FLUG REVUE May 2005: Asteroids threaten Earth

May 2005

ASTEROIDS AND METEORITES THREATEN THE EARTH

By Matthias Gründer

Danger is lurking in the expanses of the universe in the form of meteorites and asteroids of all sizes, and the probability it is that at some point in the future a massive collision is bound to occur. Research programmes aimed at detection and early warning have been running for some time at ESA.

Impact craters all over the world provide impressive testimony to this kind of catastrophe, the only visible signs of which are the scars left behind on the Earth's surface. Up to now there has been no survey of impact points in the oceans. Probably the most well-known crater is the canyon in the Arizona desert caused by the meteorite Diablo, which created a hole 1.2 kilometres wide and 150 metres deep, even though its own diameter was only around 30 metres. The size of the impact was the equivalent of a 175 million tonne volume of rock. Thousands of fragments of the iron meteorite with an overall mass of around 30 tonnes have been found to date, and experience suggests that a lot more of the mass of the cosmic shell was vaporised on impact.

Missiles from the universe have actually left vast crater deserts behind not just on the Earth but on all known, Earth-like celestial bodies. “Earth-like” refers to all planets and moons constructed out of rock. Only in the gaseous giants do the lumps of rock disappear, never to be seen again, albeit not all at once. When the comet Shoemaker Levy 9 came too close to Jupiter in 1992, the immense gravitational pull of the giant planet tore it apart into 21 large fragments and countless smaller ones. As if threaded out on a string of pearls, these then crashed into the boiling gas envelope of Jupiter, causing a breathtaking display of fireworks, and the “impacts” caused as a result were visible for a whole week as giant, coloured eddies in the atmosphere.

Some ten tonnes of cosmic debris enter the Earth's atmosphere every day, but these fragments are normally much too small to cause significant damage. In most cases, the frictional heat causes them to burn out, many of them crash unnoticed to the ground or into the sea and only a few make their way into the headlines. Some years ago a cow was hit in Australia, while another time the window of a cold frame was shattered. Nothing very exciting, then.

But since the Hollywood hit, “Armageddon”, or since the flypast of the asteroid Toutatis close to the Earth on 29 September 2004 was fully documented in all the media, the public have been aware of the latent danger. Even if tiny events are often blown up into sensations, nevertheless one must assume that there is a possibility of a collision.

The first occasion on which scientists detected the approach of a foreign object at a distance of less than 50,000 kilometres to the Earth was on 17 March 2004. Asteroids like this “2004 FH”, with its diameter of around 30 metres, are small beer in the cosmic context and are normally never detected. Up to now it has not been possible to search specifically for them as they do not have a stable trajectory so that they can only be detected by chance. However, a diameter of 30 metres, one recalls, was sufficient to create the Arizona crater – reason enough to finally begin thinking about an early warning system. After all, the impact from the lump of rock would have released the energy of around 130 Hiroshima bombs.

As early as 1990 NASA commissioned a feasibility study for the development of a possible defensive system, but the results were shattering, even if they showed that the reality looks quite different from that depicted in Hollywood disaster movies. The simulated firing of an approximately one kilometre diameter asteroid with nuclear warheads did not cause annihilation, but the shattering into numerous lumps of stone would have been experienced on Earth as a carpet bombardment. Giant conflagrations and tsunamis up to four kilometres high across all the oceans would have been the consequence.

Complete annihilation is not currently possible

But other scenarios have so far yielded little in the way of satisfactory results. For example, if one were to try redirecting an asteroid away from its collision course by igniting a nuclear warhead in its vicinity, this would only cause an acceleration of two centimetres per second. In other words, for the inert colossus to adopt a different trajectory, the ignition would have to have occurred at an unimaginable distance from the Earth.

However, this only illustrates the limitations on any defensive system that might currently be feasible, i.e. based on observation, detection and advance warning. When the comet Hale-Bopp flew unexpectedly close past the Earth in 1997, at the time it was detected it had a speed of around 70 kilometres per second and was already so close that the warning time for an impact would have been much too short. If one imagines that the devastating tsunami of Boxing Day 2004 was the result of an impact, then the asteroid causing it would have had a diameter of “only” 207 metres. In other words, nothing dramatically big, and hence it would have caught us off our guard as current asteroid-searching programmes, which in any case are only carried out very sporadically, concentrate on objects with a diameter of over a kilometre.

However, someone has to make a start, and so in 2002 ESA initiated a competitive tender for the development of asteroid hunters. Initially it is a matter of gathering more information about their composition and characteristics, and especially about the impact of foreign bodies, and the space probes “Sancho” and “Hidalgo” were selected under the Don Quixote search project to be the first mission dedicated to this end. This will be launched in around six years when the financial resources required have been made available.

NASA still does not have any specific satellite projects, but four new telescopes largely dedicated to searching for near earth objects (NEOs) are to be installed on Hawaii over the next year. And finally, a draft bill under which amateur astronomers are to be paid hefty rewards for every foreign body newly discovered close to the earth has been before the US Senate for a year. The House of Representatives wants the funding for this project to come from NASA. But here lies the difficulty: NASA has hardly any budget to fund such payments and, in particular, no one is yet responsible for collecting impact warnings or for responding to them. There is virtually no authority, either national or international, to whom one could turn.

On the other hand, this should not be viewed as grounds for panic, since according to probability theory there is unlikely to be a collision for the next few hundred years. So we still have time for a warning and response system. The main thing is that a start is being made.

From page 66 of FLUG REVUE 5/2005

Last updated 14 April 2005

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