Hot Science

Understanding the Threat to Earth from Asteroids and Comets

Andrew May

• Publisher: Icon Books
• Year: 2019
• ISBN:
  9781785784941
• Topic: Physical Sciences
• Subtopic: Astronomy & Astrophysics

As end-of-the-world scenarios go, an apocalyptic collision with an
asteroid or comet is the new kid on the block, gaining respectability only in
the last decade of the 20th century with the realisation that the dinosaurs had
been wiped out by just such an impact. Now the science community is making up for lost time, with
worldwide efforts to track the thousands of potentially hazardous near-Earth
objects, and plans for high-tech hardware that could deflect an incoming object
from a collision course – a procedure depicted, with little regard for
scientific accuracy, in several Hollywood movies.Astrophysicist and science writer Andrew May disentangles
fact from fiction in this fast-moving and entertaining account, covering the
nature and history of comets and asteroids, the reason why some orbits are more
hazardous than others, the devastating local and global effects that an impact
event would produce, and – more optimistically – the way future space missions
could avert a catastrophe.

1

ASTEROID APOCALYPSE

In the popular imagination, an asteroid or comet impact is one of the top ‘end of the world’ scenarios. It’s a relative newcomer to the field, only having entered mainstream consciousness in the last decade of the 20th century – thanks in large part to the Hollywood films Armageddon and Deep Impact – but it took hold quickly and has stayed with us ever since.

If the gold standard of cliché-dom is to be satirised in The Simpsons cartoon series, then cosmic impact makes the grade. It’s right there in episode 492 from 2011. Bart says: ‘No matter what we do, an asteroid is going to wipe us out. So we should party hard and wreck the place!’ – to which Homer replies: ‘Yeah, why should the asteroid have all the fun?’

Popular culture aside, impacts from outer space really do pose a serious, ever-present threat. There are thousands of asteroids travelling on orbits that cross our own – and any of these could, in theory, end up in exactly the same place as the Earth at the same time. The same is true of many comets – and new comets are falling into the inner Solar System all the time. From a cosmic perspective, impacts aren’t a rarity – they’re business as usual.

It’s happened before …

Let’s start by looking at the Moon. Its appearance has been shaped almost exclusively by impact events. Aside from the thousands of obvious impact craters, virtually all its other visible features – such as mountain ranges and the large dark areas that are misnamed ‘seas’ – were caused by impacts too. So why has the Moon suffered from cosmic collisions so much more than the Earth?

The answer is that it hasn’t. The Earth has suffered just as much as the Moon, but various atmospheric and geological processes – not to mention the water that covers almost three-quarters of the planet – conspire to cover up the evidence over the course of centuries. Today, there’s only one easily recognisable impact crater on Earth, the kilometre-wide Meteor crater (also known as Barringer crater) in Arizona. It’s relatively fresh-looking (see page 60) because it’s less than 50,000 years old – the blink of an eye in the planet’s 4.5-billion-year history. However, if you know the signs to look out for, the Earth has plenty of other impact craters. They’re generally older, and sometimes larger – much larger.

Chicxulub crater, for example, sprawls for almost 200 km across the northern tip of Mexico’s Yucatan peninsula and out into the Gulf of Mexico. It’s the relic of a catastrophic collision that occurred 66 million years ago, when a rocky object about 10 km across plummeted into the Earth from outer space. Now that’s a big chunk of rock, and it would obviously have caused tremendous destruction on a local scale. On the other hand, it’s not really that big compared to a planet that’s almost 13,000 km in diameter. So the Earth as a whole would have hardly noticed the impact, right?

Wrong. The Chicxulub impactor was travelling at around 20 km/s (kilometres per second), which translates to an enormous amount of kinetic energy. When it hit, all that energy was transferred to the Earth in the form of a huge explosion. The biggest explosion that most people can visualise is the one that destroyed the city of Hiroshima in 1945. So let’s try to imagine something five billion times worse than that, all concentrated in a single instant and at a single point on the Yucatan peninsula. That was Chicxulub.

The impact produced a huge cloud of dust and ash which enshrouded the planet and changed the global climate for centuries to come. The result was the extinction of around 75 per cent of all plant and animal species then living on Earth. Chicxulub’s most famous victims were the dinosaurs – those giant vertebrates that had dominated the biosphere for 150 million years. It wasn’t the first time something like this had happened. The dinosaurs themselves came to prominence in the wake of an earlier mass extinction, and there were at least three others prior to that. They weren’t all necessarily caused by impact events – there are other possible causes – but it’s likely that at least some of them were.

If a 10-km object can cause so much devastation, what about a 1-km one – or even 100 metres? That’s not going to wipe out whole species, but it could still be enormously destructive. The object that created Arizona’s Meteor crater was only about 50 metres in size. It doesn’t need much imagination to visualise what would happen if a similar object hit a densely populated city today. An object of similar size – probably slightly larger – entered the atmosphere over Russia in 1908. This one exploded at high altitude, so there wasn’t any crater, but the resulting fireball scorched a huge area of forest, and the blast flattened millions of trees for a radius of 30 km. By a stroke of luck, this happened over the sparsely populated Tunguska river valley, so there were virtually no casualties. If the same thing had happened over Moscow, 3,600 km to the west, it would have been a different story.

In earlier times, before the notion of cosmic impacts was fully understood, an event like Tunguska would probably have been mis-recorded as some other type of natural disaster, such as an earthquake. Did people notice any correlation between such events and ‘things seen in the sky’? There has always been a strong association between comets and impending disaster, though more likely this was out of pure superstition.

In many cultures, a comet was once the archetypal ‘bad omen’. To give just one famous example, a bright comet appeared in the year 1066, shortly before the Norman invasion of England. One of the scenes in the Bayeux tapestry – created by the Normans after their victory – depicts the English king Harold being warned about it. His fate was sealed; the comet was taken as a certain portent of defeat (conveniently ignoring the fact that the Normans would have seen the comet too).

Halley’s comet depicted in the Bayeux tapestry.

In hindsight, we now know that the comet of 1066 was Halley’s comet – the largest and most spectacular of the regularly appearing ‘short-period comets’. Its orbit brings it back to the inner Solar System time and again, roughly every 76 years. It has been seen since antiquity, but the fact that each appearance was the same object was only worked out after Isaac Newton developed his theory of gravity in the 17th century. Among other things, this explains how objects in the Solar System all move on regular orbits.

Newton was a professor of mathematics at Cambridge University, but it was a professor at Cambridge’s great rival, Oxford, who first demonstrated the tremendous predictive power of Newton’s theory. This was Edmond Halley, from whom the famous comet takes its name. After observing a comet in 1682 and calculating its orbit, he realised that it was exactly the same comet that had been recorded on at least two previous occasions, in 1531 and 1607. He extrapolated the orbit forwards to work out that the comet would appear again in 1758 – which it duly did, 17 years after Halley’s death.

This new understanding hardly made cometary paranoia go away – it just changed its nature. To quote Carl Sagan and Ann Druyan:

The brand of mischief that comets are said to bring – flood, darkness, fire, rending the Earth asunder – changes with time and astronomical fashion. But the association of comets with catastrophe remains curiously steady through the generations.

Even Halley wasn’t immune. In 1694, he presented a paper to the Royal Society called ‘Some considerations about the cause of the universal deluge’. Despite wrapping it up in the technical-sounding term ‘universal deluge’, what he was talking about here was nothing other than the Biblical Flood. In his paper, he ascribes this to ‘the casual shock of a comet or other transient body’ – using the word ‘casual’ in its original sense of occurring by chance.

For modern readers, the reaction to any talk about the Bible outside a theological context is likely to be a rolling of the eyes – but Halley was a product of his time. Nevertheless, his description is surprisingly modern, even hinting at two of the currently accepted effects of impacts: tsunamis (‘the great agitation such a shock must necessarily occasion in the sea’) and cratering (‘such a shock may have occasioned that vast depression of the Caspian Sea and other great lakes in the world’). In common with most of his contemporaries, Halley accepted the Bible as an accurate account of historical events. From that perspective, he concludes that the impact hypothesis ‘may render a probable account of the strange catastrophe we may be sure has at least once happened to the Earth’.

… and it can happen again

Another follower of Isaac Newton was William Whiston – a literal follower in this case, since he succeeded Newton as professor of mathematics at Cambridge. Like Halley, Whiston was convinced that a comet had caused the Biblical Flood – and he went a step further. He thought the world was due for another disaster of similar proportions. With a comet on its way in 1736, Whiston predicted that it would collide with Earth and destroy civilisation on 16 October that year. As scaremongering exercises go, this one was quite effective. It’s said that people fled London for the countryside, banks were so inundated by people wanting to withdraw money they had to close, and in the end the Archbishop of Canterbury was forced to issue a call for calm.

A much more accomplished follower in Newton and Halley’s footsteps was the great French physicist Pierre-Simon Laplace. His Wikipedia page lists more than 30 scientific theories and methods under the heading ‘known for’. In his book The System of the World, first published in 1796, Laplace speculated that cometary impacts might result in global extinctions:

The greater part of men and animals drowned in a universal deluge, or destroyed by the violence of the shock given to the terrestrial globe; whole species annihilated; all the monuments of human industry reversed; such are the disasters which a shock of a comet would produce.

Although Laplace was taken seriously on most subjects, this proved to be an exception. The scientific consensus in his day, and for almost two centuries afterwards, was that there was no place for sudden catastrophes – caused by comets or anything else – in Earthly affairs. Ironically, this particular dogma originated as a reaction against outdated religious narratives like the Biblical Flood. Having dismissed such things as superstition, science embraced a new paradigm called ‘gradualism’ – the deliberate polar opposite of catastrophism. As recently as 1972, The Penguin Dictionary of Geology boasted the following entry:

Catastrophism. The hypothesis, now more or less completely discarded, that changes in the Earth occur as a result of isolated giant catastrophes of relatively short duration.

Having been tossed aside by mainstream science, catastrophism found a new home in the realm of pseudoscientific cranks and religious doom-mongers. This simply created a vicious circle, further hindering its consideration by serious scientists. The most notorious case was that of Immanuel Velikovsky in the 1950s. A qualified psychologist – but not a qualified astronomer – he ascribed a wide range of historically recorded disasters to cosmic collisions, using a narrative that was almost wilfully ignorant of the way the Solar System actually works. The timescales he talked about were those applicable to human affairs, not astronomical or geological processes. A good (and mercifully brief) summary of his theory was provided by science fiction author and pseudoscience-debunker John Sladek:

Between 1500 and 700 BC, the Earth was visited by a series of comet-induced catastrophes, which Velikovsky has choreographed thus: Jupiter collides with Saturn, knocking a piece out of itself which becomes a comet. The comet collides with the Earth several times (causing earthquakes, floods, meteor showers, etc). It then collides with Mars, knocking it out of orbit. Mars bears down on us (more quakes etc). Finally, Mars and the comet collide again, very near the Earth. Small comets are pulled off the comet’s tail; they become the asteroid belt, while Mars is knocked back into orbit, and the comet settles down to become the planet Venus.

That was such utter nonsense it hardened the science community more strongly than ever against catastrophism. At the same time – and frustratingly for scientists – Velikovsky’s ideas proved enormously popular with a certain section of the general public. A whole new sub-genre of pseudoscience grew up around it, on a par with – and catering to the same audience as – flying saucers and alien abductions.

Velikovsky-style catastrophism is still alive and well today, in the form of scaremongering rumours that pop up every now and then on the internet. In 2012, for example, a number of people became convinced that a collision with a non-existent planet called Nibiru was imminent. Such stories, originating in small online communities, can sometimes reach much wider audiences thanks to irresponsible reporting by tabloid newspapers. To take just one example, in January 2017 the Daily Mail carried the headline: ‘A doomsday asteroid will hit Earth next month and trigger devastating mega-tsunamis, claims conspiracy theorist’.

An important point needs to be made here. The Daily Mail didn’t run that headline simply for the benefit of other conspiracy theorists – there just aren’t enough of them to make it a commercially viable proposition for a large-circulation newspaper. Instead, they ran it for the millions of ordinary people who see the whole thing as an object of humour. It was an entertainment piece, not a scaremongering one. For scientists, this ‘giggle factor’ – that’s the term they use – is just as much of an annoyance as Velikovsky-style crankery. That’s because, at some point towards the end of the 20th century, the scientists themselves stopped laughing at cata...

Table of contents

• Cover
• Title Page
• Contents
• 1: Asteroid Apocalypse
• 2: Rocks in Space
• 3: Collision Course
• 4: Death from the Skies
• 5: Cosmic Connections
• 6: Mapping the Threat
• 7: Planetary Defence
• 8: A Question of When, not If
• Further Reading
• Index
• About the Author
• Copyright
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