December 20, 2011
The way our planet
temporarily captures asteroids suggests Earth should
have at least
one extra moon at any one time.
Back in 2006, the
Catalina Sky Survey in
Arizona noticed that a mysterious body had begun orbiting the Earth.
This object had a spectrum that was
remarkably similar to the titanium white paint used on Saturn V
rocket stages and, indeed, a number of rocket stages are known to
orbit the Sun close to Earth.
But this was not an object of ours.
2006 RH120, as it
became known, turned out to be a tiny asteroid just a few meters
across - a natural satellite like the Moon. It was captured by
Earth's gravity in September 2006 and orbited us until June 2007
when it wandered off into the Solar System in search of a more
2006 RH120 was the first reliably
documented example of a temporary moon.
But there should be many more examples, say Mikael Granvik
and buddies at the University of Hawaii in Honolulu. Today these
guys say they have modeled the way the Earth-Moon system captures
these objects to understand how frequently we can expect to have
additional moons and how long they should stay in orbit.
The answer is straightforward to state.
"At any given time, there should be
at least one natural Earth satellite of 1 meter diameter
orbiting the Earth," say Granvik and co.
These objects should hang around for
about 10 months and make about three revolutions of the planet. That
means Earth ought to have a meter-sized moon right now.
This is of more than academic interest. NASA has repeatedly said it
is interested in sending humans to a near Earth asteroid. What
better than to kick off with one that is in orbit here?
Finding a suitable candidate will be tricky though. Asteroids that
are likely to become temporary satellites in the near future will be
small and therefore hard to see. What's more, they will be subject
to many forces pushing and pulling them so that predicting when and
if they will ever be captured will be next to impossible.
But improved monitoring might help spot them when they get here,
which might allow a launch to be planned in advance.
Granvik and co conclude:
"The scientific potential of being
able to first remotely characterize a meteoroid and then visit
and bring it back to Earth would be unprecedented."