PLEASE NOTE:
*
NEW OUTER SATELLITE OF JUPITER DISCOVERED
From MPC PRESS INFORMATION SHEET:
http://cfa-www.harvard.edu/cfa/ps/pressinfo/S1999J1.html
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).
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