73P/Schwassmann-Wachmann is proving to be a far more interesting object than first anticipated. The comet is closing toward the Sun and will swing around it on June 7, passing the Earth along the way at a distance of 11.7 million kilometers. The fascination comes from watching its ongoing disintegration, which has broken the comet into more than 30 separate fragments. Nor is the show over. The larger fragments appear to be continuing their breakup. In the image below, taken by the Hubble Space Telescope, you can see one of the major fragments breaking into smaller chunks, dozens of which trail behind the main piece. The chunks are evidently pushed back along the tail by outgassing from their Sun-facing surfaces, and the smallest of them seem to be dissipating completely over a multi-day period. Image: The second image from a three-day observation with Hubble showing the breakup of Comet 73P/Schwassmann-Wachmann 3's Fragment B. Credit: NASA, ESA, H. Weaver (APL/JHU), M. Mutchler and...

We know little enough about our system's tenth planet (once called 2003 UB313 and now unofficially called Xena), but as new observations come in, we're learning that it is a mysterious place indeed. The latest news comes from the Space Telescope Science Institute, which pegs the object's size at five percent larger than Pluto. That would make Xena roughly 2400 kilometers in diameter, a smaller world than originally thought based on how much light it reflects. Not exactly startling news, given Xena's distance from the Sun and the difficulty in observing it, but the newly confirmed size means that the planet is so reflective that it bounces back fully 86 percent of the light that reaches it. No other planet can match this, and in the entire Solar System, no object other than Enceladus is so reflective. The latter is apparently constantly re-coated with ice by its active geysers. Image: An artist's concept of Kuiper Belt object 2003 UB313 (nicknamed "Xena") and its satellite...

The sky seems to be full of interesting objects that are breaking apart. They're always worth studying, as we learned through the impacts of the famous 'string of pearls' comet -- Shoemaker-Levy 9 -- on Jupiter in 1994. For one thing, the celestial display they afford is uncommonly interesting; for another, they are a reminder of the kinds of debris that we need to track just in case any of it turns out to be on an Earth-crossing trajectory. No apparent chance of that with 60558 Echeclus, a 50-kilometer 'centaur' out beyond the orbit of Saturn. Centaurs are icy planetoids that, in the case of both 60558 Echeclus and Chiron (the first centaur to be discovered), seem to display cometary properties. 60558 Echeclus is now even more interesting with the news that a large chunk of the object seems to have broken away. The resultant dust cloud is 100,000 kilometers across as both centaur and breakaway fragment blow off dust and gas, with more coming from the fragment than the main body (and...

That the Pioneer spacecraft have experienced some kind of anomalous acceleration is now well established. Just what the cause of that acceleration is remains an open question. But we do know that the anomaly appears as "...a constant and uniform acceleration directed towards the Sun...", as described in a paper called "What do the orbital motions of the outer planets of the Solar System tell us about the Pioneer anomaly?" The effect shows up in data from both spacecraft from the moment they passed the 20 AU mark on their journeys. The authors, Lorenzio Iorio and Giuseppe Giudice (Dipartimento di Progettazione e Gestione Industriale, Naples) note that recent work has not resolved the question of whether there is some internal factor aboard the spacecraft that is causing the anomaly, or whether its origin is external. In any case, the situation is intriguing enough that dedicated space missions to explore it have been proposed. This paper investigates whether there is an "...external,...

Gas giant moons like Europa offer the tantalizing hint of life-sustaining conditions, with oxygen supplied by their abundant ices. But without sufficient heat, how is the oxygen to be coaxed from their frozen surfaces? So far, the explanation has been that high-energy particles bombarding such a moon's surface could help to release the gas, which would have already been molecularly bound with hydrogen. But a study at the Pacific Northwest National Laboratory suggests a different explanation. Simulating high-energy bombardment of a moon's surface, researchers there found that the process is much more complex. "We found that a simpler two-step could not account for our results," said PNNL staff scientist Greg Kimmel. "Our model is a four-step process." Here's how a PNNL news release explains what's going on: First, the energetic particle produces what is known as a common "reactive oxygen species" called a hydroxyl radical, or OH. Next, two OH molecules react to produce hydrogen...

How did a newly formed Earth, supposedly hot and dry, wind up with oceans? Comets have been the leading candidate for the needed delivery mechanism, given their large ice content. But ice from the asteroid belt may make a better fit to Earth's water supply, and the discovery of a new class of comets there may mean that at least some objects in that part of the Solar System have ice at their surfaces. Asteroids and comets, in other words, may in some cases be more closely related than anyone realized. These conclusions come from work performed by University of Hawaii graduate student Henry Hsieh and professor David Jewitt, who have christened the newly discovered objects 'main-belt comets.' These are comets with asteroid-like orbits; they seem to have formed in the inner Solar System rather than in the frigid reaches of the Kuiper Belt or the Oort Cloud. The evidence? An object called asteroid 118401 is ejecting dust, just like a comet. And so is another mysterious comet known as...

There seems to be an emerging maxim in deep space studies: every new mission will overturn at least one enshrined assumption Thus the early Stardust results, studying the cometary debris from Wild 2. Because comets come out of the outer dark in their long arc toward the inner system, one would expect them to be made of materials that were born in cold temperatures. But Stardust has brought us cometary dust that's packed with minerals formed at high temperatures. What a fascinating set of challenges now face the Stardust researchers. They've found olivine in the Wild 2 materials; it's a compound of iron, magnesium and various other things (the Stardust sample is primarily magnesium). Most astronomers believe that olivine crystals are formed from glass that has undergone heating near stars. So how crystals of olivine can show up in the Wild 2 samples bears scrutiny -- after all, Wild 2 is thought to have formed well beyond the orbit of Neptune. And olivine isn't the only oddity about...

A few years ago, the idea of life on Enceladus would have seemed preposterous, but the Cassini orbiter has sent back images suggesting that the Saturnian moon houses reservoirs of liquid water near the surface. And liquid water is intriguing indeed in any discussion of life. "We realize that this is a radical conclusion - that we may have evidence for liquid water within a body so small and so cold," said Dr. Carolyn Porco, Cassini imaging team leader at the Space Science Institute in Boulder, Colo., and the lead author of the report in the journal Science. "However, if we are right, we have significantly broadened the diversity of solar system environments where we might possibly have conditions suitable for living organisms. It doesn't get any more exciting than this." Take a look at the high-resolution Cassini image above (and be sure to click to enlarge it), where a spray of material is clearly visible above the moon's southern polar region. Then compare to the image below, where...

Centauri Dreams is following the Pioneer 10 story with great interest, and not just in terms of the anomalous effects that continue to keep this mission in the news. Ponder that Pioneer 10 was launched in 1972 and consider that even with the technologies of its day, the probe may still be able to communicate with Earth. We have learned so much in the interim about hardened electronics and autonomous self-repair that there is reason to believe probes to even remoter locations in the Kuiper Belt and beyond are feasible providing we can solve the propulsion conundrum. The next attempt to contact the venerable spacecraft would occur in March, if it occurs at all, and you can hear more about it in an interview conducted by Planetary Radio. The guest is JPL senior research scientist John Anderson, who discusses the mission, its current communications challenges, and the possible reasons for what appears to be its deceleration as it moves away from the Sun. Or is the effect really a...

Centauri Dreams now returns to its normal publication schedule, after a brief hold to allow the recent post on the creation of an interstellar foundation to receive maximum visibility. The good ideas that came in both through comments as well as e-mail now go to the founding members of the foundation, and as further news develops, I will publish it here. If you would like to make a suggestion about the name of the foundation, feel free to send it either as a comment on the original post or as an e-mail message to me. But now we move back to research news, and an issue that needs updating. The Pioneeer Anomaly has caught our attention before -- both Pioneer spacecraft seem to be slowing down as they depart our Solar System in ways that challenge conventional theory. The Planetary Society's Pioneer Anomaly Team has launched an effort to recover Pioneer datasets and analyze their contents which has now examined over eleven years of Pioneer 10 data and about four years from Pioneer 11....

Hard on the heels of the recent Hubble photograph of 2003 UB313 comes further news on the size of this increasingly interesting object. While Hubble showed that the newly discovered '10th planet' was only slightly larger than Pluto (and therefore smaller than originally thought), a German team has now provided further data suggesting that the object has a diameter of about 3000 kilometers, roughly 700 kilometers larger than that of Pluto. Measuring distant objects is tricky enough at Pluto's distance, but 2003 UB313 can reach 97 AU at the most distant point in its orbit, almost twice as far as Pluto ever gets from the Sun. To get an accurate reading, astronomers must know something about the reflectivity of the object. But what Frank Bertoldi (University of Bonn and the Max-Planck-Institute for Radioastronomy) and Wilhelm Altenhoff (MPIfR) managed to do was to combine optical observations with heat measurements at a wavelength of 1.2 mm, where reflected sunlight is negligible. The...

2003 UB313, the '10th planet' discovered by Michael Brown (California Institute of Technology), continues to fuel the debate over what constitutes a planet and where the division between planet and Kuiper Belt object should be. A new Hubble photograph shows the object to be slightly larger than Pluto, but nowhere near the 25 to 50 percent larger that Brown originally estimated. But Brown was the first to state, early in the game, that we needed better data to get an accurate size estimate. And you can see why his original view made sense: if 2003 UB313 really is not much larger than Pluto, then it reflects over 90 percent of the light that hits it. What causes the additional brightness (Pluto, for example, reflects just 60 percent of incoming light) remains conjectural. But this must be an icy surface, and the distinctions between the new world and Pluto will continue to spur controversy. Meanwhile, we have a new paper on another Kuiper Belt find, the object called 2003 EL61. The...

While we're on the subject of Pluto -- and we will be off and on as the New Horizons launch approaches -- it's intriguing to see how much we have already learned about the Pluto/Charon pair from Earth-based telescopes. We've just found out, for instance, that Charon's radius is 606 kilometers, with a fudge factor of plus or minus 8 kilometers. That's a pretty remarkable measurement for an object this distant, but it proceeded through a reliable and time-honored astronomical method: stellar occultation. If you know when and where to look, an occultation can provide reams of information. What's happening is that the nearer object, in this case Charon, passes in front of a distant star; observations of that event give us not just accurate size estimates but useful data on the object's density and possible atmosphere. For by combining the occultation data with measurements from the Hubble Space Telescope, the team (from MIT and Williams College) was able to establish a density for Charon...