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Following the early bombardment
phase, the Earth has been a relatively tranquil place.
Although from time to time it is hit by asteroid and comet fragments, the
last impact that had a major effect on the global environment and
species evolution was 65 million years ago, at the end of the
Cretaceous period. At that time, approximately 60% of all known
species became extinct, most famously, of course, the dinosaurs. Although the Solar System is still a dangerous place, our planet is
now protected from the worst excesses of bombardment by the presence
of the giant planets. Jupiter, in particular, has been responsible for
accreting (swallowing) many inward-travelling bodies before they could
reach the Earth. The most recent and most spectacular example is the
impact of comet
Shoemaker-Levy 9. on this giant planet in 1994. Recently, there has been an increasing scientific and political
interest in the question of how great the risk of such impacts really
is. A detailed
report on this subject has been compiled and issued by the UK
Government. Other related sites include the Spaceguard
Foundation and the NeoDys (NEO Dynamic Site),
from where many other links will be found. Attention has mostly been directed towards the so-called
Near Earth Objects (NEOs), asteroids and comets, the orbits of
which cross that of the Earth. Several hundred such objects are known
at this moment, some with diameters larger than about 1 km, a size
that is considered to be sufficiently large to provoke a global
disaster in the event of impact. Extensive searches for NEOs are carried out with dedicated
astronomical telescopes, especially in the U.S. and Japan, and have
resulted in a rapidly increasing discovery rate of these
objects. Although the estimates are still quite uncertain, it is
believed that perhaps one third of all of the most dangerous objects
(i.e., those with diameters above 1 km) have been found by now. But
smaller objects may also cause dramatic damage by impacting the
Earth. There is general agreement that these searches must continue in
order to provide a complete inventory from which it will be possible
to predict possible impacts in the future. There has also been much discussion about how a possible impact,
once predicted, could then be avoided. In the case of smaller objects,
it may be sufficient to evacuate the population in the geographical
area where the impact is predicted to happen. However, if a larger asteroid is approaching, this would not be
enough. The only viable defence appears to be to deflect it into an
orbit that does not cross that of the Earth. The possibility of
destroying such an object, e.g. by a nuclear explosion, is less
attractive, in particular because some of the pieces may still hit the
Earth. The associated scenarios have been amply described in recent
science fiction films, e.g., "Deep Impact" and "Armageddon". The Tunguska
event in Siberia in 1908 was a reminder that even a
comparatively small body (some 50-70 meters) may cause great damage
(about 2000 square kilometres were severely affected).Impacts
Near-Earth Objects (NEOs)
Avoiding an impact
Last updated June 30, 2001