Gotcha! Jupiter Turned Comet into a Moon
This Hubble picture, taken on 23 July, is the sharpest visible-light picture taken of the atmospheric debris from a comet or asteroid that collided with Jupiter on 19 July. This is Hubble's first science observation following its repair and upgrade in May. The image was taken with the new Wide Field Camera 3.
Credit: NASA, ESA, and H. Hammel (Space Science Institute, Boulder, Colorado) and the Jupiter Comet Impact Team

Jupiter already has an abundance of moons, but from 1949 to 1961 it had another, temporary satellite in the form of a comet trapped in the gas giant's gravitational grip.

Comet 147P/Kushida-Muramatsu was captured as a temporary moon of Jupiter in the mid-20th century and remained trapped in an irregular orbit for about twelve years, astronomers announced today.

There are only a handful of known comets where this phenomenon of temporary satellite capture has occurred and the capture duration in the case of Kushida-Muramatsu is the third longest.

The discovery was presented today at the European Planetary Science Congress in Potsdam by David Asher of Armagh Observatory in Northern Ireland.

An international team led by Katsuhito Ohtsuka of the Tokyo Meteor Network modeled the trajectories of 18 ?quasi-Hilda comets,? objects with the potential to go through a temporary satellite capture by Jupiter that results in them either leaving or joining the ?Hilda? group of objects in the asteroid belt. Most of the cases of temporary capture were flybys, where the comets did not complete a full orbit.

But Kushida-Muramatsu was different: The team used recent observations tracking the comet over nine years to calculate hundreds of possible orbital paths for it over the previous century. In all scenarios, Kushida-Muramatsu completed two full revolutions of Jupiter, making it only the fifth captured orbiter to be identified.

"Our results demonstrate some of the routes taken by cometary bodies through interplanetary space that can allow them either to enter or to escape situations where they are in orbit around the planet Jupiter," Asher said.

Asteroids and comets can sometimes be distorted or fragmented by tidal effects induced by the gravitational field of a capturing planet, or may even impact with the planet. The most famous victim of both these effects was comet D/1993 F2 (Shoemaker-Levy 9), which was torn apart on passing close to Jupiter and whose fragments then collided with that planet in 1994. Previous computational studies have shown that Shoemaker-Levy 9 may well have been a quasi-Hilda comet before its capture by Jupiter.

"Fortunately for us Jupiter, as the most massive planet with the greatest gravity, sucks objects towards it more readily than other planets and we expect to observe large impacts there more often than on Earth. Comet Kushida-Muramatsu has escaped from the giant planet and will avoid the fate of Shoemaker-Levy 9 for the foreseeable future," Asher said.

The object that impacted with Jupiter this July, causing the new dark spot discovered by Australian amateur astronomer Anthony Wesley, may also have been a member of this class, even if it did not suffer tidal disruption like Shoemaker-Levy.

"Our work has become very topical again with the discovery this July of an expanding debris plume, created by the dust from the colliding object, which is the evident signature of an impact. The results of our study suggest that impacts on Jupiter and temporary satellite capture events may happen more frequently than we previously expected," Asher said.

The team has also confirmed a future moon of Jupiter. Comet 111P/Helin-Roman-Crockett, which has already orbited Jupiter three times between 1967 and 1985, is due to complete six laps of the giant planet between 2068 and 2086.