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 CircularNo. 7460 (issued July 20).
Financial support for the Spacewatch Project is summarized on its web site.
The MPC computations were carried out using the Tamkin Foundation Computer Network.
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