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.

Discovery image of S/2003 J 21. Credit: CASCA

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".

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."

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.

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 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.