So far this year, Jupiter has gained 21 new distant satellites, with
the most recent announcement of satellite S/2003 J 21 on June 3rd at
the annual Canadian Astronomical Society (CASCA) meeting in Waterloo
(ON). This puts Jupiter far ahead of the all other planets, with 61
known moons. Astronomers from the University of British Columbia
(Professor Brett Gladman and postdoctoral researcher Lynne Allen) and
the National Research Council (Dr. JJ Kavelaars) of the National
Research Council of Canada are the discovery team.

The work has been made possible by the new Megaprime mosaic
(http://www.cfht.hawaii.edu/News/MegaPrime/) of CCD cameras at the 3.6m
Canada-France-Hawaii telescope.

Information on the discoveries, the tracking effort, and new results
are provided below. See also

http://www.astro.ubc.ca/people/gladman/jup2003.html

SIFTING THE SAND AT JUPITER

Astronomers in Canada have been busy the last few months peering around
the planet Jupiter to search for small new moons of that planet,
extending a spree of moon discoveries begun in 1997. The current team
of jovian searchers consists of UBC astronomers Brett Gladman and Lynne
Allen, and JJ Kavelaars of the National Research Council of Canada. The
international team includes Cornell University astronomers Phil
Nicholson, Joseph A. Burns, and Valerio Carruba, Jean-Marc Petit of the
Observatoire de Besancon, and Brian Marsden and Matthew Holman of the
Harvard-Smithsonian Center for Astrophysics.

Detecting satellites around gigantic Jupiter is a difficult undertaking
because they can occupy a large patch of sky around the planet (making
searching time consuming) and the feeble amounts of light they reflect
back to us must compete against the glare of brilliant Jupiter. New and
bigger detectors have made more complete searches possible, and twenty
one new jovian satellites have been discovered this year.

This boosts the number of known satellites of jupiter to sixty one. It
now appears that each giant planet’s irregular satellite population is
the result of ancient collisions between former moon and passing comets
or asteroids. “These collisions result in the production of families of
satellites in similar orbits,” said Gladman, “which seem to be the
rule”.

NEEDLE IN A HAYSTACK

The new satellites were a challenge to detect because most are only
about 1-5 kilometers in size. Their small size and distance from the
Sun prevent the satellites from shining any brighter than 24th
magnitude, about 100 million times fainter than can be seen with the
unaided eye. To locate these new moons, the canadian team has been
using the brand new Megaprime mosaic of CCD cameras at the 3.6m
Canada-France-Hawaii telescope on Mauna Kea in Hawaii. Covering all the
sky in which satellites could be found required this new large mosaic
camera, enabling them to quickly obtain images of the entire sky around
the planet in which the moons could be living. They used computer
algorithms to search the images for the faint points of light moving
across the sky in the manner expected for Jupiter moons. “Searching by
eye through the 50 gigabytes of images each night would be an
impossible task,” says UBC postdoctoral fellow Lynne Allen, “so we must
use powerful computers to sift through the data.”

THIS YEAR

Because moons can sometimes appear in front of distant stars or lost in
the light scattered from the planet, to really find them all requires
painstakingly repeating the search several times. The team has been
doing this in Febuary, March, and April of 2003 and has announced 9 new
satellites and provided observations on all 21 of this year’s
disoveries. The lastest announcement, named S/2003 J 21, is another
body in the cluster of satellites near Jupiter’s moon Ananke (the
latter discovered in 1951). NRC astronomer Kavelaars says: “The
tracking of these extremely faint objects is extremely difficult, but
necessary for without many observations one cannot calculate their
orbits around the planet in order to learn about their origin.” The
entire region around Jupiter has essentially been re-examined many
times during this spring, `picking up’ the moons which were by bad luck
unseen on some of the nights. “Their observational strategy involving
pairs of nights of observations each month has paid off”, says Brian
Marsden who has computed the orbits of the satellites based on the
observations. Satellites S/2003 J 13 through J 21 were announced with
observations from this team and an independent team at the University
of Hawaii.

AN INTERESTING ORBIT

Of all the jovian satellites discovered in the last two years, it is
the second to most recent one, S/2003 J 20 which stands out from the
pack. The canadian team has been tracking the satellite steadily this
year, improving its orbit and yielding two surprises. First, its orbit
stands apart from all other previously known Jupiter moons, thus
appearing not to be part of one of the known ‘families’ of objects.
Secondly, Valerio Carruba (Cornell University) has confirmed that this
object is lodged in an interesting orbital resonance with Jupiter. It
is in fact this Kozai resonance which sets the maximum orbital
inclination (orbital ’tilt’ with respenct to the plane of our Solar
System) these moons can have, for if more inclined their orbits would
distort periodically every century and drop down into the dangerous
realm of Jupiter’s larger (regular) satellites, which would eliminate
these small moons. The new satellite S/2003 J 20 is right on the edge
of the stability region, barely avoiding this fate.

THE FUTURE

The region around Jupiter has now been covered several times to the
faintless levels which can be reached. This means that except for a
trickle, the spree of irregular satellite discoveries that has occurred
since 1997 will slow because all the giant planets have now been
surveyed with modern technology. Novel observational techniques will
allow astronomers to discover a few fainter satellites (as has recently
been done for Neptune,) but this will not likely produce the rush of
discoveries that the deployment of the new generation of CCD cameras
has made possible.

IMAGES

Discovery image of S/2003 J 21 (postive B/W)
http://www.astro.ubc.ca/people/gladman/J21disc1.jpg

Discovery triplet of S/2003 J 21 (postive B/W). The faint moon
(circled) can be seen moving relative to the background stars.
http://www.astro.ubc.ca/people/gladman/J21disc.jpg

Black and white image of a Megacam field near Jupiter (black/white
positive image). The brilliant planet is just off the top of the field,
producing a great deal of scattered light that confuses the search.
http://www.astro.ubc.ca/people/gladman/jup2003mpp.jpg

Negative of the above image (negative image)
http://www.astro.ubc.ca/people/gladman/jup2003mpn.jpg

Discoverers Gladman and Allen pointing out the jovian moon
http://www.astro.ubc.ca/people/gladman/jupcagnes.jpg