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Astromoners find newly discovered asteroid is Earth’s companion

Astronomers from the Armagh Observatory in Northern Ireland have found
that a recently discovered asteroid has been following the Earth in
its motion around the Sun for at least the past 250,000 years and may
be intimately related to the origin of our planet.

Their work appears in a paper in the journal Monthly Notices of the Royal Astronomical Society.

The asteroid first caught the eye of the scientists, Apostolos “Tolis”
Christou and David Asher, 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”, says
Dr. Christou, “but what really impressed me at the time was how
Earth-like its orbit was”. Most near-Earth asteroids — NEAs for short
— have very eccentric, or egg-shaped, orbits that take the asteroid
right through the inner solar system. But the new object, designated
2010 SO16, is different. Its orbit is almost circular so that it
cannot come close to any other planet in the solar system except the

The researchers set out to investigate how stable this orbit is and
how long the asteroid has occupied it. To do that, they first had to
take into account the current uncertainty in the asteroid’s orbit.
“Not knowing precisely the location of a newly-discovered NEA is quite
common”, explained Dr. Asher. “The only way to eliminate the
uncertainty is to keep tracking the asteroid for as long as possible,
usually months or years”. But the two scientists overcame that problem
by creating virtual “clones” of the asteroid for every possible orbit
that 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 remained in a so-called “horseshoe”
state with respect to the Earth. In this configuration, an object
mimics very closely the orbital motion of our planet around the Sun,
but as seen from Earth it appears to slowly trace out a horseshoe
shape in space. Asteroid 2010 SO16 takes 175 years to make the trip
from one end of the horseshoe to the other. So while on the one hand
its orbit is remarkably similar to Earth’s, in fact “this asteroid is
terraphobic”, explains Tolis. “It keeps well away from the Earth. So
well, in fact, that 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 trailing the Earth.

Currently, three other horseshoe companions of the Earth are known to
exist but, unlike 2010 SO16, these linger for a few thousand years at
most before moving on to different orbits. Also, with an estimated
diameter of 200-400 meters, 2010 SO16 is by far the largest of Earth’s
horseshoe asteroids. The team have already used the Las Cumbres
Observatory’s Faulkes Telescope in an ongoing campaign to track the
object and refine its orbit further. “It is not that difficult to spot
with a medium-sized professional telescope”, says Dr. Asher. “It will
remain as an evening object in Earth’s skies for many years to come.”

Ultimately, Christou and Asher would like to know where it came from,
and they have already thought of several possibilities. It could be an
ordinary asteroid coming from the Main Belt between Mars and Jupiter.
In that case, the random gravitational pull of the different planets
would be responsible for its present orbit; something that Tolis and
David think is an unlikely proposition. It could also be a piece of
the Moon that escaped the gravity of the Earth-Moon system and went
into an independent orbit around the Sun. However, the very stability
of its orbit means that there is currently no way to transport it from
the Moon to where it is now. Finally, 2010 SO16 could represent
leakage from a population of objects near the so-called triangular
equilibrium points 60 degrees ahead of and behind the Earth in its
orbit. Such a population has been postulated in the past 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 4.5 billion years ago.

For the time being, the astronomers would like to see the physical
properties of the object studied from the ground, especially its
color. “Color, a measure of an asteroid’s reflectivity across the
electromagnetic spectrum, can tell you a lot about its origin”, they
explain. “With this information we can start testing possible origin
scenarios with hard data. If it proves to be unique in some way, it
may be worth sending a probe to study it up close, and perhaps bring
back a sample for laboratory scrutiny.”


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