93,94, which has a similar orbit. Almost certainly, they are the debris produced by fragmentation of comet Encke, or by its parent comet – its progenitor. In its current orbit, Comet Encke is occasionally just bright enough to be observed by the naked eye on a very clear night. Although of reasonable size, thought to be around 5 kilometres in diameter, it currently has only a weak tail, presumably because much of its surface is dormant and not outgassing. Perhaps its surface is mostly covered with a thick and dark crust. Encke, like the Taurid meteor stream more generally, resides in a low inclination orbit with high eccentricity – it is very elliptical. Most importantly, it is a short-period comet whose orbit straddles Earth’s. Its perihelion (closest approach to the sun) is close to Venus while its aphelion (furthest distance from the sun) is way beyond Mars – it is therefore also an Apollo object, which makes it a threat to Earth.

As for all small bodies within the inner solar system, Encke’s orbit precesses due to gravitational interactions with the major planets, especially Jupiter. Every time Encke is near aphelion, far beyond Mars, it is pushed or pulled a little bit by Jupiter. And the cumulative effect of this, summed over many orbits, is orbital precession. The direction in which its elliptical orbit points, its axis, completes an entire cycle of apsidal precession roughly every 6,000 years. This means that, because its orbit straddles Earth’s, their orbits intersect four times every 6,000 years.

       Of course, being a small object in the vastness of space, we do not expect a collision between Encke and Earth at these times, since both Earth and Encke can be anywhere along their orbits when their orbits intersect. But on average, if we didn’t know in advance where Encke is along its orbit, then four times every 6,000 years there would be a chance they might collide, albeit a very small chance.

But, of course, Encke is not just an isolated comet. Remember, it is probably surrounded by a halo of debris, as for most comets in the inner solar system, that has dispersed along its orbit. And remember too, that Encke is likely to be just one of the largest fragments embedded within a broad meteor complex, the Taurids, that contains thousands, or perhaps even millions, of Tunguska-sized fragments. So, while we do not expect to collide with Encke itself, there is a realistic prospect of colliding with another object within the Taurid meteor stream.

We know this, because we already encounter the Taurid meteor stream on a regular basis – we see the Taurids on Earth twice a year. But currently, we only encounter a weak and diffuse portion of the stream consisting mainly of small grains of dust and larger pebbles. But in time, denser filaments embedded within the larger complex that contain larger objects will undoubtedly precess into Earth’s path.

It was because of this periodicity in risk, and the likelihood of multiple impacts with many fragments at once, that the phrase ‘coherent catastrophism’ was coined to describe this kind of cosmic impact risk97,98. There will be certain extended periods when fireball activity associated with Encke and the Taurids is expected to peak. If any of these fireball swarms are sufficiently large and/or intense they could have cataclysmic consequences for Earth’s biosphere. The question, central to this book, is whether this was the cause of the Younger Dryas event?

Encke-like near-Earth Asteroids

A link between the Taurid meteor stream and comet Encke was proposed nearly seventy years ago by one of the founders of modern cometary science, Fred Whipple, a professor of astronomy at Harvard. He originally proposed that at least a portion of the Taurid meteor stream was formed by a fragmentation event involving Encke some 5,000 years ago, with another portion formed by a fragmentation event around 1,500 years ago93,94.

Then, in 1984 while working at the Royal Observatory in Edinburgh, Victor Clube and Bill Napier suggested that several large apparently asteroidal bodies might be linked with the Taurids99 – they might be outgassed or dormant fragments of an ancient progenitor comet. Of course, true asteroids cannot be linked with comets since they form in different regions of the solar system. But dormant comets with a thick crust that no longer sport a tail can appear rather like asteroids, and, indeed, it is now suspected that many near-Earth asteroids are actually dormant or extinct comets100.

Since then, several groups have studied patterns, or correlations, in the orbits of some large near-Earth bodies and meteor streams, and today it is accepted that most, and probably all, meteor streams can be linked with a comet, whether currently active or dormant. In many cases, these meteor showers are linked with an object that is currently designated as an asteroid, because it doesn’t appear to have a cometary tail. But these supposed asteroids are almost certainly dormant comets that over many years have fragmented and degassed to produce a meteor stream and corresponding asteroid-like large body. It appears the convention is that these cosmic bodies are classed as asteroids until proven cometary.

      But, this consensus has been achieved only in the last few years. Even as recently as 2007, it was still argued by some cometary scientists that most of these large near-Earth bodies associated with meteor streams are more likely to have originated from the main asteroid belt, and are therefore true asteroids. This view was reinforced by the apparent colour of these objects: many seemed to have surface colours resembling main belt asteroids more closely than comets101. But the truth is, we simply do not know enough about the surface chemical composition, and hence colour, of dormant comets to confidently assign them to any particular class of object, and therefore we cannot decide their cometary or asteroidal ancestry on this basis alone102.

The most notable contribution to this line of work for our story is Napier’s seminal 2010 paper Palaeolithic extinctions and the Taurid complex103. In it, he argues that an ancient and very large comet entered the inner solar system, probably over 20,000 years ago, and has since decayed largely to dust, leaving behind the Taurid meteor complex along with a host of large, dormant comet fragments and comet Encke.

He searched the database of all known near-Earth objects, which has been compiled from many Spaceguard-related telescope searches, and selected from it only those objects whose orbits have a similar size, inclination and eccentricity to comet Encke. He then examined the ‘longitude of perihelion’ of all these objects. Suppose you were to look down vertically at the solar system so that you were no longer aware of the height of any orbit. In other words, suppose you ‘projected’ all orbits onto the plane of the solar system. You can then choose a reference direction in this plane, with the sun at the centre, and measure the angle the perihelion of an orbit (the point on an orbit closest to the sun) makes with that reference direction. This is the longitude of perihelion of the orbit.

      Now, if the objects shortlisted by Napier are unrelated to comet Encke, there should be no pattern, or correlation, in their longitudes of perihelion. They could have originated from any direction in the solar system, and we should find a nearly even distribution of longitudes for these objects. But this is not what he found. Instead, he found that, of the brightest, and therefore presumably largest, twenty of these objects, nineteen of them (including Encke) have similar longitudes of perihelion. I have repeated his analysis using the most up-to-date database of Apollo asteroids, and his conclusion continues to hold (see Figure 18).

Figure 18. Longitude of the perihelion of the 20 brightest ‘asteroids’ in Encke-like orbits. Although longitude is unconstrained in this search, we see that it is clustered around 190 degrees – indicating these bodies are related. Encke is the open circle at about 161 degrees.

Importantly, the probability of this happening by pure chance, assuming these bodies are unrelated to Encke, is extremely small. In fact, it is easy to calculate that this scenario has a probability of around 1 in 1.6 million of occurring by pure chance. As this is so small, we should instead conclude this arrangement of longitudes has almost certainly not occurred by pure chance, and that most of these objects are ‘genetically’ related. In other words, they are the fragmentation products of an ancient parent, or progenitor, comet, as this is the only known reason why their perihelia can have similar longitudes.

      Furthermore, because Encke is known to be a comet, this implies that most of the eighteen objects in this list are also comet fragments. It is quite satisfying that most of these objects are also associated with major meteor showers seen on Earth, practically confirming their cometary status. We can therefore be very confident that many large bodies in Encke-like orbits are the dormant comet progeny of a larger parent body.

Typically, the large objects featuring in Figure 18 are estimated to be in the range of one to ten kilometres in diameter, with the largest being Hephaistos at around six kilometres. But this assumes they are asteroidal (except comet Encke, of course), and therefore have a high albedo. However, given that they are very likely dormant comet fragments, which will tend to have dark pitch-like crusts with low albedos, they might be substantially larger than conventionally thought.

Centaurs

Having seen that evidence points strongly towards the trapping of a giant comet within the inner solar system over 20,000 years ago, which has since decayed to generate the very broad Taurid meteor complex and lots of dust, we should check how likely such an event is. Because if it is found that this scenario is extremely unlikely, it suggests our deductions are probably incorrect and the evidence has been misinterpreted. We would need to search for other, more likely explanations for the Younger Dryas event.

But, before attempting to resolve this issue, rather than repeatedly referring to this putative giant comet as ‘the progenitor of comet Encke’ or another cumbersome phrase, let’s instead give it a short and convenient name. Celestial bodies are generally named after gods of one sort or another, or is it the other way around? In any case, in the circumstances, it should probably have a name with malevolent overtones. There are plenty to choose from. For the sake of argument, let’s choose Satan. It seems appropriate, as Satan and Apollo (or the Greek Apollyon, i.e. Abaddon the Destroyer, the Angel of Death) have been linked in several religious texts.

The existence of the main asteroid belt between Mars and Jupiter has long been known. By the mid-19th century hundreds of asteroids had been discovered, and that figure now stands at hundreds of thousands. So, it is understandable that the cosmic impact threat to Earth originally focussed on asteroids deflected from the main asteroid belt.

Comets, of course, have been known for far longer than this. Chinese records of comet sightings date back to at least the first millennium BC, and ancient Greek astronomers might even have recorded a cometary splitting event104. Over the millennia comets have been viewed with fear and dismay by many cultures – they are the ‘harbingers of doom’. But modern scholarship generally attributes this attitude to superstition and religious excess rather than a deeper knowledge of their role in the solar system, or ancient lore handed down through the generations.

Despite frequent sightings of comets over the millennia, a good understanding of how comets are formed, and therefore where they come from, is much more recent. By the middle of the 20th century it was realised there must be a reservoir of comets beyond Pluto to account for their regular appearance in the inner solar system. Today, it is known they originate from far beyond Pluto in the Kuiper belt, scattered disc and Oort cloud. These are really different regions of one very diffuse and extended structure; the disc-like Kuiper belt, rather like the main asteroid belt, closer to Pluto, becoming the spherical Oort cloud far beyond the solar system, with the broadening scattered disc in between. 

At these great distances, far beyond Pluto, comets are only weakly bound to the sun, and can easily be knocked inwards into our planetary system. Most will simply pass through the solar system on very long-period orbits. But a few will pass close enough to one of the main planets, especially the massive outer planets, to be captured into shorter-period orbits. Those that come to reside eventually between Jupiter and Neptune are known as Centaurs.

The first Centaur discovered, Hidalgo, was spotted in the 1920s, but it was not realised they form a distinct population of solar system bodies until the 1980s. Now there are thousands of Centaurs known over 1 kilometre in diameter, with an expected population of tens of thousands.

Because their orbits cross those of the massive outer plants, their orbits are unstable and they will not remain as Centaurs for long. This means the pool of Centaurs we currently observe must be relatively young, and must also be replenished by other comets falling inwards from the Kuiper belt and beyond. Depending on the specifics of their paths, Centaurs can either impact an outer planet directly, like famous Schumacher-Levy 9, or be flung via gravitational slingshot back into deep space again, or, if we are particularly unlucky, they can be slowed down enough by a close encounter with Jupiter to begin orbiting within the inner solar system. Effectively, Jupiter is the gatekeeper to the inner solar system.

But this possibility has only been realised relatively recently. Napier and Clube, in their seminal The microstructure of terrestrial catastrophism paper of 1984, first described the scenario whereby Centaurs can arrive in the inner solar system from unstable orbits in the Jupiter-Neptune region99. They specifically identified large Centaurs, or comets, such as Hidalgo (40 km diameter) and Chiron (around 200 km diameter), and suggested one of these is likely to adopt a dangerous Encke-like orbit every few hundred thousand to few million years.

Later, more detailed studies confirmed this scenario by performing orbital simulations for Chiron, one of the largest known Centaurs, finding its orbit to be quite unstable with a significant probability (37%) it had been captured, and then released, from a short-period orbit within the inner solar system in the last 100,000 years or so105. The nature of orbital calculations, like weather forecasting and modelling of other chaotic systems, is difficult over such long time periods, and therefore only probabilistic conclusions can be made. Nevertheless, this work suggests that a large fragment of Chiron could have remained in the inner solar system, or that another comet like Chiron could become trapped within the inner solar system in the not-too-distant future. Indeed, the Kreutz group of sun-grazing comets likely originated from the disintegration of a giant comet, around 100 kilometres in diameter, that adopted a sun-grazing orbit within only the last 2,000 years106. They are disintegrating so rapidly because they approach the sun so closely.

Although most Centaurs are much smaller than Chiron, there are around ten times as many Centaurs with sizes between 100 and 200 kilometres, than over 200 kilometres. This suggests, if their orbits are as unstable as Chiron’s, that we should expect to observe a large (over 100 kilometres in diameter) Centaur adopt an Earth-crossing orbit every 10 to 100 thousand years102. It seems Clube and Napier’s original 1984 estimate was conservative – the threat is actually greater than they thought.

Given this situation, what should we observe in the inner solar system? What can we expect to see right now? It all depends on the distribution of Centaur sizes, the rate at which comets decay to zodiacal dust within the inner solar system, and the rate at which this dust clears. But, because the mass distribution of Centaurs is top-heavy – most of their mass is in the largest Centaurs – provided large Centaurs fragment and decay faster than they arrive in the inner solar system, which is thought to be correct, we should expect a fluctuating, or flickering, scenario.

      In other words, although we can expect a steady trickle of smaller Centaurs to enter Earth-crossing orbits within the inner solar system, these hardly matter. It is the entry of very large Centaurs that dominates everything, from the zodiacal dust we observe to the risk we face on Earth. When a large Centaur becomes trapped within the inner solar system we should expect to see a cascade of fragmentation and splitting events that massively increases the risk of impact to Earth. But as the consumption or elimination of zodiacal dust from the inner solar system is somewhat slower than the process of comet fragmentation, we can later expect this impact risk to reduce substantially, and instead to see a rather massive zodiacal dust cloud remaining in the inner solar system accompanied by a steadily dwindling number of genetically related smaller fragments.

This is precisely what is actually observed right now, and it also explains the apparently higher-than-expected impact rate over the last few tens of thousands of years. Essentially, our modern understanding of the centaur population of comets is entirely consistent with current observations of dormant near-Earth comets and the massive Zodiacal dust cloud.

In fact, it seems the risk to civilisation from the largest Centaurs currently orbiting beyond Jupiter is far higher than the risk posed by all the asteroids currently inside Jupiter’s orbit102. Clube and Napier pointed this out decades ago, but it seems their warnings have largely fallen on deaf ears. Just as with the Younger Dryas impact hypothesis, their view requires the uniformitarian paradigm to be abandoned, and this was always going to be a tough fight because of the implications for pretty much all of academia. And many people have invested their careers, and millions of dollars of taxpayer’s money, in the search for asteroids, and they do not welcome the possibility that they have overlooked the main cosmic threat to civilisation.

Clearly, given the known population of Centaurs, and the nature of their unstable orbits, Satan is unlikely to be the only major demon to have terrorised Earth over the duration of human development, although it is probably the only one since the end of the last ice age. Orbital calculations based on a large sample of the known orbits of Centaurs indicate that Centaurs of all sizes are expected to become Earth-crossers at a rate slightly greater than one per millennium107, although, of course, most of these will be relatively small. Nevertheless, it is quite clear from all this detailed work that Satan’s appearance in the inner solar system in the last 20,000 years or so is not unreasonable, but instead is to be expected.

Clube and Napier’s early work in 1984 showed considerable foresight and appears to have been confirmed in almost every detail by the latest cometary science. However, one of their early predictions does not appear to have been borne out. They originally suggested that along with Encke and the Taurids, the progenitor of the whole stream (Satan) likely remains hidden within it, unnoticed due to its dark and inactive surface with very low albedo. But, the latest observations from the NeoWISE mission95, which uses a space telescope to search for orbiting bodies in the infrared and can therefore spot large comet fragments even if they are very dark, and other Spaceguard surveys, suggests this is unlikely. It appears most of the large bodies in near-Earth orbits have already been found. Despite this, the continuing existence of a large and very dark Satan cannot be ruled out, since every NeoWISE detection is only confirmed when also checked at normal (optical) observational frequencies. If Satan is too dark to be picked out visually, despite its supposedly large size, it might remain hidden still, despite the latest efforts.

Satan

It is now accepted that many, and probably all, of our major meteor showers are caused by fragmenting comets. It is also clear that many large Taurid objects in Encke-like orbits are ‘genetically’ related to Satan, formerly a very large Centaur, and are responsible for many of these meteor showers.

So, what was Satan like – in particular, how big was he? It is important to know this because the larger the comet, the more fragments it will produce, and the more dangerous it will have been.

There are two main methods for estimating the size of Satan, as it entered the inner solar system. The first involves the zodiacal dust cloud, since it is known that comets that decay within the inner solar system, like Encke, decay into the dust which forms this cloud108. By estimating the mass of the cloud, we can infer the size of Satan. The second method involves modelling cometary decay. If we know the rate at which comets decay in the inner solar system, and if we also know how long ago Satan entered the inner solar system, then we can back-calculate its original size based on the size of the remaining fragments. Let’s look closely at the zodiacal cloud method first.

If we assume that, say, half of all the current mass of the zodiacal dust cloud resulted from the decay of Satan alone, with the other half coming mainly from other cometary sources, then we can estimate Satan’s original size. This is a reasonable assumption because of the correlation in the orbits of Encke and its relatives, and because it is known that most of the mass of the Centaur system is contained in the few largest bodies. The current mass of the zodiacal cloud is consistently estimated to be somewhere between 10 and 100 thousand billion tonnes102,103,108. Using this range of values, and an estimate for the density of cometary material of 0.5 grams per cubic centimetre (assuming a ‘fluffy’ ice-ball with half the density of liquid water), and an estimate that half of a comet’s mass is due to dust with the other half being volatile ices, which don’t contribute to the zodiacal dust cloud, we obtain an estimated diameter between 35 to 75 kilometres for Satan originally. Adding the current inventory of mass within the Taurid meteor stream makes very little difference to this value. 

Now, this is a very big comet – far bigger than any currently known in the inner solar system. But even this must be an underestimate – it is a lower limit. The reason is that over the decay lifetime of Satan some of the dust it produced will already have been lost from the zodiacal dust cloud, either by falling into the Sun or by being blown outwards into outer space by the solar wind. Therefore, to get a better estimate of Satan’s original size we would need to know when Satan entered the inner solar system, the rate at which Satan decayed into dust, and the rate at which dust leaves the zodiacal cloud.

What is known about the first issue? When did Satan enter the inner solar system? This is very difficult to know accurately, but we do have a good clue to work with. Recall the remaining large fragments found by Napier that are almost certainly related. If we know how quickly they move away, or disperse, from each other, then we can work out how long ago they separated. This kind of calculation requires detailed orbital simulations using computational methods that take into account both gravitational and non-gravitational forces, since as active comets they will be subject to significant forces due to outgassing. Although there is a fair degree of uncertainty in these types of simulation, because the non-gravitational forces are difficult to know accurately, it has nevertheless been calculated that Satan likely split around 20 to 30 thousand years ago, and possibly longer if these fragments were less active than assumed109. Without any cometary activity at all they must have split from each other around 100,000 years ago to achieve the current separation seen in Figure 18. If a lifetime of at least 20,000 years for these comet fragments is considered, it indicates Satan was originally extremely massive, and much larger than the 35 kilometres found so far as a lower limit.

      The next issue to consider is the decay rate of comets in the inner solar system. Again, this is a difficult issue, but a recent model, based on observations of the decay rate of small comets within the inner solar system, predicts that the diameters of comets in Encke-like orbits decrease at a rate of about 3 kilometres per 1,000 years110. Although this model is relatively recent, it is quite simple, and has only been calibrated by observations of the decay of relatively small comets, up to 10 kilometres in diameter. As Satan would have been much larger than this, we must tread cautiously – it is always dangerous to use models beyond the bounds of their calibration. Other models of cometary decay use a slower rate of 1 kilometre per thousand years. Taking an average view of 2 kilometres per thousand years suggests each fragment that split from Satan could have been in the region of 40 to 60 kilometres in diameter when Satan ruptured over 20,000 years ago. If, like Humpty Dumpty, we ‘put these fragments back together again’ we arrive at a giant comet in the region of 130 kilometres or so in diameter.

Although this is very large, it is quite feasible. Several Centaurs of this size, or larger, are currently known in the outer solar system, including Pholus (190 km), Chiron (230 km) and Chariklo (260 km), although the dimensions of these Centaurs are uncertain. Napier, who is the leading expert on this issue, suggests Satan (he doesn’t call it this, by the way) is likely to have been around 100 kilometres in diameter103. This is entirely reasonable according to the arguments presented here. Essentially, the current massive zodiacal dust cloud suggests a comet of diameter at least 35 km, and possibly over 100 km, entered the solar system a few tens of thousands of years ago. This view is supported by the very old and broad Taurid meteor stream and the remaining massive zodiacal dust cloud that can be viewed as its fossilised remains.

The fall of Satan

Given that it appears very likely that a giant comet well in excess of 35 km in diameter, and more likely in the region of 100 km in diameter or more, did become trapped in the inner solar system at least 20,000 years ago, what are the expected consequences for Earth?

Again, Clube and Napier first outlined this scenario in their seminal 1984 publication. They suggested the signals of Satan’s presence should be observed in the geological record, including in ice cores. Since this time, Napier and Clube’s position has hardly changed, and the detailed evidence appears to be converging in their favour.

Coming only a few years after the major contributions of Alvarez and co-workers to the theory of an asteroid impact that killed off the dinosaurs, one might expect their work would have received more attention than appears to be the case, since it provided a potential mechanism for the dinosaur-ending calamity, beyond simply the idea of a huge random or ‘rogue’ asteroid. But, there appears to be a built-in scepticism towards the notion that comet-induced catastrophes can occur, especially over the course of civilisation. Since this time, Napier and Clube, along with their long-time colleagues, have patiently built their case97,98 in the face of criticism from some quarters, notably David Morrison at NASA who chaired the 1992 Spaceguard committee. They should be applauded for their gumption, as relatively few among the astronomical community, or even among the specialist cometary science community, appear to have been either as interested or determined as they have been.

      David Morrison’s reasons for objecting to their theory of coherent catastrophism seem, to me, to be misguided. Perhaps his scepticism is so strong because he is also a Fellow of the Committee for Skeptical Enquiry (CSE), just like Boslough, and to him Napier and Clube’s efforts seem too similar to those of Velikovsky, whose name and legacy is like poison within some academic circles. By the time of the 1992 Spaceguard report to Congress, Napier and Clube had already published several papers that described their theory of coherent catastrophism, and two books, The Cosmic Serpent and The Cosmic Winter, that expanded on their view and attempted to reinterpret historical events in its terms. But the Spaceguard report appears to have ignored their ideas by focusing on the threat from asteroids within the inner solar system. Despite the latest cometary science that suggests the Taurid meteor complex and Centaurs in the outer solar system are a much greater risk, hunting for rogue asteroids in near-Earth space appears to remain NASA’s prime strategy.

Even with Clube and Napier’s efforts, it has proven very difficult to predict the consequences for Earth of the entry of Satan into the inner solar system. The reason is that the expected number and magnitude of collisions of cometary fragments with Earth over this time depends sensitively on how Satan actually fragmented, and we know so little about this process for giant comets like Satan. For example, suppose Satan only decayed through outgassing. In this scenario, the only large comet fragment produced is Satan himself. Now imagine a different scenario where Satan instead split into 1,000 different large fragments early in his life, which then each proceeded to decay only through outgassing. In this scenario, the probability of a collision with Earth is 1,000 times larger, although the magnitude of any collision event is 1,000 times smaller. Clearly, the fragmentation pathway, or ‘tree’, is extremely important in assessing the threat to Earth. Although it is likely that Satan split into many major fragments at least 20,000 years ago, beyond this initial splitting we know practically nothing about the detailed fragmentation pathway.

      Fortunately, we do know the probability of a collision with any given fragment. A simple formula, derived by the Estonian astronomer Ernst Opik in 1951 (grandfather to Lembit Opik, the former Liberal Democrat MP), allows calculation of the annual impact probability of an object in any Earth-crossing orbit111, like Encke’s. Modern studies based on precise orbital calculations have shown that Opik’s formula is pretty accurate under most situations. The value predicted by Opik’s formula for an Encke-like orbit is an average collision rate of 1 every 200 million years. This means that if there are 10,000 objects orbiting in Encke-like orbits for 20,000 years, we should expect to experience a collision with one of them over that time. Our main problem, then, is estimating a reasonable fragmentation sequence for Satan.

To date, the most detailed study of the potential consequences of this current period of coherent catastrophism, caused by Satan, is by Napier103. He used Opik’s formula and a simple model of Satan’s fragmentation tree to estimate the likely frequency of impacts of Earth with large comet fragments. His fragmentation model goes something like this. Assume Satan’s initial size is 100 kilometres, and that Satan undergoes 1,000 fragmentation events, each producing 10,000 fragments, over 20,000 years. If Satan is depleted after this time, then each fragment is roughly 500 metres in diameter, ignoring losses as dust during fragmentation. Therefore, if we assume a total population of 10,000 of these fragments for the entire lifetime of Satan, i.e. at least 20,000 years, we can expect a collision of Earth with one of them over this time. A fragment of this size is thought to be equivalent, in terms of impact energy, to around 100 Tunguskas, or equivalently 100,000 Hiroshima bombs, i.e. a 1000 Megaton event.

      Now, quite probably, this is a conservative estimate because Encke-like comets with diameters around 500 metres are expected to survive for about 250 years110, according to the estimated rate of cometary decay we are using, gradually turning to dust over this time. In Napier’s scenario, a fragmentation event occurs every 20 years. Therefore, the population of fragments is likely to be 250/20 = 12 to 13 times higher than assumed. Consequently, perhaps 10 events of this magnitude, and not just one, can be expected.

Alternatively, suppose Satan undergoes 1,000 fragmentation events, each producing 1,000 fragments, over 20,000 years. If Satan is depleted after this time, then each fragment is roughly 1 kilometre in diameter, ignoring losses as dust during fragmentation. Therefore, assuming a total population of 1,000 of these fragments for the entire lifetime of Satan, i.e. 20,000 years, Earth can be expected to collide with one of them with a probability of 0.1, or 10%, over this time. As before, this will be an underestimate, since each fragment is expected to survive, this time, for about 500 years. This means the total population of these fragments is actually 500/20 = 25 times greater than suggested, indicating that Earth can be expected to collide with one or two of them over the period of 20,000 years. A fragment of this size is thought to be equivalent, in terms of impact energy, to a 10,000 Megaton event.

Considering that Jupiter-family comets seem to undergo frequent fragmentation events, perhaps with each orbit when they are closest to the sun, and that Encke’s orbital period is just over 3 years, these kinds of estimates are not unreasonable. They suggest we can expect perhaps 10 events of around 1,000 Megatons, and possibly one or two events of 10,000 Megatons, to have occurred over the last 20,000 years. This agrees with the range suggested by Napier, and is on the scale of that proposed for the Younger Dryas event. Note that although this scenario only takes into account the initial splitting of fragments from Satan, each fragment is expected to split further, and therefore the proposed collision events need not be caused by a single body. In other words, each fragment can actually be viewed as a swarm of comet debris.

      This analysis lends strong support to the notion that an event of the size proposed by the Younger Dryas impact hypothesis is entirely reasonable given Satan’s presence in the inner solar system, which is itself entirely reasonable given the known population of Centaurs between Jupiter and Neptune. We can therefore be very confident that the view, stated in very strong terms by Boslough, that such an event is extremely unlikely to have ever occurred in the history of the universe (and even defies the laws of physics!) is plain wrong112. In fact, the truth is quite the opposite; it would be surprising if no event of this scale occurred over the course of civilisation.

Because the number of comet fragments orbiting within the Taurid meteor stream of a given size increases rapidly with decreasing size, it immediately follows that we can expect many more collisions with smaller fragments. We should therefore expect the archaeological record to be chock-full of encounters with smaller pieces of Taurid debris, which are more likely to have occurred as airbursts. This means we should not be surprised to see geochemical evidence of Tunguska and super-Tunguska-like events at a wide range of depths in sediments, but with a more local extent than the Younger Dryas event, i.e. national rather than continental in scale.

Likewise, we should expect to find numerous but more local effects on the biosphere, such as felled forests with aligned trunks, frequent bottlenecks in animal populations (for which evidence can be found in the DNA of surviving populations), the occasional extinction, and a few cases of civilisation collapse. We should also expect to see major climate change events recorded by ice cores, as well as other geological anomalies such as massive landslides and mega-tsunami. Importantly, all these signals of cosmic catastrophe can occur without the creation of obvious craters or shocked quartz. It seems a new scientific discipline is needed that investigates the effects of collisions with cometary debris, and not just hard, dense asteroids. This view is completely consistent with the evidence for multiple black mats found at different levels in sediments, as discussed in the previous chapter.