Replies

It was only ever weakly gravitationally bound to earth. An asteroid orbiting the sun in the same direction as the earth and the other planets can be captured by a planet when it's orbit crosses near the orbit of the planet in front of the direction in which the planet is orbiting the sun. The gravitational attraction drags the asteroid towards the planet (and the planet slightly towards the asteroid). The orbital velocity of the asteroid decreases and it can be kept in orbit around the planet. The amount of deceleration experienced by the asteroid determines how tightly it is bound to the planet.

The any object orbiting a planet is perturbed by the gravity of every other particle in the solar system. For objects orbiting the earth, the significant perturbing bodies are, more or less in order: the moon, the sun, Venus and Jupiter. Depending on their relative locations, they can provide just enough of a push to free a weakly bound object from the grasp of earth's gravity and continue in orbit around the sun.

Jupiter, of course is the king of capture, remember Shoemaker-Levy? It was captured in 1968, but not observed until 1994, shortly before its perturbed orbit caused it to collide with Jupiter.

The opposite of capture occurs, too. An asteroid approaching a planet's orbit from behind can be given a boost that increases its velocity enough to hurl it out of the solar system. Numerous solar system missions take advantage of the gravitational boost available from Jupiter and the other planets.

any object orbiting a planet is perturbed by the gravity of every other particle in the solar system. For objects orbiting the earth, the significant perturbing bodies are, more or less in order: the moon, the sun, Venus and Jupiter.

If you look for accurate ephemerides on the web, you will find instead of earth, a mysterious object called EMB. This is the earth-moon barycenter, which must be regarded as the object orbiting the sun as a single mass.

To speak of 2006 RH120 as being "perturbed by the moon" is like saying that Apollo 8, e.g., was orbiting the earth and "perturbed by the moon". I notice that the cited article is ambivalent on this point, which is amusing in light of the flap over the planetary status of Pluto. The embarrassing fact is that the earth-moon system is a double planet.