Jupiter had temporary moon for 12 years

Jupiter snared a passing comet in the middle of the last century, eventually releasing it 12 years later, astronomers reported on Monday.

Data presented at the European Planetary Science Congress in Potsdam, near Berlin, showed that the biggest planet of the Solar System gained a temporary satellite, a comet called 147P/Kushida-Muramatsu, between 1949 and 1961.

It is only the fifth captured comet to be identified, a press release said.

Comets are lonely wanderers of the Solar System, sometimes taking decades or even centuries to complete a long orbit around the Sun.

On rare occasions, though, these enigmatic bodies of ice and dust can wander into the vicinity of a planet, where they are netted by its gravitational pull.

Sometimes, the comet breaks up and smashes into the planet, as was famously the case with Comet Shoemaker-Levy 9, whose fragments smashed into Jupiter in 1994.

Most of the observed temporary captures have been flybys, in which a comet does not complete a full orbit before wresting itself free.

Kushida-Muramatsu, though, completed two full revolutions of Jupiter, following an irregular orbit, before it gained its freedom, according to calculations led Katsuhito Ohtsuka of the Tokyo Meteor Network.

"The results of our study suggests that impacts on Jupiter and temporary satellite capture events may happen more frequently than we previously expected," said David Asher of Northern Ireland's Armagh Observatory, who presented the data in Potsdam.

The study has a bearing for understanding the risk of a cometary impact on Earth, an event that would wreak catastrophic damage. A colliding asteroid or comet is believed to have ended the long reign of the dinosaurs some 65 million years ago.

Jupiter is often seen as a "goalkeeper" that takes hits from comets that could possibly endanger Earth.

But it can deflect passing comets, too, altering their track around the Sun.

Thus understanding how this process works will help astronomers trying to evaluate the risk to Earth from rogue rocks.

Comet Kushida-Muramatsu appears to have avoided the fate of Shoemaker-Levy 9 "for the foreseeable future," said Levy.