Close up view of Prometheus

On 27 January 2010, the Cassini spacecraft acquired this close-up view of the small 60km Saturnian satellite Prometheus. Shadows cast by its irregular surface suggests that this satellite might have the `flying saucer' shape that is also exhibited by Saturn's other small satellites Pan and Atlas (as detailed in Porco et al 2007). Note that this peculiar shape can result when a satellite forms while orbiting inside a dense planetary ring (such as perhaps Pan, which does orbit in Saturn's main A ring) while also under the influence of Saturn's strong gravitational tide. This in turn suggests that Prometheus (as well as its cohorts Atlas and Pandora) might actually have formed at the outer edge of the A ring and then migrated to its current position just beyond the A ring's outer edge. This kind of radial migration is in fact expected to be driven by the strong gravitational torques that the A ring exerts on the nearby satellites. Check the CICLOPS website for more details.

Recent collision in the asteroid belt?

This dust trail was imaged by the LINEAR (Lincoln Near Earth Asteroid Research) survey on January 6. The trail is named P/2010 A2, and the arrows point to a faint 200 meter object that is the likely source of this dust. Ordinarily, dusty streaks such as this are comet tails, which form when as the comet's icy surface sublimates (melts). That process also liberates small dust grains from the comet's surface. Pressure due to sunlight then sweeps that dust cloud out into a long tail. Comets are known to inhabit the asteroid belt, but only in the outer part. But this dust trail lies in the inner asteroid belt, where comets are not known to reside. So the current thinking is that this trail, which might only be weeks old, could instead be debris from a recent collision between two asteroids. So this picture could be the first view of the aftermath of a never-before-seen astronomical event---the collision between two asteroids. See this New Scientist article for more details.