MITIGATION 
            POSSIBILITIES

For the first time in the history of the Earth there are possibilities for mitigating the effects of impacts by Near Earth Objects and even, in the longer term, for deflecting them entirely from collision with the Earth.

But all this depends on first improving our ability to detect such objects well in advance and to measure accurately their orbits and physical properties - our key science priorities - and on having in place mechanisms, international, national and local, to take the necessary action.

Once an asteroid or comet on a collision course is identified and its orbit tracked, its likely point and time of impact can generally be predicted. Such accurate prediction is rarely possible for most other natural hazards, such as earthquakes. If the impact is on the sea, roughly twice as likely as on land, the resulting tsunamis could affect vast numbers of people living near coastlines. Wherever the impact, people could, in principle, be moved to safety, given sufficient warning and appropriate logistical support, although the degree of success would depend crucially not only on the size of the object but also on its composition, speed and angle of approach, and on the size of the population in the affected area. The great majority of impacts will be of smaller objects of less than a few hundred metres in diameter, for which moving people should significantly reduce loss of life. However, extensive material damage would nevertheless arise.

After impact from a large object - fortunately very much less common than the smaller ones just mentioned - it would be difficult to sustain the population during the long period which might follow when the Sun’s rays were blocked by dust injected into the atmosphere at the time of impact. The only realistic course would be to try to avert the predicted collision.

A number of possible mechanisms have been considered for deflecting or breaking up potentially hazardous Near Earth Objects; most would require the use of a spacecraft with some means of transferring energy or momentum to the object, for example by kinetic energy transfer (by heavy projectiles carried on the spacecraft or by causing a collision between asteroids), by chemical or nuclear explosives, or even by mounting “sails” on the object to harness the Sun’s radiation pressure. Some of these mechanisms are more realistic than others. Given warnings of decades or centuries, new technological developments would almost certainly emerge. The Task Force believes that studies should now be set in hand on an international basis to look into the practical possibilities of deflection.

To try to destroy an asteroid or comet in space by a single explosive charge on or below its surface would risk breaking it uncontrollably into a number of large pieces which could still hit the Earth, doing even more damage. A more promising method would be to fly a spacecraft alongside the object, perhaps for months or years, nudging it in a controlled way from time to time with explosives or other means.This relatively gentle approach is particularly important because many asteroids and comets are held together only by their own very weak gravitational fields.The longer the time before impact, the more effective even a small nudge would be. This is not science fiction. When NASA launches its Deep Impact mission to comet Tempel 1 in 2004, the spacecraft will eventually release a half tonne lump of copper to cause a huge crater in the comet. Although this is not the objective, the result will also be to deflect the comet’s orbit. However, the deflection in this case will be small in comparison to that required to deal with a real threat.

Long period comets present new dimensions of difficulty. By definition, such comets have never been seen before. They come unpredictably at all angles from the outer reaches of the Solar System, but can usually only be seen when at a distance of about 5 AU from Earth. Warning of the approach of such a body could well be less than a year. Urgent measures and even more powerful rockets and explosives would then be essential.

Any proposal to use nuclear explosives to deflect an asteroid or comet could well prove politically unacceptable in a world that seeks to reduce or abandon nuclear weapons, even though such explosives could probably be designed and controlled to prevent abuse. Indeed, the use of nuclear explosives might only be contemplated as a last resort if a major impact were otherwise inevitable.

In considering the prospects for deflection, it would be necessary to take into account a range of international treaties and principles including the original Outer Space Treaty of 1967.The work of the United Nations Committee on the Peaceful Uses of Outer Space is also relevant as is the work of the Inter-Agency Debris Co-ordination Group, comprising representatives of space agencies of the United States, Russia, China, India, Japan, Ukraine and Europe. Some of this group’s responsibilities are similar to those which would be required for any impacts by Near Earth Objects.

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