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TIE-L  July 2000

TIE-L July 2000

Subject:

New Outer Satellite Of Jupiter Discovered

From:

Ron Baalke <[log in to unmask]>

Reply-To:

Technology in Education <[log in to unmask]>

Date:

Fri, 21 Jul 2000 21:08:43 GMT

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (140 lines)

http://cfa-www.harvard.edu/cfa/ps/pressinfo/S1999J1.html

PRESS INFORMATION SHEET:

NEW OUTER SATELLITE OF JUPITER DISCOVERED

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 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.

----------------------------------------------------------------------------

This page may not be copied onto other Web sites, but other sites may place
"clickable" pointers to this page. Photo-copies of this information sheet
may be distributed for educational purposes, provided that no charge is made
in doing so; such photocopies must include full credits. Journalists and
writers may quote from this "information sheet", provided that proper credit
is given. Further information is available by writing to Mail Stop 18;
Smithsonian Observatory; 60 Garden St.; Cambridge, MA 02138; U.S.A.

----------------------------------------------------------------------------

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