Jupiter has been caught ‘kidnapping’ comets when they venture too close, forcing them to become temporary satellites of the great planet before they are slingshot away or spiral into Jupiter, according to new data presented at this week’s European Planetary Science Congress (EPSC) in Potsdam, Germany.

Recent evidence of a collision on Jupiter, possibly by a comet, came to light in July (see our report here). In his presentation at EPSC, Dr David Asher of Armagh Planetarium will today show how he and an international team of astronomers discovered that comet 147P/Kushida-Murumatsu had been trapped in an orbit around Jupiter for twelve years between 1949 and 1961 before being flung out of the Jovian system.

Comet 147P/Kushida-Murumatsu is only the fifth comet to have had its orbit traced back to reveal it has been circling Jupiter. Comet Shoemaker–Levy 9, which was broken into fragments by Jupiter’s gravity before colliding with the planet in July 1994, is of course the most famous of Jupiter’s ‘cometary hostages’. By tracing its path back, astronomers discovered that Shoemaker–Levy 9 had first entered a loose orbit around Jupiter as far back as 1930.

Those comets that don’t hit Jupiter, but are able to escape like 147P/Kushida-Murumatsu did, may join the Hilda group of objects in the Asteroid Belt (named after asteroid 153 Hilda), which exist between 3.7 and 4.2 astronomical units from the Sun, and are in a 2:3 orbital resonance with Jupiter (meaning their orbital periods around the Sun are two-thirds that of Jupiter). Asher, team leader Dr Katsuhito Ohtsuka of the Tokyo Meteor Network, and colleagues modelled the trajectories of 18 so-called ‘quasi-Hilda comets’ and found that while most made only short fly-bys of Jupiter, 147P/Kushida-Murumatsu made two complete orbits of the planet. In addition, they discovered that comet 111P/Helin–Roman–Crockett made three laps of Jupiter between 1967 and 1985, and will enter orbit around Jupiter again in the year 2068, leaving 18 years later after making six more orbits of the planet.

“The results of our study suggests that impacts on Jupiter and temporary satellite capture events may happen more frequently than we previously expected,” says Asher. This has implications for impacts on Earth. According to research by Barrie Jones and Jonti Horner of the Open University, Jupiter flings as many comets our way as it collides with, and hence isn’t the protector of the inner Solar System that everyone thought it was (read one of their papers here). Understanding how comets are interacting with Jupiter will allow us to better assess the risk of future impacts on Earth.