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Horseshoe orbit
*** Shopping-Tip: Horseshoe orbit
Image:2002aa29-orbit-3.png thumb|250px| The orbit of [[2002 AA29|2002 AA29 and the Earth. Image:
Jet Propulsion Laboratory JPL]]
Image:2002aa29-orbit.png thumb|250px| The horseshoe orbit of AA29 with respect to the Earth every 95 years. Image: JPL
A '''horseshoe
orbit''' appears when a viewer on an orbiting body (like Earth) watches the movement of another orbiting body, whose orbit is skinnier (more eccentric), but has about the same period. As a result, the path appears to have the shape of a kidney bean.
The loop is not closed but will drift forward or backward slightly each time, so that the point it circles will appear to move smoothly along Earth's orbit over a long period of time. When the object approaches Earth closely at either end of its trajectory, gravitational exchange of energy conspires to change the object's appearant direction. Over an entire cycle the loops will fill out a horseshoe shape, with the Earth in the horseshoe's gap.
Several
asteroids (such as
3753 Cruithne and
2002 AA29 2002 AA29) are known to occupy horseshoe orbits with respect to
Earth. The moons of
Saturn (planet) Saturn Epimetheus (moon) Epimetheus and
Janus (moon) Janus occupy horseshoe orbits with respect to each other (in their case, there is no repeated looping: each one traces a full horseshoe with respect to the other).
----
In the following, "'''A'''" refers to the object that is tracing out the horseshoe with respect to the planet or moon. The primary is the object the planet/moon is itself orbiting.
If '''A''''s orbit is slightly inside the orbit of the planet/moon, A orbits slightly faster than that planet/moon. If '''A''' started in front of the planet, it would slowly drift ahead because it's orbiting faster.
Conversely, if '''A''''s orbit is slightly outside the orbit of the planet/moon, it orbits slightly slower. If '''A''' started a bit behind the planet, it will slowly drift behind because it's orbiting slower.
Starting from the high orbit (slower), '''A''' falls behind the planet and drifts away. But it is still in almost the same orbit. So eventually '''A''' drifts all the way around the orbit and approaches the planet from its front.
As '''A''' gets closer, the planet's gravity starts pulling. This reduces '''A''''s orbital velocity, so it drops inward with respect to the primary. The drop releases energy, so '''A''' speeds up as it settles in its new, faster, inner orbit. So '''A''' starts drifting away from the planet, because it is now moving faster.
'''A''' drifts around the orbit and catches up to the planet from behind. Now the planet's gravity increases '''A''''s velocity, which causes it to rise outward with respect to the primary. This absorbs energy, and '''A''' slows down as it settles in its new, slower, outer orbit. '''A''' now drifts away behind the planet (more accurately, the planet pulls away ahead of '''A''').
If you observe this cycle from the Earth, it looks like the asteroid moves slowly toward the Earth over a period of years. Then it mysteriously starts moving toward or away from the Sun. Finally it starts moving away from the Earth, without ever going around the Earth.
The cycle happens over the span of hundreds of actual orbits around the Sun or
other central body. But from the perspective of someone standing on the planet, it looks
like the asteroid is traveling in a horseshoe shaped orbit.
Category:Astronomy
Category:Celestial mechanics
{{astro-stub}}
de:Hufeisenumlaufbahn
fi:Hevosenkenkärata
*** Shopping-Tip: Horseshoe orbit
