horseshoe orbit

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The gravitationally perturbed path traced out by a body in a similar orbit to another body of comparable or greater mass. Whichever of the two bodies is moving on the slightly smaller and faster orbit will eventually catch up with the other. If the two bodies are of comparable mass, their mutual gravitational pull will raise the first object into a larger and slower orbit so that it falls behind again, while the second body will be pulled into a smaller and faster orbit so that it moves ahead. In effect, the two have swapped orbits. Next time it will be the turn of what was the second body to catch up the first, whereupon the two will exchange places again. Neither body actually passes the other during these manoeuvres; rather, they describe a horseshoe-shaped orbit relative to each other. Such behaviour is exhibited by the Saturnian satellites Janus and Epimetheus, whose orbits are only 50 km apart. If one object is much more massive than the other, such as a planet and an asteroid or a moon and a ring particle, only the orbit of the smaller body is significantly altered by the encounters, being alternately raised or lowered by the gravity of the more massive body.

Subjects: Astronomy and Astrophysics.

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