Catastrophes from Space: Prospects for Planetary Defence
University of Plymouth - 2001-01-03
Annual Conference of the Royal Geographical Society with the Institute of British Geographers
The idea that the Earth is liable to catastrophes from space may be disquieting but it has been around for a very long time. Nearly every culture has its creation myth, and its apocalypse myth; and many include interventions, some divine and others not so divine, from the sky and the stars. In Julius Caesar by William Shakespeare, Calpurnia urged her husband not to go to the Senate on the Ides of March in 44 BC.
"When beggars die, there are no comets seen;
The heavens themselves blaze forth the death of princes."
At the beginning of the 19th century the arguments crystallized. Different schools of thought lined up against each other, among them the Catastrophists versus the Uniformitarians. Embedded in the Judeo-Christian tradition is the notion that God interferes from time to time in the affairs of the Earth, and in particular those of humankind. If humans do not control the Earth's destiny, then God does. From this comes thinking about the Creation, the Flood, the Last Day, and divine punishment for human aberrations manifest in disease, famine, earthquakes and other natural disasters.
As this range of ideas became discredited with the rise of science - time became longer and the Earth smaller in the scheme of things - so also was discredited the idea of catastrophes coming from outside the Earth. The Uniformitarians believed in slow and accumulating change as the norm. James Hutton wrote of "no vestige of a beginning, no prospect of an end". Charles Darwin saw evolution by natural selection as a process covering hundreds of millions of years.
As so often in ferocious argument of this kind, there is something to be said for both sides. Of course there are catastrophes, or sudden changes of direction, even if they do not happen very often. For the most part, and certainly within the brief compass of a human life, change is gradual even if it does not always go in straight lines.
Particularly repellent is the notion of catastrophes from space. Whatever the natural or human-induced problems we face as we scurry around on its surface, the Earth is our only home. Perhaps this is why it has taken us so long to understand the realities. We all know how difficult it is to change ways of thinking. People went through agonies trying to reconcile the Bible with geology in the first half of the 19th century. Great ingenuity was shown in linking catastrophes with the Flood. To this day there are people who deny tectonic plate movement and the human contribution to climate change, and seek to explain impacts on the Earth or the Moon by any means other than what is now obvious.
In the last half century we have extended the human sphere into space. Advances in rocketry, space launches (almost 4000 since 1958), the first man on the Moon in 1969, and even more potent, those photographs of the Earth from space, have added the spatial dimension to human thinking. But the notion that the Earth was subject - is still subject - to extra terrestrial impacts was not generally accepted. Perhaps the turning point for most people was when the Alvarez family - father and son - suggested in 1980 that an enormous object had struck Chicxulub in the Yucatan peninsula of Mexico with devastating effects some 65 million years ago.
Thereafter the search for other traces of extra terrestrial impacts was on. Events which were well known, like the Tunguska meteorite of 1908, suddenly fitted in. History has had to be rewritten. Once people knew what they were looking for, they soon found craters and possible historical events in plenty. I remember pointing out to a startled US Congressman that near where we were sitting in Washington was the vast impact crater of Chesapeake Bay, which was carved out some 34 million years ago. Even if wind, weather and erosion have blurred the traces, there are craters on every continent, and more are being found all the time. A marvellously clear specimen is the Barringer Crater in Arizona, well preserved in the desert for 49,000 years. Records suggest that a Chinese city was struck by stones from space with a death toll of 10,000 in the 15th Century.
In the present decade knowledge has increased exponentially. In July 1994 came the spectacle of the comet Shoemaker-Levy colliding with the planet Jupiter. As the comet entered Jupiter's gravitational field, it broke up into 21 fragments, resulting in multiple impacts. One fragment alone created a fireball as big as the Earth. In 1997 many of us saw the comet Hale-Bopp as it travelled magnificently and harmlessly across our skies. Less than two weeks ago an object with a diameter of around 50 metres came within two lunar distances of the Earth, a mere 769,000 kilometres (a tiny step on cosmic scales). We now have a much better but still very limited understanding of the hazards the Earth faces from extra terrestrial intrusion. It is a problem of high concern for geographers of all kinds.
It was in these circumstances and following a parliamentary debate that the British government decided to set up a Task Force on Potentially Hazardous Near Earth Objects on 1 January 2000. It consisted of three people: Harry Atkinson as Chairman (former Under Secretary, and Director of the Science and Engineering Research Council); David Williams (Perren Professor of Astronomy at University College London, and former President of the Royal Astronomical Society); and myself as the interested amateur. Our remit was to make proposals to the government on how Britain should best contribute to the international effort on Near Earth Objects: and in particular how to "confirm the nature of the hazard and potential levels of risk; identify the current contribution to international efforts; advise the government on what action to take … and on the communication of issues to the public; and to report … by the middle of 2000".
We had a fascinating six months in which we drew on the knowledge and wisdom of the astronomical community. Everyone we consulted wanted to help, and our eventual report, published in September 2000, represents far more than the Task Force could have achieved by itself. Particularly interesting was our visit to the United States where in somewhat dispersed fashion more has been done than in the rest of the world put together. We were kindly received by NASA, the Executive Office of the President, the Pentagon, the National Science Foundation, Congress (Congressional support has been a vital element in the US effort), the Jet Propulsion Laboratory in Pasadena, and most exciting the Kitt Peak Observatory in Arizona, where we were able to see Near Earth Objects as moving points of light across the night sky.
Near Earth Objects
What are these objects? So regular is the Earth's procession around the Sun, with a wobble or two in space and time, that we forget its origins as an aggregate of material from a disk, with some of the disk material still in eccentric circulation. Asteroids and comets may have made life possible in the very early history of the Earth by depositing a thin layer of carbon-based molecules and water.
There are four different types of extra terrestrial objects that can collide with the Earth. First are asteroids. Most asteroids are believed to be the remnants of material from the circumstellar disk that coalesced to form the planets. They therefore represent samples of the primordial solar system, largely unchanged since its formation 4,600 billion years ago. They come in all shapes, sizes and composition, from pebbles, lumps of ice, slabs of iron, and agglomerations of rocks to icy worlds up to 1,000 kilometres in diameter.
Most asteroids orbit the Sun in a fast moving cloud between Mars and Jupiter. But there are some less well behaved ones which intersect with the orbit of the Earth and can be dangerous. Some of the bigger ones have been identified and charted, but most, which emit no light and reflect only the light of the Sun, are hard to detect. They travel at speeds of around 20 kilometres a second.
Next are long period comets, which are thought to originate in the spherical cloud of debris that surrounds the Sun at distances of almost two light years (or about 150 million kilometres). Their orbits can be perturbed by gravity, causing the bodies to leave the cloud and approach the Sun. Some five to ten significant cometary bodies approach the Sun each year. Orbital periods are longer than 200 years and they will often only return after thousands or millions of years, if at all. As a result most long period comets are new to science when they appear in the inner solar system.
Next are short period comets, which are thought to originate in the Kuiper Belt beyond the orbit of Neptune. All comets are believed to have roughly the same composition. They have been described as 'dirty snowballs' although 'icy mudballs' might be more accurate. The average short period comet has an orbital period of up to 200 years. Once detected, orbits can be predicted with some accuracy, but such factors as the gravitational influences of major bodies, in particular Jupiter, in the solar system can cause chaos and uncertainty. Comets travel much faster than asteroids: sometimes at around 70 kilometres a second.
The final category is cometary debris. All comets leave a trail of debris; some of it large and some of it small, as they travel through the solar system. A recent example is Shoemaker-Levy. If a large comet breaks up, it will produce a stream of fragments in its wake, varying in size from a few microns to a few hundreds or thousands of metres in diameter. A chunk fell off the comet Hale-Bopp in 1996. Such annual meteor showers as the Leonids may cause no problem, but the bigger objects - comets turned into asteroids - can pose a significant and recurring threat to the Earth. With good reason comets were seen as harbingers of doom in ancient times.
Impacts
Fortunately for us the atmosphere of the Earth acts as a protective shield for most Near Earth Objects which burn up or explode at high altitudes. By contrast the larger ones can hit the Earth with devastating effect. The astronomer Edmond Halley drew attention to this possibility as long ago as 1694. In the Task Force report, we give lists of the main impacts so far known over hundreds of millions of years, and, perhaps more telling, of the relatively minor impacts of the last ten years. We also give lists of possible close approaches or near misses past, present and future. These lists make interesting reading.
The main effects of impacts are blast waves, tsunamis (or huge ocean waves), injection of material into the atmosphere, and electro magnetic changes near the surface. Obviously such effects vary enormously, and depend on the character of the object, and its speed and angle of entry. More than two thirds of the Earth's surface is ocean, and much of the rest is desert, uninhabited mountain and polar ice cap. Blast waves can be so intense that they are more important than the material behind them, and affect both land and sea. Tsunamis can be 70 metres or more in height, and are probably more destructive - and more frequent - than anything else. Edmund Haley wrote with some prescience about "the great agitation that such a shock must necessarily occasion in the sea." Changes in the atmosphere affect the temperature at the Earth's surface, and also the chemistry of the troposphere, with consequences for its carbon dioxide content and the protective ozone layer. Electromagnetic effects can be compared with those caused by the explosion of nuclear devices.
It may be worth taking a look at some specific impacts of which we now have knowledge. First come the big hits of which the Chicxulub event of 65 million years ago is a good example; then the Tunguska event of 1908; then an event above Lake Tagish in the Yukon on 18 January last year; and finally the possible effects of cometary dust on the Earth's climate.
The Chicxulub event changed the history of life on Earth. An object with a diameter of 10 kilometres hit Yucatan at a shallow angle, digging out a crater traceable today with a diameter of around 180 kilometres. It threw up a cloud of vaporized and molten rock over north America. The consequent dust in the upper atmosphere darkened and drastically cooled the whole Earth, damaging the process of photosynthesis on the surface. When the dust settled the temperature swung the other way. Water vapour and carbon dioxide in the atmosphere caused a runaway green house effect. It has been calculated that the surface temperature of the Earth could have risen by as much as 10oC for at least 500,000 years. It is no wonder that all living creatures were affected, and that a high proportion, including the dinosaur family, perished.
Extinctions of this magnitude are a disaster for some, but an opportunity for others. Indeed the rise of the mammals, with humans a very late arrival among them, would not have happened without Chicxulub. Fortunately such major events are extremely rare. But they have occurred at very roughly 100 million year intervals throughout the history of the Earth, and could, at least in theory, happen at any time.
Let us now look at the Tunguska event of 1908. An object with a diameter of 60 metres broke up over Siberia, destroying some 2,000 square kilometres of forest. It lit up the night sky across most of the northern hemisphere, and in Belgium was likened to a great red glow after sunset as if from a huge distant fire. Had it struck St Petersburg or London, there would have been little left of either. Although the area was inhabited by very few people, a witness described the sky as being split in two, with the northern part covered with fire, then a great heat and a mighty crash. The devastation can still be detected today, almost a century later. Events of this kind are relatively frequent of the order of once every 250 years.
More frequent still are events of the kind that happened over Lake Tagish in the Yukon on 18 January last year. An object of around five metres in diameter exploded at an altitude of 25 kilometres, causing a long and bright fireball, a loud bang, a shower of fragments, and an electro magnetic pulse which caused a temporary loss of power transmission on the ground below. There were several witnesses. A curious effect was that many noticed smells, frequently described as sulphurous (although hot metal and rock were also mentioned) around the area of the shower. Analysis of the fragments has shown that they come from the asteroid belt between Mars and Jupiter, and derive from the early history of the solar system.
There is a fairly constant hail of small objects into the upper atmosphere, most unnoticed below. Examples of small impacts include the Mbale impact in August 1992, when a boy was hit by a stone after its deflection by a banana leaf. Then there was the so called Peeskill object which stove in the back of an old Chevrolet in upstate New York on 9 October 1992. The owner is said to have made a fortune out of it.
My last specific instance relates to the trails of dust left behind by comets though which the Earth passes from time to time. They are more serious than the fireworks displays with which they are often associated. Such dust can provide the nuclei for ice crystals, sometimes known as diamond dust, to form at the top of the troposphere some 10 to 15 kilometres above the surface of the Earth. This has the effect of reflecting solar radiation back into space, and could, with such other factors as the Earth's ever changing relationship with the Sun (the so called Milankovitch effect), help to trigger the beginning and the end of the ice ages, with their many fluctuations, over the last two million years.
The Risks
Together these factors point to one central conclusion. We and all living creatures live dangerously on Earth. It is only the shortness of our lives which shields us from understanding how vulnerable the Earth really is. From the 17th century onwards there have been theories about the effects of extra planetary activity on human society: revolutions have been correlated with fireballs or meteorite showers, and the crash of civilizations and spread of disease with comets. While a healthy scepticism is always required, we cannot afford to dismiss these ideas altogether.
Examination of them leads into consideration of risks which I know is one of the central themes of your conference. It was the hardest thing that the Task Force had to do. Comparison with other sorts of risks tends to be artificial. There can be no real comparison of like with like. An individual's chance of being killed by the effects of an asteroid or comet impact is very small, but the risk increases with the size of the impacting body, with the greatest risk associated with global catastrophes resulting from the impacts of objects larger than a kilometre. Calculations have been made relating to the probabilities of a citizen of the United States dying from a variety of causes. The probability of dying from a motor vehicle accident is 1 in a 100 and from a homicide is 1 in 300, while the probability of dying from an aircraft accident is 1 in 20,000, from an asteroid impact 1 in 25,000, and from a flood 1 in 30,000. By contrast the probability of dying from food poisoning is only 1 in 3 million.
While the probability of being killed by an asteroid impact is comparable to that of being killed by an aircraft accident, the main difference is that aircraft accidents kill small numbers of people with high probability while asteroid impacts kill huge numbers of people with low probability.
Perhaps the nearest analogy is with the risks arising from a major nuclear accident. So far governments are much better prepared for a nuclear accident than they are for an impact from space.
In trying to reach some kind of useful judgement, the Task Force distinguished between the risks arising from objects whose Earth crossing orbits were fairly well known, and those from objects which were unknown and could only be guessed at from statistics. We also distinguished between impacts with global consequences, and those with regional or local consequences. So far most work, nearly all in the United States, has been done on objects of over a kilometre in diameter. From this we concluded that none of these was likely to hit the Earth in the next 50 years. But there could be more in this category, and a great many more - thousands more - with diameters of between 5 metres and a kilometre. As we have seen, even a 5 metre object has effects, and a Tunguska sized object of 60 metres even greater ones. It was one of these which sailed past just before Christmas.
Our biggest handicap is simply lack of knowledge. The Task Force's strongest recommendation was for a concerted international effort to discover and track most of the dangerous objects (at the same time improving our statistical knowledge of the remainder), and to study further the consequences of impacts and the possibilities for mitigation.
Mitigation
The question many people ask is: Why all this fuss when there is nothing anyone can do about it anyway? I believe this to be profoundly wrong, and the Task Force produced robust recommendations on the subject.
First let me make a general observation. In January 1995 the American Institute of Aeronautics and Astronautics published a report entitled Response to the Potential Threat of a Near-Earth Object Impact. It contained the following passage:
"If some day an asteroid does strike the Earth, killing not only the human race but millions of other species as well, and we could have prevented it but did not because of indecision, unbalanced priorities, imprecise risk definition and incomplete planning, then it would be the greatest abdication in all of human history not to use our gift of rational intellect and conscience to shepherd our own survival, and that of all life on Earth."
What then can we do? Let us suppose that with the help of a greatly improved telescope network, we could with reasonable accuracy predict the next range of extra terrestrial events. The range would of course be wide. We would have to reckon not only with the size and composition of an incoming object, but also with the possibility that it might enter into ever diminishing orbit round the Earth before colliding with it. Our response would fall into two categories:
- The first would comprise conventional measures of civil defence. Depending on the length of notice and the size and composition of the incoming object, people could move out of target areas to relatively safer areas elsewhere. Such countries as Britain and Japan would be particularly vulnerable to tsunamis arising from oceanic impacts. Both have suffered from tsunamis in the long past. A big problem would be how to feed a displaced population. A big hit could lead to a darkened Earth which could affect growing seasons for food the world over.
- The second is the more exotic prospect of planetary defence, either through destruction or through deflection of incoming objects. This is not a fantasy of such films as Deep Impact or Armageddon. Even now there is an American satellite in orbit around the asteroid Eros and there is a project to release a half tonne lump of copper to cause a crater on a comet and incidentally to change its orbit. Destruction of an object by high yield nuclear devices might be technically feasible, but would carry enormous risks of its own. Incomplete destruction of an object could subject the Earth to multiple impacts from pieces of the original body. We saw for ourselves what happened to Jupiter when Shoemaker-Levy broke into 21 pieces with 21 impacts.
More promising are the possibilities of deflection through modification of the object's orbit. The amount of modification required is inversely proportional to the time available before impact. So early warning would be vital. Methods considered include detonation of nuclear or chemical devices close to the body to change its orbit. But such devices would have to be used with care. There is a danger that they could deliver a huge amount of energy with very little momentum, whereas to deflect a massive object effectively, it is momentum that is needed: a steady gentle push, not an explosive jolt. This could be achieved by irradiation of the near side surface of the object which might give it the little shove required.
Other possible methods include the mounting of sails on the object to harness the Sun's radiation pressure to push it from its course. Another would be the use of mass drivers. These would consist of magnetic accelerators and fuel from the material of the object itself. The source of power would again be the Sun. In order to make the system work a number of engineering problems would have to be solved. With adequate warning time this should be possible. According to the astrophysicist Freeman Dyson, for a comet of a diameter of one kilometre and a warning time of a hundred years, the power required to miss the Earth would only be 10 kilowatts. It seems remarkable that 10 kilowatts in the right place at the right time might save a billion lives.
While agreeing that all reasonable measures should be taken to reduce the hazards, Carl Sagan and Steven Ostro have concluded that:
"premature deployment of any asteroid orbit-modifying capability in the real world and in the light of well established human fragility and fallibility may introduce a new category of danger that dwarfs that posed by the objects themselves."
The potential for misuse of a deflection system is known as the "deflection dilemma". Perhaps the best way of avoiding it is to continue the research on the engineering problems involved but set nothing in place until a real threat is identified. So the current focus should be on identification.
Conclusions
The Task Force's report contained fourteen recommendations. They covered the science, the social and economic aspects, mitigation, and the necessary organization to give effect to what was proposed.
Obviously the main effort must be international. Here we suggested something analogous to the UN-sponsored Intergovernmental Panel on Climate Change to co-ordinate the science, consider the social and economic effects, and recommend the necessary action. On the European level we recommended co-ordination of the somewhat dispersed efforts of the main European countries so that they could take a complementary role to that of the United States in specific areas. We underlined the need to work out a comprehensive European strategy.
At national level we recommended the appointment of a single government Department to take overall responsibility and co-ordinate the work of other Departments and the Research Councils concerned. We also recommended the creation of a British Centre for Near Earth Objects to promote and co-ordinate work in this country, and to provide an information service for the general public as well as for the academic, scientific and environmental communities. The Minister concerned - Lord Sainsbury - has promised the Government's reply to the Task Force report by the turn of the year.
I have been astonished by the interest shown in our report not only in this country but in the United States and elsewhere. We hope it will be a catalyst for action. It means looking after our own future as well as that of generations to come. We are the only animal species which could have this amazing capability within its grasp. Yet before we get too big a conceit of ourselves, let us remember how small and vulnerable we are. We are like microbes on the surface of an apple, on an insignificant tree, in an insignificant orchard, among billions of other orchards stretching over horizons beyond our sight or even our imagining.
