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Moon Mania
Moon Mania
10 Apr 2003
(Source: The Planetary Society)

More Moons for Saturn and Jupiter - When Will It End?
By Melanie Melton Knocke
Planetary Society
April 9, 2003

A team of astronomers announced today that they have discovered a new moon orbiting around Saturn, bringing that planet's moon count up to 31. Late last week, the same team announced the discovery of 6 new moons around Jupiter, bringing Jupiter's moon count to 58.

The latest addition to Saturn's family of moons is a small chunk of rock about 5 miles (8 km) in size. It follows a very elliptical, retrograde orbit that carries it far away from the ringed planet.

The moon was first discovered on February 5, 2003 by Scott Shepard and David Jewitt of the University of Hawaii and Jan Kleyna of Cambridge University. Follow up observations were done using the telescopes on Mauna Kea with Brian Marsden of the Harvard Smithsonian Center for Astrophysics providing orbital calculations. The discovery of this moon was formally announced on April 8, 2003.

The same team of astronomers is responsible for the discovery of six new moons of Jupiter announced last week. All of these Jovian moons appear to be very small with large retrograde orbits.

None of these newest moons - or any of the other 12 Jovian moons discovered earlier this year - resemble a traditional moon. All are very small, ranging in size from just a few miles across to as small as 0.62 miles (1 km). All of them travel in large, very elliptical (egg-shaped) orbits and all follow a retrograde path (they orbit in the opposite direction of the planet's rotation). As a result, these latest discoveries are classified as irregular moons.

Astronomers believe that irregular moons were formed elsewhere in the solar system and captured when they passed too close to Jupiter or Saturn. 14 of Saturn's 31 moons are classified as irregular. 50 of Jupiter's 58 moons are irregular as well.

With the whirlwind of moon discoveries, there may have to be changes to the definition of a moon, or at least some clarifications. Currently, anything that orbits a planet falls into the "moon" category - no matter how big or small. Under this definition, a tiny pebble would be considered a moon if it were in orbit around a planet.

Before a few years ago, we were unable to see anything as small as a few miles across, let alone a tiny pebble, orbiting around our neighboring planets. Now, however, with the increased sensitivity of telescopes and cameras, astronomers are seeing more and more. And, it is turning out that there is a lot of stuff out there!

Astronomers may have to decide if they want to stick with the current moon definition - absolutely everything orbiting a planet is a moon - or come up with some new categories.

Until then, every small rock astronomers discover is still a moon. So, don't expect a final moon count for quite some time. Just keep checking back here and we will try to keep these pages updated!

Below is the updated moon count - as of April 9, 2003.

Planet Moons
Mercury 0
Venus 0
Earth 1
Mars 2
Jupiter 58
Saturn 31
Uranus 21
Neptune 11
Pluto 1

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Last Updated: 28 Apr 2003