Sunday, May 08, 2005
Phoebe Featured
... in this month's Nature. You can see the image of Phoebe here. This was pieced together from two images from last summer's fly-by on the way into Saturn.
"Once in a while, these things get lucky and get captured by one of the giant planets, says Jonathan Lunine, an astronomer with the Lunar and Planetary Laboratory in Tucson AZ. The evidence is clear that objects move about in the solar system, and that Phoebe looks a lot like objects from way farther out in the solar system, but scientists have yet to come up with an actual mechanism for this moon to have been captured into an orbit around Saturn.
Moon-size objects don't just check out a planetary neighborhood and decide to slow down of their own volition. An object must lose speed to be captured by gravity. One mechanism is by grazing the top of an atmosphere. The air will cause drag on the object and if the conditions are just right, voila: a new moon! Problem with Phoebe is that it's several million miles out from Saturn. Once a captured object has grazed the atmosphere of a planet and been captured by gravity, it will return to that orbital low point and continue to spiral in closer. Sometimes the object will break up, as was the case with the comet Shoemaker-Levy prior to its 1994 plunge into Jupiter.
So the mystery remains: how did Phoebe get captured? The outer four giant planets of our solar system each have a small population of moons in irregular orbits. We know they got there from somewhere else. But how?
