A Jovian moon lost and found

In September 1975, astronomer Charles T. Kowal had his eye on Jupiter, trying to pinpoint the position of a Jovian satellite he had discovered the year before. Sky conditions were perfect, and when Kowal examined his photographs, he found what appeared to be yet another moon of the giant planet. A week later, another astronomer photographed the same object.

That was the last time anyone spotted the body.

On Nov. 21, 2000, however, David C. Jewitt and his colleagues from the University of Hawaii in Honolulu found an intriguing object near Jupiter. Because of its slow motion relative to the stars and its location, the researchers suggested the object was a previously unseen moon.

When Brian G. Marsden of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., examined the body’s orbital data, he pegged it as the very object Kowal had found and lost 25 years earlier.

“The missing Jupiter satellite is one of the classic stories of planetary astronomy,” says Jewitt. “I had always assumed that the 1975 object had been some sort of misidentification.”

Jewitt’s team, which announced the find in a Nov. 25 circular of the International Astronomical Union, estimates that the moon has a diameter of only 16 kilometers and an orbit that’s large, elliptical, and inclined relative to Jupiter’s equator. Nine other Jovian moons have similar characteristics.

“I was amazed and delighted at the recovery of this satellite,” says Kowal, now at the Johns Hopkins Applied Physics Laboratory in Laurel, Md. “I had come to believe it was a passing comet.”

In a Jan. 5 IAU circular, Jewitt’s team reports finding 10 previously unknown Jovian moons, bringing the total to 28.

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