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: http://star.arm.ac.uk/annrep/annrep2003/node61.html
Дата изменения: Tue May 18 17:48:07 2004 Дата индексирования: Tue Oct 2 05:04:09 2012 Кодировка: Поисковые слова: crab nebula |
First, the rate at which meteorites strike the Moon can be inferred from
the number and age distribution of lunar meteorites found in Antarctica
and elsewhere. Coupled with hydrodynamic code simulations of
crater-forming impacts on the Moon, Napier finds that the small-body
impact rate on the Earth is substantially higher than most current
estimates, with a Tunguska-like impact expected roughly once every
100-200 years. One or more impacts in the thousand-megaton range may
occur over time-scales 1,000-5,000 years. These impacts would
eject a considerable mass of dust into the stratosphere and so be
accompanied by brief, global coolings. They may constitute the greatest
short-term celestial hazard to civilization.
Secondly, the case of multi-km bodies has been studied building on work by
Bailey & Emel'yanenko, and Wickramasinghe & Hoyle. Dynamical models by
Bailey & Emel'yanenko had previously shown that the population of comets
captured from the Oort cloud and thrown into short-period orbits (orbital
periods 20-200 years) should be about 100 times greater than is actually
observed. They suggested that the missing comets may comprise a large,
dormant population. If so, this population would constitute a significant
hazard. However these hypothetical bodies would still be detected through
ground-based telescopic surveys 50-100 times more often than they
actually are: at least 400 should by now have been discovered, as against
the handful actually found. Thus it has been argued that these dormant
comets do not exist in significant numbers, and that the active comets
have not become extinct but have instead simply disintegrated. However
Napier, in collaboration with Chandra and Janaki Wickramasinghe at Cardiff
University, has shown that the surfaces of inactive comets, if comprised
of loose, fluffy organic material like the comet dust which enters the
Earth's atmosphere, may developed reflectivities which are effectively
zero in visible light. The NEO population may therefore include a
substantial population of `stealth comets': multi-km bodies, undetectable
with current surveys. These bodies may constitute the dominant impact
hazard in the 100,000 megaton range.