Re: All About Phoebe

Phoebe is the outermost of Saturn's recognised and well-characterized moons. I am not going to be a lot of help about the myths concerning Phoebe, and perhaps a bit of a wet blanket about the mysteries.

The reason that it is hard to find much information about Phoebe is simply that not a lot of information is known. Moons of outer planets in the solar system are a long way from us, and it is very difficult to get much detailed information from the Earth. A surprisingly large proportion of what is known comes from the Voyager space probes.

When the space probes have flown past Saturn, they have given us good information about the planet and the moons. But Phoebe is in an orbit a very long way away from the planet. There was a good view of seven of the nine major moons, which have orbits between 180 000 km (Mimas) and 1.4 million km (Hyperion) from the planet. One of the probes also got a view of Iapetus, 3.5 million km from the planet. But Phoebe spends its time in an elliptical orbit between 10 and 16 million km from Saturn, and was simply not in a position where it could be closely examined.

Phoebe is a smallish moon -- diameter about 250 km, compared with about 3500 km for our moon as a typical large moon, or 10 and 25 km for the two really small moons of Mars. Its orbit goes backwards around Saturn (it revolves from east to west) unlike all of Saturn's other moons, and is quite eccentric, that is elliptical, when most moon orbits are close to circular. These facts, together with the fact that its orbit is so unusually far from the primary planet, strongly suggest that it is a captured asteroid rather than one of Saturn's original flock of satellites.

A satellite of this size is too small to have an atmosphere, and there really is not much else about Phoebe that makes it particularly interesting to planetary astronomers. It is probably very much like a typical asteroid of the same size -- say Juno or Hebe -- though it may be icy rather than rocky. I do not think that even that is known.

For any other information, you will have to search astronomy references in the library, and do a web search. But do not be surprised if you find little or nothing more than this.

Note added by the astro moderator:

The Cassini Mission will observe Phoebe in 2004 upon arrival at Saturn: see http://www.jpl.nasa.gov:80/cassini/Mission/near.html

Marc Herant