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Planetary News: Moon Discoveries (2005)

Twelve New Moons for Saturn

With the powerful Cassini orbiter in place at Saturn, it's easy to forget that scientists can still study the ringed planet from the Earth. But astronomers at the University of Hawaii have followed up on many previous moon discoveries with a dozen new satellites at Saturn, bringing the total count to 49 (although the status of three of the new moons identified by Cassini is in doubt). The satellites were discovered on December 12, 2004 using the 8.2-meter (27-foot) Subaru telescope on Mauna Kea. They were confirmed in observations conducted from January through March using Subaru and the 8-meter (26-foot) Gemini North telescope.

Unlike the moons recently discovered by Cassini, which are closely associated with Saturn's ring system, these 12 new moons are "irregular" satellites. They orbit very far from Saturn (around 20 million kilometers or 12 million miles away), and have inclined, elliptical orbits. All but one orbit Saturn in a retrograde fashion, opposite to the direction that Saturn's larger moons and rings travel around the giant planet.

Here are the basic facts about the 12 new moons. All are approximately 5 kilometers (3 miles) in diameter.

Of Saturn's 49 moons:

• Eleven are tiny bodies orbiting among or just outside the main ring system. These may just be big clumps of ring stuff. In fact, the identification of three of them as moons is not yet certain; they could have just been transient clumps in the rings, long-lived enough for Cassini to spot them in a sequence of ring images, but dissipating later. The nine ring-related moons are: Pan, Atlas, S/2004 S3, S/2004 S4, S/2004 S6, Prometheus, Pandora, Janus, and Epimetheus. Methone and Pallene are two more tiny bodies orbiting a little further beyond these.
• Eight are large, spherical or nearly spherical icy moons, worlds in their own right. These are Mimas, Enceladus, Tethys, Dione, Rhea, Titan, Hyperion, and Iapetus.
• Four are tiny bodies that accompany larger moons in their orbits, sitting in dynamically stable "Lagrange points" 60 degrees ahead of or behind the bigger moons. These are Telesto and Calypso (companions of Tethys) and Helene and Polydeuces (companions of Dione).

The remaining 26 are "irregular satellites," which fly around Saturn in elliptical, inclined orbits, suggesting that they may be captured bodies. One of these, Phoebe, is quite large and was studied closely by Cassini in June of 2004. Cassini's measurements suggest that Phoebe is an object that originally formed in a more distant reach of the solar system. The other 25 moons may also be captured bodies. Fourteen of the irregular satellites have names: Kiviuq, Ijiraq, Phoebe, Paaliaq, Skathi, Albiorix, Erriapo, Siarnaq, Tarvos, Mundilfari, Narvi, Suttungr, Thrymr, and Ymir. The remaining 12, just discovered, await their naming.

Here is the current moon count (as of May 3, 2005).