Helene at Saturn

The Cassini spacecraft captured this closeup image of Helene on June 18, 2011. Helene is a small satellite of Saturn about 20 miles across. Helene is a coorbital satellite, which means that it shares an orbit with the much larger satellite Dione that is ~30 times larger and ~30,000 times more massive. Helene resides at Dione's L4 Lagrange point, which is a stable niche in Dione's orbit that leads that satellite by 60 degrees. The other stable niche is of course the L5 Lagrange point that trails Dione by 60 degrees. The orbit of a coorbital satellite is analogous to the Trojan asteroids that lead or trail Jupiter by 60 degrees in its orbit about the Sun.

No one knows how a coorbital satellite like Helene came to reside in such a special orbit. But it is conceivable that a coorbital satellite is debris that was excavated when the larger satellite Dione was stuck by a comet long ago. If this scenario is correct, then a lucky fraction of that debris managed to find and settle into one or both Lagrange points where it could have reassembled into a small coorbital satellite like Helene.

To see more images of Helene, as well as the rest of the Saturnian system, visit Cassini's CICLOPS website.

The Dawn spacecraft approaches asteroid Vesta

The is the Dawn spacecraft's view of the asteroid Vesta. Dawn is the NASA mission that will visit two asteroids, beginning with Vesta in a few weeks, and then Ceres in 2015. Dawn will go into orbit about Vesta on July 16 to study that asteroid's surface for about a year. Vesta is about 300 miles across and is the fourth largest asteroid. Vesta's surface is composed of basaltic rock, or lava, which makes this asteroid quite unique and very interesting. Evidently Vesta was volcanically active in the past, likely when it first formed 4.5 billion years ago.

In July 2012, Dawn will then fire up its ion engine and depart Vesta for its 3 year trip to asteroid Ceres, which is the largest asteroid, one that appears to be quite rich in water. Meanwhile, keep an eye on the Dawn mission website for better pictures of Vesta that will soon get much more interesting when the spacecraft goes into orbit.