Expert: Tom Whiting - 4/7/2011

Question
This asteroid orbit makes absolutely no sense to me. How could it arise and exist in this way? It looks impossible. Could you clear it up a bit? Thanks very much for your thoughts and time.

http://www.space.com/11316-asteroid-earth-horseshoe-orbit.html

Answer
Hi Reg,
The horseshoe diagram is misleading as it keeps the Earth stationary... therefore the horseshoe pattern is the way we see the body tracking across our night sky.
But in reality (from either the viewpoint of the asteroid, or better still, from a vantage point above the solar system) the body is REALLY paralleling the Earth. For a while it is inside our orbit say just behind the earth, traveling faster around the sun, then it crosses over (either slightly above or below the Earth - no fear of collision at a distance of over a million miles at closest approach).... and moves into a slightly more distant (and slower) orbit around the sun.
So basically, first it's inside our orbit traveling faster around the sun, then outside our orbit traveling slower around the sun... all in the same direction... counter-clockwise around the sun,
as seen from above... see
http://www.technologyreview.com/blog/arxiv/26608/
Note the phrase "from the asteroids point of view, it orbits the sun in the same direction sometimes in a quicker orbit and sometimes outside our orbit in a slower orbit"...
So forget the horseshoe pattern, think of a auto racetrack where another car is sometimes inside your track traveling around the circle faster, and sometimes outside your track traveling slower.
That's the way to look at it.
This may not be unique... Jupiter has it's "Trojan asteroids" following ahead of and behind Jupiter in a stable Langranian equilibrium point, so it's not unusual to have small bodies in gravitational equilibrium orbits around, or in equilibrium with a parent body.
Hope this helps,
Clear Skies,
Tom Whiting
Erie, PA USA

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Tom