SPACEGUARD

By Jonathan Tate

The startling realisation that the Universe is a violent and unpredictable place has caused one of the most important paradigm shifts in scientific thought in the last half century. Impacts have played an important, perhaps even dominant part in the physical and biological evolution of the solar system, and the Earth has had a long and violent history of collisions with other solar system bodies. Darwin did not get it wrong, but certainly missed the whole story.

Catastrophism is no longer a dirty word.

This new reality was starkly demonstrated in July 1994 when the fragments of the shattered Comet Shoemaker-Levy 9 smashed into the planet Jupiter, and was brought sharply to the public's attention in March 1998 when it appeared that asteroid 1997 XF11 might collide with the Earth in October 2028.

It has been one of the most significant discoveries of the space age that all of the bodies in the solar system with visible, stable surfaces show extensive evidence of impacts - they are covered with craters. But on Earth we see very few obvious examples, because geological and weathering processes soon destroy the evidence. But they are there. There are now more than 150 structures on the Earth that have been positively identified as impact craters, and more are coming to light each year. The identities of many have been masked by heavy erosion over the centuries, and others have been buried deep under ocean sediments, but the geological evidence clearly confirms their impact origins.

So what's doing all this damage? There are four groups of extraterrestrial objects that can potentially threaten the Earth:

The majority of asteroids are confined to the main asteroid belt, orbiting the Sun between Mars and Jupiter. But, there are significant groups of asteroids found elsewhere. Of particular concern are the Aten and Apollo families that have orbits that cross that of the Earth. Another group, the Amor family, do not yet have Earth crossing orbits, but as their paths evolve impacts will become quite possible in the future.

Short period comets are thought to originate in a wide band of solar system debris called the Kuiper Belt that starts just beyond the orbit of Neptune. It is only in the last few years that Kuiper Belt objects have been discovered, and it’s beginning to look as though Pluto might be just the biggest of them.

Short period comets have orbital periods measured in years or decades (by definition, up to 200 years), and there are two major families, the Jupiter family with periods of less than 20 years, and the Halley family with periods between 20 and 200 years. Most stay close to the plane of the solar system, and once found their orbits can be predicted with some accuracy.

Long period comets on the other hand originate in a spherical cloud of debris that surrounds the Sun at distances between 20,000 and 100,000 AU (almost two light years - half way to the nearest star). Every now and again the movement of a nearby star or massive dust or molecular cloud will cause a few to fall inwards towards the Sun. Whatever the mechanism, five to ten significant cometary bodies approach the Sun each year, while an unknown number of smaller bodies pass undetected.

Unlike short period comets, their long period cousins have orbits that are randomly orientated on the celestial sphere. Their orbital periods are longer than 200 years, and they will often only return after thousands or even millions of years (or, indeed, not at all). As a result, most long period comets will be new to science when they reappear in the inner solar system, and they can do so with little or no warning.

All comets are believed to have similar compositions and have been described as “dirty snowballs,” but recent research indicates that “Icy mudball” may be a more accurate description. The main constituents of the nuclei are volatile ices, mainly water, mixed with dust and hydrocarbons. The one nucleus that has been closely studies (Comet Halley) is blacker than coal, probably due to a coating of dark hydrocarbons. Until the Giotto mission in 1986 that photographed Halley it was assumed that cometary nuclei would be bright reflective objects because of their icy composition. The darkness of Halley has caused a reassessment, and a consequent upward revision of size and mass estimates of cometary nuclei.

It now seems almost certain that the disruption of cometary bodies, like Comet Shoemaker-Levy 9, is far more common than previously suspected. When a large comet nucleus shatters, either because of a collision or because of internal stresses as its volatile components vaporise, it produces a stream of fragments and debris in its wake. This will, over time, spread out along the orbital path, and the fragments will vary in size from a few microns to a few hundreds or thousands of metres in diameter. Such a debris stream could pose a significant and recurring threat to the Earth not only from the impacts of the larger fragments, but also from the dusting of the upper atmosphere by fine dust particles. This atmospheric dusting will reduce the amount of sunlight and heat reaching the ground, and looks as though it could be a significant factor in climate change. There is serious research being done into the links between historical periods of atmospheric dusting and the onset of the ice ages, and, in more recent times, periods of social unrest.

There is increasing evidence that there might be a significant population of “dormant” comets occupying Halley type orbits. Once a comet has outgassed all of the available volatiles, or it has developed a total crust of hydrocarbons and dust, its coma and tail will disappear, and the remaining, inert nucleus will take on the appearance of a dark asteroid. Indeed, a substantial number of the objects currently thought of as asteroids may well be dormant comet nuclei.

Currently there are about 250 asteroids known to have orbits that cross that of Earth, and they range in diameter from a few metres up to the largest, 1620 Ivar, which has a diameter of 9 kilometres. There are probably about 2,100 Earth crossing asteroids larger than one kilometre in diameter, and perhaps 320,000 larger than 100 metres. An impact on the Earth by one of the smaller bodies would be a local or regional catastrophe, but it would not be globally threatening. After analysing the Shoemaker-Levy 9, it is clear that an impact by an object larger than about one kilometre would significantly degrade the global environment to the extent that the survival of a significant proportion of the human population would be put at serious risk. An impact by an object larger than about five kilometres would inevitably lead to mass extinctions on a large scale.

So what is going to do the killing after a major impact? Immediate effects will include the obvious explosive effects at ground zero and local firestorms raised by the superheated air from the impact. A crater, about 20 times the diameter of the impacting body, will be excavated in a matter of seconds, and debris will be ejected into sub orbital trajectories. This debris will later re-enter the atmosphere – the meteor shower from hell - possibly all over the globe raising massive fires that destroy a significant proportion of the biomass. Intense acid rain would result from the ionisation of the air as the impactor entered the atmosphere, as would the production of pyrotoxins. The ozone layer would be obliterated, and major volcanism and seismic activity can be expected as the shock wave of the impact ripples through the planet. All of this will cause a global environmental disaster of extreme severity.

In addition to most or all of these effects, an impact at sea will produce a significant "tsunami," capable of travelling considerable distances, and possessing enormous energy. Such surges will pose a substantial threat to low lying and coastal areas. The United Kingdom, with much of its population and economic infrastructure located in precisely such areas, would be at particular risk from an impact anywhere in the Atlantic Ocean.

However, that is far from all; the main killing mechanism will be the vast amount of dust and debris injected into the upper atmosphere, combined with the smoke from the firestorms (witness last year's events in Indonesia). These will block the Sun and cause a phenomenon similar to, but much more severe than the “nuclear winter” that became such an issue during the Cold War. It is this that is likely to pose the greatest threat to the ecosphere on a global scale as food chains collapse and darkness, cold and starvation set in.

After a few months or years the atmosphere will clear, but the surface of the Earth, now mainly white in colour, might reflect too much of the Sun's radiation to prevent a new ice age. However, there are other mechanisms at work. The atmosphere will contain a substantial excess of CO2, resulting from the global fires and vulcanism. The Earth could be in for a massive overdose of greenhouse effect. The balance between sweltering and freezing is a very fine one.

Such globally threatening events are expected at statistic intervals of 100,000 years.

Smaller strikes, in the 50-100 metre range, though not globally threatening, have in the past caused massive damage to the area of impact, and often at considerable distance. We saw this at Tunguska in 1908 and in the Amazonian Rain Forest in 1930. The spread of human settlement, civilisation, and particularly urbanisation, makes it much more likely that a future impact, even relatively small, could result in the massive loss of human life and property. The timescale for such impacts is between 50 and 100 years.

Even much smaller impacts can have significant effects. Typically a 10-metre diameter body will have the kinetic energy of about 100 kilotons, and is likely to detonate at an altitude above 10 km, causing little or no damage on the ground, but considerable alarm to those who witness it as we saw on 9 October in El Paso, Texas. Such events have been recorded by US surveillance satellites at the rate of one or two per month, and smaller Kiloton sized explosions happen every 1 to 10 days.

An individual's chance of being killed by the effects of an asteroid or comet impact is small, but the risk increases with the size of the impacting body, with the greatest risk associated with global catastrophes resulting from impacts of objects larger than one kilometre. Statistics on the causes of unnatural deaths in the United States look like this:

The one thing that these figures fail to demonstrate is the qualitative difference between individual events, such as the majority of cases in the table above, and the sudden, but massive damage and loss of life caused by a major impact event. After an average natural disaster, be it floods, volcanic eruption, earthquake or whatever, help soon arrives from unaffected areas. With a major impact event nowhere is unaffected. Help won’t be coming.

So, what can be done to prevent this sort of catastrophe?

Detailed studies have been conducted in both the United States and Russia on ways of avoiding catastrophic NEO impacts.

Firstly, destruction. The possibility of destroying potential impactors, probably with high yield nuclear weapons, has been studied in some detail.

With the current lack of knowledge of the exact composition of particular objects, and their structural strength, there is an element of doubt as to the effectiveness of this plan. The fear would be that incomplete disruption of the object would subject the Earth to multiple impacts from pieces of the original body.

The effects of transforming a cannon ball into a cluster bomb could be just as far-reaching as the original threat.

So, destruction may not be the best answer. Assuming that the potential impactor can be identified early enough, its orbit could be modified sufficiently to ensure that a collision could not occur. The amount of modification required is inversely proportional to the time available before impact, so early warning of a potential threat will be crucial.

Methods that have been studied include the detonation of a nuclear weapon close to the body to change its orbit, or the use of propulsion units or mass drivers (using the material of the object itself as fuel) to physically drive it from its path.

Any system designed to deflect an Earth threatening body from a collision course could also, conceivably, be used to direct a non-threatening body towards the planet. This potential for misuse is known as the “Deflection Dilemma”.

This can, however be avoided with enough notice of an impending impact, because the countermeasures need not be constructed or deployed until actually required.

Whatever method chosen, the basic fact is that we need an effective surveillance system to give us adequate warning of impact, and the time to react. Detection and tracking are the key to our survival.

Despite the urgency advocated by the scientific community the current situation is that very little is being done on national scales, except to some extent in the United States.

US Air Force Space Command, with some support from Congress, was developing a number of Planetary Defence related projects. However, funding has recently been severely cut back.

NASA funds ground based research to the tune of approximately $1.5 million per year, which now includes the NEAT programme which has been in operation since 1995, utilising a sophisticated CCD system attached to a USAF 1-metre GEODSS telescope originally designed to track Soviet satellites. Recently NASA has announced that they are doubling this figure to $3 million per year. This extra money will help with programmes such as LINEAR, another GEODSS system, located in New Mexico, and the LONEOS project using a 0.6 metre Schmidt near Flagstaff.

The privately funded Spacewatch programme, directed by Tom Gehrels, has been using a 70-year-old 0.91 metre telescope, but in June commissioned a new 1.8 metre instrument, paid for by private donations.

In Japan there is a follow-up programme using the Kiso Schmidt telescope, and a major new detection programme has just been announced, In Japan there is a follow-up programme using the Kiso Schmidt telescope. In April 1998, it was announced that 2 billion yen (around 15 million dollars) will be spent on the development of a radar telescope and a 1-metre optical telescope for NEO and space debris studies. The construction of the 1-metre telescope will start in autumn 1998, and it will be ready for operation in about three years.

In South America Argentina, Uruguay and Brazil plan to collaborate over two 1-metre telescopes to be located in western Argentina.

In Europe Sweden has a plan to build a 2.7 metre Spaceguard telescope in the Canaries, while Finland is planning a large telescope in Namibia.

Italy will soon have a search and follow-up programme, using a 1-metre reflector, closely modelled on the Spacewatch programme, and in France the OCA Schmidt telescope is being fitted with a new CCD system in collaboration with DLR Berlin.

In the United Kingdom a debate in parliament on the Spaceguard project was held in March 1999. As a result, it would seem that the British government are at last taking the threat seriously, and the various agencies concerned are looking forward to a new, co-operative partnership with government. The main effort in UK is now directed towards the establishment of a National Spaceguard Centre. The UK has tremendous intellectual resources, and such a centre of excellence would be the best contribution that the country could make to the international effort, short of building a new telescope in the Southern Hemisphere.

To try to pull all of these projects together the Working Group on Near Earth Objects of the International Astronomical Union established the Spaceguard Foundation in March 1996 with the following aims:

This initiative has already received support from the Council of Europe, which issued a declaration in March 1996 encouraging member states to support the Foundation's efforts.

In the United Kingdom, prompted by Spaceguard UK, the British National Space Centre hosted a meeting in November 1996 to discuss the NEO threat.

The aim of the meeting was to bring together as many interested parties as possible to assess exactly what the United Kingdom is doing in the field, and to discuss what should be done in the future.

Dr Jasper Wall, the Director of the Royal Greenwich Observatory, summed up the conclusions:

Despite all of this the government decided that the United Kingdom would not fund additional studies into the threat from NEOs, or Planetary Defence matters over and above the national contribution to ESA which is, in turn, providing a modest grant to the Spaceguard Foundation.

A second meeting, with wider participation was held in July of last year at the RGO. A lot of ground was covered, but Dr Nigel Holloway, a risk analyst from the Atomic Weapons Establishment Aldermaston, made one of the most significant comments in his paper:

“The risks from intermediate and large NEO impacts, associated with tsunami and impact winter effects respectively, fall close to the UK national limits of tolerability, without regard for the fact that the overall risks to the world are a hundred times higher again. Were the risks "owned" then the owner would be put under strong pressure to reduce the risk, even at considerable expense.

By comparing other risk management expenditure decisions, the NEO risk is found to be sufficient in magnitude to warrant substantial action. Astronomers should consider the relative importance of NEO detection and their other projects in this light.”

It is some time since astronomers have been called upon to serve in a directly useful fashion at the expense of their more theoretical aspirations. The discovery of the NEO risk changes that.”

In addition, the meeting endorsed Spaceguard UK as the prime non-governmental organisation concerned with the impact threat in the United Kingdom.

Spaceguard UK was formally established on 1st January 1997 with the following aims:

Spaceguard UK is affiliated with the international Spaceguard Foundation, and has links with many scientific and government organisations both here, and abroad. And we are proud to have Sir Bernard Lovell, Dr Patrick Moore, Sir Arthur C Clarke and Sir Crispin Tickell as our Patrons.

Our current activities are concentrated on ensuring that the consensus achieved at the two UKNEO Working Group meetings is transformed into meaningful action by the British government and organisations world-wide. To do this we have prepared an action plan as follows:

Collaborative development of mitigation strategies

The United Kingdom should collaborate with the international community on the development, and, if necessary, the deployment of mitigation measures.

The last two years have been an extraordinary time, and I believe that in its first year Spaceguard UK has managed to significantly raise perceptions of Planetary Defence issues in the UK. This slide shows are current professional membership, which covers the majority of the key players in the field.

The threat of a catastrophic impact is real, demonstrable and inevitable. But, as a danger to the biosphere it differs from other natural disasters in three critical respects:

It is worth pointing out that the government is spending £758M on the Millennium Dome; they might want to consider how much the protection of our entire civilisation is worth.

Jonathan Tate is the founder and Director of Spaceguard UK , a member of the Board of Directors of the Spaceguard Foundation, a consultant to the IAU Working Group on Near Earth Objects, a fellow of the Royal Astronomical Society and an Associate of COSPAR.

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