Jupiter's protective role for Earth has long been assumed, the theory being that the giant planet deflects asteroids and comets away from the inner Solar System. But studies on the subject are sparse, and focus on long period comets in extremely elliptical orbits. What about short period comets, and in particular the Jupiter Family of Comets (JFC)? These are comets thought to originate in the Kuiper Belt whose orbits are now controlled by Jupiter. Comet 81P/Wild 2, encountered by Stardust, was one of these, as was Comet Shoemaker Levy-9. We all saw in 1994 what a planetary impact from a comet could do when Shoemaker Levy-9 struck Jupiter. And a new study suggests that Jupiter's presence offers Earth no real protection from such objects. A research team at the UK's Open University set up computer models that examined the Jupiter Family, defined for these purposes as comets whose semi-major axes are smaller than Jupiter's and whose orbital period is less than twenty years. Some studies...

We're used to astronomical phenomena being relatively stable over the course of a human lifetime, which is why unexpected activity catches the eye. Think of the surprise over Io's volcanoes when they were first revealed by Voyager. And now we learn that the ring system around Uranus has changed significantly since Voyager 2 photographed it some twenty-one years ago. The inner rings have become more prominent, showing dusty material moving in otherwise empty regions of the rings. So reports Imke de Pater (UC-Berkeley), who led the team of astronomers investigating these changes, and presented the results at the European Planetary Science Congress today in Potsdam. Says de Pater of the activity: "People tend to think of the rings as unchanging, but our observations show that not to be the case. There are a lot of forces acting on small dust grains, so it is not that crazy to find that the arrangement of rings has changed." Assisting the investigation is the edge-on view we now have of...

Have scientists found a new category of asteroid? The evidence for basalt of a hitherto unseen composition on two small objects in the outer asteroid belt points to the possibility. An igneous rock, basalt would indicate that the asteroids were once part of a larger body, one that underwent some form of internal heating. The problem is that basalt is unusual for this part of the asteroid belt, nor is it clear whether the two fragments under study came from the same parent body. Add to that an unusual reflectance spectrum and the picture gets interesting indeed. The asteroids in question are (7472) Kumakiri and (10537) 1991 RY16, and therein lies a tale. The two were chosen from a group of six candidate asteroids thought to be classified as V-type, a name deriving from Vesta, the second largest of the asteroids. Not long ago it was thought that all basaltic V-type asteroids were simply fragments of Vesta, but in the past few years several V-type objects not belonging to this family...

With the European Planetary Science Congress now in session in Potsdam, we should have several interesting presentations to discuss this week, the first of which involves Titan. Huygens and Cassini have shown us a frigid, rocky surface under a surreal orange cloud of hydrocarbons, a place where liquid methane seems to flow in lakebeds and rivers. Methane in these circumstances plays much the same role that water plays here on Earth. Image: The surface of Titan as seen by the Huygens probe. Credit: ESA/NASA/JPL/University of Arizona. But what's under that interesting surface? The permafrost crust seems to sit on a soggy layer of ammonia, methane and water, beneath which an icy layer should surround a rocky core. It's that icy layer that interests Vasili Dimitrov (Tel-Aviv University), who is discussing its nature at the Potsdam conference. Dr. Dimitrov is hoping to put limits on the methane reserve available within Titan, which will tell us, among other things, whether that thick...

We always knew the surface of Enceladus was cold, but those tantalizing plumes breaking out of the Saturnian moon's south polar region gave hope of warmer things within. Liquid water fits with one model, pockets of which could account for the occasional geysers of ice crystals mixing with methane, nitrogen and carbon dioxide that Cassini has measured. Fill Enceladus with an internal ocean and the possibility of some kind of biology becomes an attractive study. But an alternate take on the plumes has been on view for some time now. Coming out of the University of Illinois, it's based on the idea that stiff compounds of ice called clathrates may cover Enceladus to a considerable depth. Whereas the warm interior model could produce such geysers, coined 'Cold Faithful' out of analogy to Yellowstone National Park's Old Faithful geyser, so could the clathrate model. But the latter, dubbed 'Frigid Faithful,' could operate far below the freezing point of water, with obvious implications for...