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.

Another impact at Jupiter!

Anthony Wesley does it again! This time, by recording a video what appears to be another small impact at Jupiter, presumably by an unseen comet or asteroid. Recall that Anthony was also the amateur astronomer who spotted as asteroid impact at Jupiter in July 2009. See New Scientist for a video of the explosion by another amateur astronomer, Christopher Go.

Hubble image of dust trail in asteroid belt

David Jewitt (UCLA) acquired this new Hubble image of the mysterious dust trail that appeared recently in the asteroid belt. This trail is also discussed in this January 20 post. Current thinking says that this debris from a recent collision among two asteroids. Check the Hubble page for more details.

Recent impacts on Mars exposes subsurface ice

The Mars Reconnaissance Orbiter (MRO) took these images of a 6m crater on Mars in October 2008 (left) and again in January 2009; see this press release for more details. This crater is absent from images acquired in 2007, so it must be due to a relatively recent impact. Note also the bright material in the crater that fades over time. This is to be expected if this were subsurface ice that was suddenly exposed to the surface. Water ice is not stable at the surface of Mars, and will sublimate (vaporize) over time. The MRO spacecraft has discovered several new craters where fresh ice appears to fade over time. This particular crater has a latitude of 43 degrees, which indicates that subsurface ice on Mars extends all the way from from the poles to Mars' mid-latitudes.

New Hubble image of impact site on Jupiter

This optical HST image of the impact site on Jupiter was collected by Heidi Hammel (SSI) and others using HST's Wide Field Camera 3. This camera is still new and not fully calibrated yet, since it was installed only two months ago by Shuttle astronauts. Nonetheless, it is still able to produce a magnificent image. The current estimate for the impactor diameter is about a third of a kilometer. See this press release for more details.

Keck image shows likely impact on Jupiter

Above is an infrared image of Jupiter, as observed by Paul Kalas, Michael Fitzgerald and Franck Marchis at the Keck telescope. Note the bright spot, which overlaps the mysterious black spot that recently appeared on Jupiter (see earlier post, below), indicating that this spot is indeed hot. This supports the notion that Jupiter was indeed hit by an impacting comet or asteroid. See New Scientist for more.

New dark spot on Jupiter:due to asteroid or comet impact?

Anthony Wesley from Murrumbateman Australia reports seeing a black spot in Jupiter's atmosphere. Black spots on Jupiter are reported from time to time, the most famous example of which was due to the impact of comet Shoemaker-Levy 9. That comet orbited Jupiter unseen until 1992, when it was disrupted by that planet's gravitational tide into about 20 fragments during a particularly close approach to the planet. Those fragments then struck Jupiter in 1994, resulting in similarly black (but much larger) bullseyes. Followup observations of this new spot will hopefully reveal whether it might be due to an impact by a comet or asteroid, which is a very rare astronomical event. Stay tuned.