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May 16, 2002

New Satellites of Jupiter


Dr. David Jewitt 808-956-7682
Mr. Scott S. Sheppard 808-956-6098
Mrs. Karen Rehbock 808-956-6829

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University of Hawaii astronomers announce the discovery of 11 new satellites of Jupiter. These new satellites, when added to the eleven discovered the previous year by the Hawaii team, bring the total of known Jupiter satellites to 39. This is more than any other planet.


The new satellites were discovered during mid-December of 2001 by a team led by Scott S. Sheppard and David Jewitt from the University of Hawaii's Institute for Astronomy and including Jan Kleyna of Cambridge University, England. They used the Canada-France-Hawaii (3.6 meter) telescope with one of the largest digital imaging cameras in the world, the "12K", to obtain sensitive images of a wide area around Jupiter. The digital images were processed using high speed computers and then searched with an efficient computer algorithm. Candidate satellites were monitored in the succeeding months at the University of Hawaii 2.2-meter telescope to confirm their orbits and to reject closer asteroids masquerading as satellites. Orbits of the new satellites were fitted by both Robert Jacobson at the NASA Jet Propulsion Laboratory and Brian Marsden at the Minor Planet Center. The satellites were formally announced by the International Astronomical Union on Circular No. 7900 (May 16, 2002).


The 11 new objects all belong to the so-called "irregular satellite" class, meaning that they have large semi-major axes, eccentricities and inclinations. All are retrograde (they orbit in the direction opposite to the rotation of the planet), and possess similar semi-major axes (about 300 Jupiter radii or 20 million km) The estimated diameters are between about 2 and 4 kilometers, assuming a 4% albedo. As yet, nothing is known about their surface properties, compositions or densities, but they are presumed to be rocky objects like the asteroids.

The new discoveries bring the known total of Jupiter satellites to 39, of which 31 are irregulars. (The 8 regular satellites include 4 large objects discovered by Galileo and 4 small objects on circular orbits interior to that of Io). Jupiter's nearest rival for having the largest number of known satellites is Saturn, with 30 (of which 13 are irregular).


The large, elongated and inclined orbits of the irregular satellites strongly suggest an origin by capture. Since no efficient contemporary capture mechanisms are known, it is likely that the irregular satellites were acquired when Jupiter was young, possibly still in the process of condensing down to its equilibrium size. The precise mechanism of capture remains unidentified but there are two leading theories for the capture process. In the gas drag hypothesis, passing asteroids are slowed by friction with proto-Jupiter's bloated atmosphere. Those which do not burn up in the atmosphere like meteors are trapped in looping orbits like those of the new satellites. In the mass growth hypothesis, the rapid growth of Jupiter leads to capture of nearby, co-moving planetesimals. Both processes would have operated in the first million years of the solar system.

The irregular satellites are grouped into distinct dynamical families or clusters. This suggests that individual satellites are pieces of a few precursor bodies that have been shattered. The disruptions occurred either during the process of capture or possibly after capture due to collisions with Jupiter-crossing comets. Future measurements of the size distribution, surface properties and orbits of the satellites will help determine how they formed.

The Institute for Astronomy at the University of Hawaii conducts research into galaxies, cosmology, stars, planets, and the Sun. Its faculty and staff are also involved in astronomy education, deep space missions, and in the development and management of the observatories on Haleakala and Mauna Kea. Refer to for more information about the Institute.