Outburst by comet Holmes

Comet 17/P Holmes suffered an outburst in 1892, which led to its discovery then by Edwin Holmes. Causes for a cometary outburst are uncertain, but they might be due to solar heating after close passage around the Sun, or perhaps are due to a collision with another small unseen body. The left image shows comet Holmes' spherical dust coma that was produced after an outburst in October 2007, with the right image showing the comet after its obscuring coma is digitally filtered out. This filtering shows that the coma also hides several cometary fragments that drift away from the main nucleus at speeds of about 100 m/sec. White dots are background stars. These comet fragments also produce dusty streamers as they crumble and fade away, and sixteen such fragments were seen. This image was acquired at the Canada-France-Hawaii Telescope (CFHT) by Rachel Stevenson (UCLA) and colleagues, and a copy of their paper can be found at the arXiv preprint server.

Closeup of Helene

Cassini acquired this interesting image of the satellite Helene on March 3. Helene is a small 30km-wide satellite of Saturn. Its orbit is also very curious, since it resides at the L4 Lagrange point of the much larger 1000km-wide satellite Dione. The L4 Lagrange point leads Dione's motion by 60 degrees in longitude. Such satellites are known as coorbitals, since they co-orbit with another larger body, and only a few such coorbital satellites are known. This coorbital motion is analgous to the Trojan asteroids, which lead/trail Jupiter by 60 degrees. Note also Helene's smooth surface. Although the outlines of large craters are clearly evident, their filled-in appearance suggests that this satellite is also being bombarded and coated by dust that might also be present in this part of the Saturnian system. Keep an eye on the CICLOPS website for information about this small but interesting satellite.