Nice Orbital Simulations of Earth's Second Moon

Thanks, Jay. I had asked what distinguishes Cruithne from other NEOs
with semi-major axes almost exactly equal to 1 AU. The URL you quote
provides the answer

"
(Sept 18 2001) There are now two more near-Earth asteroids known to be
currently in resonant states similar to those of Cruithne. These are 1998
UP1 and 2000 PH5. We are currently in the process of preparing these
results for publishing.
"

Ron

"Huebner, Jay" jhuebn@...
09/17/02 05:46 PM

Second Moon

"Can you be more specific?"
Okay, I got this from
http://www.astro.queensu.ca/~wiegert/3753/3753.html. There is not universal
agreement of web sites on the orbits, but I believe this is a good one.
Some are clear about the fact that Cruthne's orbit is hard to understand.
But this site has nice graphics. The text (in part) reads;
"Cruithne is on an spiraling horseshoe orbit, similar to the one
sketched
above, but its behaviour is even stranger. First, it is an even more
tightly-wound spiral, with kidney-bean shaped loops. Secondly, one part of
the horseshoe actually overlaps the position of the Earth when viewed from
above. A series of four frames (to be read left to right) showing a time
lapse of Cruithne's horseshoe is shown below. Note again that Cruithne's
horseshoe is composed of tightly wound kidney-bean shaped spirals which
are
almost impossible to distinguish individually in the picture. Remember,
there are two types of motion going on: 1) every year, the asteroid traces
out a kidney bean, 2) over time, this kidney bean drifts along the Earth's
orbit, tracing out a spiral which, when complete (after 385 yrs) fills in
an
overlapping horseshoe".
Click for a larger image (60 Kb)
"Though it appears (in the small image above) that a simple ring is
formed, that is just a result of the asteroid's path overlapping the
position of the Earth before reversing direction (Click on the picture to
get a bigger and much clearer picture). The asteroid's tightly wound
spiral
horseshoe almost forms a complete ring before reversing direction, but a
small gap does remain. This overlap is just another interesting and
unexpected facet of the asteroid's behaviour, and is part of what makes
this
horseshoe the most complex known by far."
"The apparent overlap of the asteroid with the Earth does not result in
any danger of collision because the asteroid's orbit is highly inclined to
that of our planet. The overlap is caused by the asteroid passing below
the
Earth's position. To illustrate this point, two edge-on views of the Solar
System are shown below."
Click for a larger image (25 Kb)
"The high inclination of Cruithne's orbit relative to that of the other
planets is clear from these vantage points, and is what causes it to avoid
collisions. In fact, Cruithne doesn't even get particularly close to the
Earth: at its closest approach it only gets to within 0.1 astronomical
units
(about 15 million kilometers or 40 times the Earth-Moon distance). The
asteroid is currently just leaving the "overlapping" portion of its
horseshoe, and it will not get any closer to us than 0.3 astronomical
units
(45 million km = 28 million miles) in the near future. Each year, it is
at
its closest in the autumn and at this point it will pass almost directly
beneath the Earth's South Pole!"
etc. Jay Huebner