Although we've been preoccupied largely with theoretical matters this week, I don't want it to close without reference to the new Cassini imagery of Enceladus. This shot was made at a phase angle of 145 degrees when Cassini was about 14,000 kilometers from Enceladus, during the flyby of November 21. The remarkable jets spraying from the fractured surface in the south polar region are clearly visible. Image: Dramatic plumes, both large and small, spray water ice out from many locations along the famed "tiger stripes" near the south pole of Saturn's moon Enceladus. The tiger stripes are fissures that spray icy particles, water vapor and organic compounds. More than 30 individual jets of different sizes can be seen in this image and more than 20 of them had not been identified before. At least one jet spouting prominently in previous images now appears less powerful. Credit: NASA/JPL/SSI. I keep thinking about Project Orion, back in the crazy days before the Test Ban Treaty of 1963...

The Stardust spacecraft recently completed a course adjustment maneuver as it continues on its way to comet Tempel 1. The burn began at 2221 UTC on February 17 and lasted 22 minutes and 53 seconds. The net result: A change of the spacecraft's speed by 24 meters per second. That may not sound like much, but it has big ramifications for this interesting mission. Tempel 1 is a rotating object, and mission scientists want to have a look at places previously imaged by the Deep Impact mission of 2005 (and yes, this is the first time we've revisited a comet). An adjustment to Stardust's encounter time of eight hours and 20 minutes should maximize the chances of seeing the right surface features on the 2.99-kilometer wide potato-shaped mass. The burn took place almost exactly a year before the spacecraft will reach Tempel 1. You may remember Stardust as the first spacecraft to collect cometary samples and return them to Earth for study. After the sample return capsule was retrieved in 2006,...

New measurements from Cassini, made on a flyby through the plume of Enceladus on March 12, 2008, bolster the case for liquid water in the Saturnian moon. Cassini found negatively charged water ions in the plume, and its plasma spectrometer also traced other kinds of negatively charged ions including hydrocarbons. That adds Enceladus to a fairly select list -- the Earth, Titan and the comets -- where negatively charged ions are known to exist in the Solar System. They're found in Earth's ionosphere, and also in places at the surface where liquid water is in motion, such as waterfalls. Image: Is there liquid water beneath this surface? Portions of the tiger stripe fractures, or sulci, are visible along the terminator at lower right, surrounded by a circumpolar belt of mountains. The icy moon's famed jets emanate from at least eight distinct source regions, which lie on or near the tiger stripes. However, in this view, the most prominent feature is Labtayt Sulci, the approximately...

Here's something to consider re the recent Pluto news: The Hubble maps of the tiny world that were released yesterday show a resolution of roughly 300 miles per pixel. When New Horizons flies by Pluto/Charon in 2015, it will send images with a resolution of 300 feet per pixel. And we've been reminded once again that every time we look deeper into something hitherto unexplored, we're likely to be surprised. The surprise in this case was the significant reddening of the dwarf planet and the time frame in which it occurred, a mere two years. I thought the liveliest part of the teleconference on Pluto yesterday was Marc Buie's response to what had appeared in his datasets. Buie (Southwest Research Institute) was looking at imagery collected by the Hubble Space Telescope from 2002 to 2003 and comparing it with the results of earlier ground-based observations, as well as with Hubble pictures taken in 1994. The dramatic reddening seems to have occurred between 2000 and 2002, even as the...

Collisions between asteroids should be highly energetic affairs, with an average impact speed of close to 5 kilometers per second. We may be looking at the debris of a head-on collision between two asteroids in imagery provided by the Hubble Space Telescope. The object in question, originally thought to have been a comet, is P/2010 A2, discovered by the Lincoln Near-Earth Asteroid Research (LINEAR) sky survey on January 6 of this year. The follow-up Hubble imagery dates from late January, and shows an unusual filamentary pattern near the nucleus. Image: HST picture of the comet-like object called P/2010 A2. The object appears so unusual in ground-based telescopic images that discretionary time on Hubble was used to take a close-up look. This picture, from the January 29 observation, shows a bizarre X-pattern of filamentary structures near the point-like nucleus of the object and trailing streamers of dust. The inset picture shows a complex structure that suggests the object is not a...