WEEKLY WHINE

Planetary eclipse

The solar system keeps getting more and more interesting every day. This week, the University of Arizona's Spacewatch program and the International Astronomical Union's Minor Planet Center identified a new satellite of Jupiter, the first satellite of the largest planet found in the past twenty years.

The Voyager probes detected Jupiter's inner satellites Metis, Adrastea, and Thebe in 1979. Before that, the most recent satellite to be discovered from Earth was Leda, located in 1974 by Charles Kowal. The new satellite, which has the provisional designation S/1999 J1 [a Satellite detected in 1999 photographs orbiting Jupiter, the 1st to be observed in that time]. The object was first detected in images taken at Spacewatch's .9 m [36 in] telescope on Kitt Peak, AZ in OCT 1999, during Jupiter's opposition. This afforded the best viewing of Jupiter and its satellites.

One object was given the designation 1999 UX₁₈ as an asteroid. On TUE 18 JUL 2000, Spacewatch's Tim Spahr caught up with the observations of 1999 UX₁₈, identifying it on WED 06 OCT 1999, TUE 19 OCT 1999, SAT 30 OCT 1999, and THU 04 NOV 1999. Spahr, though, just couldn't fit an orbit to the four observations, until he recognized that the images were adjacent to Jupiter; the object could well have been a Jovian satellite.

He talked to the MPC's associate director, Gareth Williams, who then talked to his boss, Brian Marsden. Independently, Marsden tried a Jupiter-orbiting solution and succeeded. The MPC partners also checked S/1975 J1, a possible satellite that Kowal observed but then lost. They also looked for other possible sightings of the same object but couldn't find any.

S/1999 J1 orbits Jupiter with a semimajor axis of 24,000,000 km [15,000,000 mi] on an orbit with eccentricity .130, according to IAU Circular 7460. It is part of the outer family of Jovian satellites, which have eccentric, retrograde orbits. Its partners are Ananke, Carme, Pasiphae, and Sinope. With an expected diameter of a mere 5 km [3 mi], S/1999 J1 would be the smallest satellite in the solar system.

Jupiter is just now coming out of opposition and will soon be available for viewing in the early morning. At that point, if other observers can recover S/1999 J1, it will earn official recognition from the IAU as a satellite of Jupiter. It will then receive the name Jupiter XVII until the IAU settles upon a permanent name, which the discoverers will have the opportunity to suggest. Most other Jovian moons are named after mythological lovers of Zeus or Jupiter.

S/1999 J1 is the seventh planetary satellite to be detected in the past three years. Brett Gladman, Phil Nicholson, and friends detected Uranian moons Caliban and Sycorax in 1997. Uranus XVIII was discovered last year in Voyager 2 images by Erich Karkoschka; it inhabits an orbit adjacent to the small moon Belinda. Uranus XIX-XXI were discovered last year by JJ Kavelaars and friends.