17th Moon of Jupiter Discovered

By Keith Cowing
July 22, 2000
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Artist's ConceptAccording to a joint press release, “a collaboration between the Spacewatch program at the University of Arizona and the Minor Planet Center at the Smithsonian Astrophysical Observatory in Massachusetts has netted the first new outer satellite of Jupiter to be discovered in a quarter of a century.” The tiny moon, dubbed S/1999 J, is Jupiter’s 17th known natural satellite.

This same object was originally observed by the Spacewatch telescope at Kitt Peak in October 1999 and thought be an asteroid. As such, it was given the designation 1999 UX18. Further observations showed it to be pursuing an orbit a bit odd for an asteroid – one more like that of a comet. Additional observations, and the realization the Jupiter was rather close to this asteroid’s location in the sky, led to the realization that this object could be in orbit around Jupiter. Calculations showed this to be the case.

Astronomers then checked historical data to see if this object had been seen before so as to confirm what calculations had now led them to suspect. After failing to link it with previously observed objects in years past, they eventually found observations of the same portion of the sky earlier in October 1999. While existing observations of this object span only one month, they are enough to verify the location of the object in orbit around Jupiter. Further observations will be made soon to further refine the object’s orbit and size. At the present time, S/1999 J is thought to be exceedingly small – as little as 3 miles across. This is the smallest known satellite of any of the 9 major planets in our solar system.

Discovery image animation Animated image of S/1999 J [right] courtesy of Spacewatch. Click on image or here for larger version.

Related Links

° New Outer Satellite of Jupiter Discovered, press release,
Harvard-Smithsonian Center for Astrophysics

° Images of S/1999 J, Spacewatch

Background Information

° Harvard-Smithsonian Center for Astrophysics

° Spacewatch

° Asteroids and Comets, SpaceRef Directory

SpaceRef co-founder, Explorers Club Fellow, ex-NASA, Away Teams, Journalist, Space & Astrobiology, Lapsed climber.