A recently discovered asteroid has probably been following Earth around
the Sun for over a quarter of a million years, and may be made of
similar stuff to our planet, two researchers say. The space rock caught
their eye two months after it was found by the WISE infrared survey
satellite, launched in 2009 by the United States. “Its average
distance from the Sun is identical to that of the Earth,†said
Apostolos “Tolis†Christou of Northern Ireland’s
Armagh Observatory. The research, by Christou and David Asher of the
observatory, appears in the journal Monthly Notices of the Royal
Astronomical Society.
“What really impressed me at the time was how Earth-like its
orbit was,†Christou said. Most near-Earth asteroids have very
eccentric, or egg-shaped, orbits that take the asteroid right through
the inner solar system. But the new object, designated 2010 SO16, was
found to move on an almost circular path so that it can’t come
near any planet but ours.
Since astronomers often don’t initially know the precise
location of a newfound asteroid, the two scientists estimated it by
creating creating computer-simulated “clones†of the
asteroid for every orbit it could conceivably occupy. They then
simulated the evolution of these clones under the gravity of the Sun and
the planets for two million years into the past and in the future. They
found that all the clones closely mimicked our orbital motion around the
Sun, although as seen from Earth, they would seem to slowly trace out a
horseshoe shape in space.
Asteroid 2010 SO16 probably takes 175 years to make the trip from one
end of the horseshoe to the other, the investigators said. So while on
the one hand its orbit is remarkably similar to Earth’s,
“it keeps well away from the Earth,†Tolis said.
“It has likely been in this orbit for several hundred thousand
years, never coming closer to our planet than 50 times the distance to
the Moon.†This is where it is now, near the end of the
horseshoe.
These “horseshoe†orbits are not uncommon: Earth has at
least three other “horseshoe companions,†according to
the astronomoers. But, unlike the newly studied one, the others linger
for a few thousand years at most before moving on to different orbits,
Christou and Asher said. Also, the newfound one is much larger, with an
estimated width of 200 to 400 metres or yards. Where the asteroid came
from in under investigation, the pair said. One is that it could have
“leaked†from a population of objects near a so-called
triangular equilibrium points 60 degrees ahead of and behind the Earth
in its orbit. Such a population has been postulated but never observed
as such objects are always near the Sun in the sky. If they do exist,
they may represent relic material from the formation of Earth, Moon and
the other inner planets.
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