Smithsonian.com

Comet Shoemaker-Levy 9 broke into pieces before hiting Jupiter in 1994. The dark spots mark the impact sites. (Credit: Hubble Space Telescope Comet Team and NASA)

How many moons does Jupiter have? The answer may not be as simple as it sounds. Jupiter, the largest planet in our solar system, is so big that it can deflect the paths of comets and other objects, some of which might otherwise hit the Earth. Some of those comets hit the surface of the gas giant. Others, though, may circle Jupiter for years as temporary moons before continuing on their way through the solar system or meeting their end on the planet’s surface.

The most famous object to impact Jupiter is probably Comet Shoemaker-Levy 9, which crashed down in 1994. However, the comet first circled the planet as a temporary moon, trapped by Jupiter’s gravitational pull. And it isn’t the only one.

Astronomers from Japan and Northern Ireland, presenting their findings today at the European Planetary Science Congress, used observations of Comet Kushida-Muramatsu—from when it was discovered in 1993 and when it returned in 2001—to calculate the comet’s path over the previous century. They determined that the comet became a temporary moon when it entered Jupiter’s neighborhood in 1949. It made two full, if irregular, orbits around the planet, and then continued its travels into the inner solar system in 1962.

The researchers also predict that Comet 111P/Helin-Roman-Crockett, which circled Jupiter between 1967 and 1985, will again become a temporary moon and complete six loops around the planet between 2068 and 2086.

“The results of our study suggests that impacts on Jupiter and temporary satellite capture events may happen more frequently than we previously expected,” David Asher of Northern Ireland’s Armagh Observatory told the AFP.

So how many moons does Jupiter have? Depends on when you ask.