[meteorite-list] Earth Must Have Another Small Moon, Say Astronomers

[Ron Baalke]

http://www.technologyreview.com/blog/arxiv/27425/      

Earth Must Have Another Moon, Say Astronomers
MIT Technology Review
December 20, 2011

A study of the way our planet temporarily captures asteroids suggests
that 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. Instead, 2006 RH120, as it became
known, turned out to be a tiny asteroid just a few metres 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 interesting neighbour.

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
modelled 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 metre-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."

Ref: arxiv.org/abs/1112.3781 <http://arxiv.org/abs/1112.3781>: The
Population Of Natural Earth Satellites