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.
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