Institute for Astronomy
University of Hawaii
Honolulu, Hawaii

May 16, 2002



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.

Full illustrated story.

Univeristy of Arizona

July 21, 2000


Astronomers with the Spacewatch project at the University of Arizona in Tucson and the Minor Planet Center (MPC) at the Smithsonian Astrophysical Observatory in Massachusetts have discovered the first new moon of Jupiter in more than a quarter of a century.

Spacewatch astronomers Jim Scotti, Jeff Larsen, Tom Gehrels, Joe Montani and Spacewatch director Bob McMillan took images of the object, previously designated asteroid 1999 UX18, during their Spacewatch shifts in October and early November 1999.

MPC astronomers Gareth Williams and Tim Spahr, and MPC director Brian Marsden this week confirmed by orbital calculations that the object is not an asteroid orbiting the sun, but a previously unknown moon of Jupiter, the first found since 1974.

The newly found satellite is perhaps 3 miles across - the smallest moon ever found for any of the major planets, the discovers said. The moon, designated S/1999 J1, belongs to a subgroup of outer satellites that make complete, eccentric orbits around Jupiter at an average distance of 15 million miles every two years, the MPC astronomers calculate.

Spacewatch and the MPC have detailed the discovery of the satellite, S/1999 J 1, and images on the web at :

Further observations are needed to more reliably calculate the moon's orbit, after which MPC would assign the moon a permanent number, McMillan said.

Jupiter and its moons will be too close to the sun to be detected by Spacewatch for another few months, although larger telescopes might recover it sooner, McMillan said.

Spacewatch uses a 79-year-old, 36-inch telescope on Kitt Peak, Ariz., to survey the solar system for asteroids and comets. If the discovery is confirmed, S/1999 J 1 will be the 17th moon found at Jupiter and the first unknown moon detected by Spacewatch, McMillan said.

July 21, 2000

Communicated by Tom Gehrels:



A Joint Press Release from the Minor Planet Center at the Smithsonian Astrophysical Observatory and the Spacewatch Project of the Lunar and Planetary Laboratory of the University of Arizona.

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 Spacewatch program, which uses a 79-year-old, 36-inch telescope on Kitt Peak to survey the solar system for asteroids and comets, was concentrating in particular on the region of the sky near Jupiter. This was in October 1999, when Jupiter was about as close to the earth as it gets (less than 370 million miles) in its 12-year cycle about the sun. The planet was then at its very brightest, and the same would likely be true for any undiscovered satellite it may have. Spacewatcher Jim Scotti understood that this was therefore the time to look, so he scheduled electronic scans of the field on several nights over the course of a month.

Following the usual practice, measurements of the images of asteroids found in the scans were forwarded to the Minor Planet Center (MPC) for further study. There, MPC associate director Gareth Williams had recognized already last November numerous asteroids that were present in scans that included those obtained by Jeff Larsen on October 30 and by Scotti on November 4. The linked observations were published in the extensive electronic supplement to the monthly batch of Minor Planet Circulars.

Nobody noticed that one of the objects, given the asteroid designation 1999 UX18, was moving in a slightly unusual manner--a manner in fact suggesting that it might be a comet, except that it didn't look like a comet. And the work of both Spacewatch and the MPC had to move on, with numerous more electronic scans by the former and the processing by the latter including night-to-night linkages of data also from observing programs that nightly cover a much greater area of sky than Spacewatch.

The addition of Tim Spahr to the staff of the MPC in May 2000 allowed a more detailed inspection of some of the earlier data. On July 18, while testing a new computer program written by Williams on Spacewatch measurements made earlier in October 1999, Spahr suspected that he recognized observations of 1999 UX18 in data obtained by Tom Gehrels on October 19. With now a possible 16-day span, he hoped to be able to confirm this linkage by finding the object in Bob McMillan's data from October 6. There was indeed a candidate at about the right place, but on trying to put an orbit through the observations on the four nights he just could not get a satisfactory fit. At that point, it dawned on him that Jupiter was nearby, some two degrees, or four moon-diameters, away in the sky, and he wondered if the unsatisfactory fit were due to his having assumed the object to be traveling around the sun, when it reality it was traveling around Jupiter.

Spahr mentioned his dilemma to Williams, who in turn asked MPC director Brian Marsden to try his hand at the orbit calculation--but did not mention the Jupiter hypothesis. On seeing the bad fit, and realizing that Jupiter was nearby, Marsden also immediately suspected that the object was a satellite. Within a few minutes he had produced a jovicentric orbit that fitted the data very well. This calculation ignored the gravitational effect of the sun, however, so Marsden handed the problem back to Williams, providing him with the needed initial approximation for a calculation that did allow for the influence of the sun, Saturn and other planets.

In the mean time, Williams had confirmed that the object was not one of the known jovian satellites. Armed with a perturbed, jovicentric orbital solution, he also then searched the observational database in the hope of finding measurements of the new satellite, now given the designation S/1999 J 1, in data from 1998 (when it should have been about as bright as in 1999) and earlier. He and Marsden also examined the possibility that the object was identical with S/1975 J 1, a suspected satellite found by Charles Kowal on photographic plates taken in 1975 with the 48-inch Schmidt telescope at Palomar and lost after one week. It did not prove possible to make the linkage to 1975, and no observations of S/1999 J 1 were found from earlier years. Although Spacewatch covers less sky per night than other patrols that also regularly collaborate with the MPC, it has the advantage of routinely recording fainter objects. The new jovian satellite was too faint for these other surveys.

A check with the Spacewatch team in Arizona brought the information that the field had also been scanned by Joe Montani on October 12 last year. Arianna Gleason quickly inspected the scans and found the satellite's images, which had not been reported by the software because one of the three images was merged with a star. She measured the two usable images manually and forwarded the data to the MPC. Williams then worked this fifth night of data into his orbit solution. Since the total span of the observations was unchanged, the October 12 data had little effect on reducing the uncertainty of the calculation, but it was reassuring to see that the data from all five nights did fit together very well.

Williams' calculation shows that the new satellite belongs to the subgroup of outer satellites that travel around Jupiter in irregular orbits around an average distance of 15 million miles from the planet and take some two years to do so. The sun's gravitational influence makes these orbits highly erratic. The satellites orbit Jupiter in the opposite direction to the other jovian satellites and have undoubtedly been captured long ago in the past from orbits about the sun.

S/1999 J 1 is the first reasonably established outer satellite of Jupiter to be found since Kowal discovered Jupiter XIII, a member of the other subgroup of outer satellites, in 1974. An estimated 5 to 10 miles in diameter, that 1974 discovery, named Leda, has been held by some authorities to be the smallest confirmed satellite. With observations covering only one month, to be fully confirmed S/1999 J 1 will need to be observed again. A window of opportunity for reobservation is just opening, as Jupiter can now again be seen in the morning sky after its conjunction with the sun in May. The new satellite is rather fainter than it will become toward the end of this year, but the fact that its position can be better pinpointed at present makes it worthwhile to search for it now with a larger telescope. Granted that the Voyager mission in 1979 allowed Jupiter to be blessed with the recognition of three new inner satellites, reobservation of S/1999 J 1 will assure the largest planet a total of 17 confirmed satellites. At perhaps three miles across, the new satellite would clearly then be the smallest established for any of the major planets.

Technical information about S/1999 J 1 is contained on IAU Circular No. 7460 (issued July 20).

For images and Web links for these items, visit
Drive around town or to the nearest star party and show your support
of dark skies with the International Dark-Sky Association license-
plate frame. The IDA -- an alliance of astronomy enthusiasts,
lighting engineers, and city planners -- is leading the drive to
replace bad lights with good ones. For every "Stars Up, Lights Down"
license-plate frame sold, Sky Publishing (a lifetime corporate member
of the IDA) will donate $3 to the IDA. To order yours, visit our
online store at or call 800-253-0245.


The king of planets has a new member to its royal court, as astronomers announced the discovery of a new satellite of Jupiter on Thursday. The 20th-magnitude object was found as part of a survey program in the hopes of locating such small moons. Last year, James V. Scotti (University of Arizona) used the Spacewatch telescope atop Kitt Peak and found an object then designated 1999 UX18. Timothy B. Spahr (Harvard-Smithsonian Center for Astrophysics) was able to find the object on additional images taken even earlier. These and other follow-up observations were sufficient for astronomers to conclude that this object could not be orbiting the Sun. Brian G. Marsden (Minor Planet Center) determined that it instead orbits Jupiter.

Jupiter's 17th moon -- provisionally designated S/1999 J1 -- is perhaps only 10 kilometers wide. It orbits the planet at an average distance of 24.3 million km. It is the fifth member of a family of small moons that have inclined and retrograde (backward) orbits. The first of these outer bodies was discovered in 1908; they lie twice as far from Jupiter as the next inner satellites. The last moon to be found by ground-based observations was Leda by Charles Kowal in 1974. (The Voyager spacecraft spotted three moons close to Jupiter in 1979.)

SKY & TELESCOPE, P.O. Box 9111, Belmont, MA 02478 * 617-864-7360

Copyright 2000 Sky Publishing Corporation. S&T's Weekly News Bulletin and Sky
at a Glance stargazing calendar are provided as a service to the astronomical
community by the editors of SKY & TELESCOPE magazine. Widespread electronic
distribution is encouraged as long as these paragraphs are included. But the
text of the bulletin and calendar may not be published in any other form
without permission from Sky Publishing (contact or
phone 617-864-7360). Updates of astronomical news, including active links to
related Internet resources, are available via SKY & TELESCOPE's site on the
World Wide Web at

In cooperation with the American Association of Amateur Astronomers (,
S&T's Weekly News Bulletin and Sky at a Glance are available via electronic
mailing list. For a free subscription, send e-mail to and
put the word "join" on the first line of the body of the message. To
unsubscribe, send e-mail to and put the word "unjoin" on
the first line of the body of the message. If you should have any problems
either subscribing to or unsubscribing from the list, send a message to list
administrator John Wagoner at for assistance.
SKY & TELESCOPE, the Essential Magazine of Astronomy, is read by more than
200,000 enthusiasts each month. It is available on newsstands worldwide. For
subscription information, or for a free copy of our catalog of fine astronomy
books and products, please contact Sky Publishing Corp., 49 Bay State Rd.,
Cambridge, MA 02138-1200, U.S.A. Phone: 800-253-0245 (U.S. and Canada);
617-864-7360 (International). Fax: 617-864-6117. E-mail:
WWW: Clear skies!

Back to ASTRONET's home page
Terug naar ASTRONET's home page