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...

Centauri Dreams admits to being less than absorbed by the debate over whether Pluto is or is not a planet. Let the logic-choppers have their day; what's interesting is that whatever we call it, Pluto remains an enigmatic, unexplored world that has much in common with other Kuiper Belt objects. And its enigmatic nature seems reinforced by recent studies showing that the surface of Pluto is actually colder that it ought to be. Measuring surface temperatures on a body currently 30 AU from the Sun is tricky business, to be sure, and remember that in the course of a Plutonian year, that distance can vary from 30 to 50 AU, causing what little atmosphere is present to freeze and fall to the surface in the form of snow or ice. But studies of how sunlight is reflected from Pluto and its largest moon Charon -- using the Submillimeter Array (SMA) on Mauna Kea in Hawaii -- have enabled the first thermal measurements that could discriminate between the two objects. Image: In this artist's...

Finding new objects in the Kuiper Belt is getting to be almost routine. But what makes the latest find intriguing is the shape of its orbit. Designated 2004 XR 190 by the International Astronomical Union and nicknamed 'Buffy,' the new object is currently 58 AU from the Sun, about twice the distance to Neptune. But an analysis of its orbit shows that it does not approach closer than 50 AU, complicating theories on how Kuiper Belt objects wind up in the positions they occupy. Here's the problem: the few objects discovered beyond 50 AU (where the main Kuiper Belt seems to end), have all been in extremely eccentric, or non-circular orbits. Bear in mind that these high eccentricity orbits have been assumed to be the result of gravitational interactions with Neptune or some other outer Solar System body. The encounter was assumed to have acted as a gravitational slingshot to fling the KBO objects into the deep. But Buffy confounds these theories by coming nowhere near Neptune; another...

With the launch of the New Horizons mission to Pluto, Charon and beyond a scant month away, it's fitting to acknowledge the 100th birthday of Gerard P. Kuiper, who predicted the existence of the band of debris and minor planets we now call the Kuiper Belt in 1950. It would take forty years for confirmation of the prediction, but the study of objects large and small beyond the orbit of Neptune now has high visibility, and is one of the reasons for the New Horizons mission. Kuiper's work was hardly limited to the now famous belt. He was also a pioneer in the study of Cepheid variables, those highly useful 'standard candles' that allow us to assess stellar distances (the period of a Cepheid variable being related to its intrinsic luminosity). Other objects of Kuiper's interest included eclipsing binaries, and he played a key role in early work on Titan's atmosphere. Add to this that his students included the likes of William Hartmann, Carl Sagan, and New Horizons Science Team...

Back in the heady early days of Project Orion, Freeman Dyson was already thinking about an advanced interplanetary vehicle that could take a 1300-ton payload to Saturn. His target was Enceladus. "We knew very little about the satellites in those days," Dyson said. "Enceladus looked particularly good. It was known to have a density of .618, so it clearly had to be made of ice plus hydrocarbons, really light things, which were what you need both for biology and for propellant, so you could imagine growing your vegetables there..." The quote is from George Dyson's Project Orion: The True Story of the Atomic Spaceship (New York: Henry Holt, 2002), which belongs on the shelves of anyone interested in the human future in space. And it always comes back to me when I hear more Cassini news from Enceladus, and think how feasible it once seemed (in the 1960's!) to go straight to the outer planets. Talk about audacity -- Orion would set off atomic bombs behind a pusher plate to drive a ship so...

The Japanese spacecraft Hayabusa evidently managed to land on asteroid Itokawa several days ago after all, according to this from the Japan Aerospace Exploration Agency: "At the timepoint of Nov. 21, Hayabusa was judged not to have landed on the surface. According to the replayed data, however, it was confirmed that Hayabusa stayed on Itokawa by keeping contact with the surface for about 30 minutes after having softly bounced twice before settling. This can be verified by the data history of LRF and also by attitude control record..." For more, you can read the complete JAXA statement here. The spacecraft is now being maneuvered for a second landing (and surface sampling) attempt. Note the shadow in this photograph, much more clearly visible than in the previous images of Itokawa from Hayabusa. There are people who shrug at this sort of thing, but to Centauri Dreams images like these are breathtaking. They remind us that a human presence has now encountered objects hitherto...

The Hubble Space Telescope, in operations designed to support the upcoming New Horizons mission to Pluto, has discovered two new Plutonian moons. It's too early to speak with confidence about their size because we don't yet know to what extent light reflects from their surfaces, but the early estimates are for diameters of 32 kilometers (20 miles) and 70 kilometers (45 miles). Charon, at 1200 kilometers, dwarfs these tiny objects, provisionally designated S/2005 P1 and S/2005 P2. Their faintness makes it clear why they weren't spotted before: the new moons are roughly 5000 times fainter than Pluto itself. For more, see this Southwest Research Institute news release. "Our result also suggests that other bodies in the Kuiper Belt may have more than one satellite. We planetary scientists will have to take these new moons into account when modeling the formation of the Pluto system," says co-leader Dr. Alan Stern, executive director of the SwRI Space Science and Engineering Division. And...

What is that bright 300-mile wide patch on Xanadu, the continent-sized region on Titan, that Cassini noted last March? The area outshines everything else on the moon in long infrared wavelengths (it's described as "...spectacularly bright at 5-micron wavelengths..."), and after considerable investigation does not appear to be a cloud, a mountain or a geologically active hot spot. In visible light, Cassini saw a bright arc-shaped feature of approximately the same size in late 2004 and again in 2005. Image: Combined VIMS and ISS images of Titan's mysterious bright red spot gives researchers more information about the feature than either single view. (Credit: NASA/JPL/University of Arizona/Space Science Institute). That quotation above comes from a University of Arizona news release, one that goes on to note that subsequent radar imaging found no real temperature variation between the spot and the terrain around it. That rules out the possibility of an active ice volcano, and quickly...

Everyone is calling 2003 UB313, the Solar System's 10th planet, Xena. The name comes from a TV warrior princess of whom the curmudgeonly Centauri Dreams, never one for television, was utterly unaware. Now Xena has been found to have a moon, inevitably named Gabrielle after an equally incrutable character on the series (apparently the sidekick of Xena herself). One-tenth the size of Xena, Gabrielle is slated for further observations with the Hubble Space Telescope that will allow more accurate determination of its mass. The advent of Gabrielle is good news for those wishing to learn more about Xena. From a California Institute of Technology press release quoting Michael Brown, the 10th planet's discoverer: "A combination of the distance of the moon from the planet and the speed it goes around the planet tells you very precisely what the mass of the planet is," explains Brown. "If the planet is very massive, the moon will go around very fast; if it is less massive, the moon will travel...