From: Eric Jablow <erjablow at netacc.net>
To: WSFA members <WSFAlist at keithlynch.net>
Subject: [WSFA] Re: Jupiter hogging moons
Date: Sun, 6 Apr 2003 20:33:15 -0400
Reply-To: WSFA members <WSFAlist at keithlynch.net>

On Sunday 06 April 2003 09:28, ronkean at juno.com wrote:
> On Sun, 6 Apr 2003 08:18:34 -0400 "Wade Lynch" <wsl1 at erols.com>
writes:

> Perhaps there might be a few natural moons orbiting Earth that
> are not known.  Moons the size of a basketball, or the size of
> an egg, might not be noticed.
>

Well, there is Cruithne, discovered in 1997.  It doesn't orbit
the Earth, but its orbit around the Sun is profoundly affected
by the Earth. It has an elliptical orbit, with inclination 20
degrees, currently with an orbital period of 0.99 years; at its
closest, it's 0.3 AU from the Earth, and passes almost directly
under the Earth's South Pole.

But in a couple of hundred years, the Earth will perturb its
orbit into one with a period of 1.01 years, slightly outside the
Earth's orbit.  Then, 385 years later, it gets pulled back.

However, computer simulations show that Cruithne might be
perturbed into a more nearly standard orbit around the Earth,
and might be perturbed back a few thousand years later.  But,
no, it will not collide with the Earth.

Why had Cruithne not been discovered and analyzed earlier?
Because the three-body problem is hard!  By the way, it's
pronounced "Croo-een'-ya".

Two asteroids discovered in 1998 and 2000 have similar behaviors.

And, 2002 AA29 definitely has that behavior; its horseshoe around
the Earth takes 95 years to describe, but it occasionally gets
captured for about 50 years at a time.  The next time this will
happen is around 2600 AD.

See