NEW OUTER SATELLITE OF JUPITER DISCOVERED
From MPC PRESS INFORMATION SHEET:
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
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
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
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).
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