Many moons are locked in synchronous rotation with their mother
planets. Examples include the Galilean moons of Jupiter, Neptune’s
moon Triton and our own Moon.
In the 80s and 90s astronomers noticed that the distribution of
craters on these objects was asymmetric: they were more heavily
cratered on their leading hemispheres which makes sense since it
seems obvious that these areas should be struck more often.
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It wasn’t until 2003, however, that the same asymmetric crater
distribution was measured on our Moon. Now Takashi Ito at the
National Astronomical Observatory in Japan and Renu Malhotra at the
University of Arizona have asked an interesting question. of the
data. Can the asymmetric distribution of craters on the Moon be
explained by the known distribution of near Earth asteroids
that are thought to have caused them? Their answer is a cautious
“no”.
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To properly explain the crater distribution, Ito and Malhotra say
some other factor must have been involved. One possibility is that we
simply haven’t seen all the craters yet: the ongoing lunar mapping
missions may help on that score.
Another idea is that the Earth’s tidal forces tear Earth-crossing
asteroids apart, creating a higher number of impacts than might
otherwise be expected.
But the most exciting and potentially worrying possibility is that
there exists a previously unseen population of near Earth asteroids
that orbit the Sun at approximately the same distance as the Earth.
These have gone unnoticed because they are smaller or darker than
other asteroids, say Ito and Malhotra.
“More complete observational surveys of the near-Earth
asteroids can test our prediction,” they say.
And let’s not waste too much time about it. By some reckonings,
asteroid impacts represent the greatest threat to humankind that we
are able to calculate.
