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Millions of new Comets Coming - Part 2 of 2

News Update, Ramble through the journals, by Professor Chris Kitchin,
ASTRONOMY NOW, Aug 1999, p. 8:

Close encounters of the stellar kind:

The possibility of a torrent of comets descending on the inner solar
system and producing a spate of disastrous impacts with the Earth, as a
result of the close passage of another star disturbing the Oort cloud,
has been suggested many times. The rather weak evidence for
periodicities of some 20 to 30 million years in the fossil record of
mass extinctions has even been used to suggest the existence of a
companion star orbiting the Sun at a distance of a few tens of thousands
of AU. The high precision of the data on positions and distances of
nearby stars obtained from the Hipparcos spacecraft observations, has
now allowed some quantitative investigations to be made of the
likelihood of such encounters (Garcia-Sánchez J. et al., Astron. J. 117,
1042, 1999). The authors combined ground-based data on radial velocities
with the Hipparcos data to determine the close passages of stars with
the solar system from eight million years ago to two million years in
the future. The Oort cloud which contains some 10^12 to 10^13 comets
extends out to about 1.5 ly (0.5pc, 100,000 AU), so a star would need to
approach the Solar System to within that distance before many cometary
orbits would be disturbed. Over the ten million year period studied by
the authors, only one star will penetrate the Oort cloud. That is the
star Gliese 710, which will pass by the Solar System at a distance of
about l ly (0.33pc) about 1.4 million years from now. The authors
estimate that this will result in an increase by about 50% in the number
of Earth-orbit crossing comets for some two million years after the
star's passage. No other stars will significantly affect the flux of
comets in the inner solar system over the selected period. A marginal
increase in the number of comets however may have resulted from the
closest approach of Algol (beta Per) which was 7.9 ly (2.4 pc) away some
seven million years ago. Extrapolating the results suggests that stellar
encounters to within 0.25 ly (0.08 pc) of the Solar System, are possible
at 60 to 70 million year intervals. The current closest star, Proxima
Centauri, is still closing on the Solar System and will have its closest
approach 28,000 years from now at a distance of 3.2 ly (0.9 pc).
Thereafter Proxima Centauri will move away from us, and our nearest
neighbour will change to the star AC +79 3888 in about 45,000 years
time.


Best regards,

Bernd

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