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Post Info TOPIC: Themisto


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In September 1974, Jupiter was in opposition, near its closest point to Earth. Astronomer Charlie Kowal was on Mount Palomar, photographing the sky around Jupiter with a 48-inch telescope on three successive nights. Since the telescope had been set to track Jupiter in its slow slide across the background star field, the stars appeared on the photographic plates not as dots but as short streaks. Among the streaks, Kowal found a tiny speck, appearing in a slightly different spot on each night's plate. It was a previously unknown moon, now known as Leda.

Thirteen months later, September 30, 1975, Jupiter was again in opposition, and Kowal was back on the moon hunt. Sure enough, another moving speck was found near Jupiter on three successive nights. Unfortunately, follow-up observations were not made, possibly due to the new object's faintness (it was twice as faint as Leda), and possibly because time on the telescope was in heavy demand for other research. By the time anyone looked for Kowal's new moon, it was nowhere to be found.

With only three observations of the mystery object's location, it was difficult to know where to look. The object didn't seem to share orbital characteristics with any of Jupiter's other moons, so it was hard to even make an educated guess about where its motion would have carried it. There was even a chance that it was not a moon at all, but an asteroid whose position and speed only made it appear to be in Jupiter's vicinity.

Kowal's mystery moon finally resurfaced in November 21, 2000, with the advent of more powerful telescopes, electronic cameras and dedicated moon-search campaigns. The moon, now known as Themisto, was a mere rock less than three miles across. In retrospect, it was astounding that Kowal had photographed it at all, a quarter-century before any Jovian moon of comparable inconspicuousness.


Themisto (S/1975 J & S/2000 J1 & Jupiter XVIII) is about 8 km in diameter.
Themisto has a prograde (direct) orbit 7,391,650 km from the planet Jupiter. It has an orbital period of 129.82761 days and an orbit inclination of 13.865 to Jupiter's equator.

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