Re: Earth's Second Moon 10/26/99

[Ron Baalke <BAALKE@kelvin.jpl.nasa.gov>]

>This article shows a very clear diagram of Lagrangian points and gives
>further insight into co-orbital dynamics.
>
>http://sci.esa.int/categories/newsitem.cfm?TypeID=5&ContentID=7331

There are some rather blatant errors in this article.   When this study
few came out 2 years ago, the authors were claiming the asteroid
was a "companion" of Earth, but now I see they're trying to make
it more than it really is.

The first blatant error: since 
Asteroid 3753 Cruithne does not orbit the Earth, it cannot be called
the Earth's second moon.   The asteroid orbits the Sun in a
separate elliptical orbit just like other asteroids.  See these
diagrams of the asteroid's orbit:

http://www.asteroid.yorku.ca/3753_5.gif

Also, since the asteroid is in its own separate orbit and does not
share Earth's orbit, it also cannot be called a trojan asteroid, which
the authors of this article are claiming.  In all other cases of trojan 
asteroids such as with Jupiter and Mars, the trojan asteroids share the 
same orbit as the planet.  Asteroid 3753 Cruithe is not an Earth trojan.

The only thing the article can accurately claim, is the asteroid is
in orbital resonance with the Earth.  Also, this orbital resonance is
tied in with the Earth's Lagrange points.  But again, since the asteroid
does not share Earth's orbit, it is not a trojan asteroid.

The authors also put a lot of emphasis on the horseshoe orbit, but leave
out the critical fact that the horseshoe orbit of the asteroid only exists 
from an geocentric view (or as appeared from Earth).  You get a lot of 
strange looking orbit shapes from an Earth-viewing only perspective.

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