A day after the news about 2003 EL61, a Kuiper Belt object originally thought to be larger than Pluto, we now have another world that appears significantly larger still. 2003 UB313 was discovered with the Samuel Oschin Telescope at the Palomar Observatory by astronomers Mike Brown (Caltech), Chad Trujillo (Gemini Observatory), and David Rabinowitz (Yale University). Evidently the lower limit of its size is Pluto, and it may be (and probably is) larger. Image: Three views of the new planet. Credit: Mike Brown, California Institute of Technology. Now some 97 AU from the Sun, the planet is the farthest-known object in the Solar System. A news release from Caltech quotes Brown on 2003 UB313 and its credentials as a planet: "It's definitely bigger than Pluto," says Brown, who is professor of planetary astronomy. Scientists can infer the size of a solar-system object by its brightness, just as one can infer the size of a faraway light bulb if one knows its wattage. The reflectance of the...

A bright, slowly moving object in the outer Solar System may be a world larger than Pluto. A team of astronomers led by Jose-Luis Ortiz at the Sierra Nevada Observatory in Baja, California found the object, called 2003 EL61, using observations made in 2003. It is some 51 AU from the Sun (one AU, or Astronomical Unit, is the distance from Earth to the Sun), and evidently comes as close as 35 AU, inside Pluto's average distance of 39 AU. An analysis of older observations shows the object in images dating back to 1995. [Note: the Sierra Nevada Observatory was mistakenly identified as being in Spain in an earlier version of this post]. Is 2003 EL61 a new planet? And for that matter, how do we define what a planet is? That debate is sure to be reignited as we weigh the possibilities here, for a world larger than Pluto surely has to be considered a planet. But size measurements this far out from the Sun are tricky, and rely on an object's albedo, a measure of how much light the object...

No body in the solar system is as reflective as Saturn's moon Enceladus. Its terrain also appears relatively young, with the early Cassini flybys revealing regions that are only lightly cratered. It seems that Enceladus has undergone a number of episodes of geologic convulsion, with the southernmost latitudes seeing the most recent activity, producing a tortured surface marked by crisscrossing faults, folds and ridges. All this comes from findings revealed by the July 14 Enceladus flyby and discussed recently in a news release from the Cassini Imaging Central Laboratory for Operations. The latest flyby brought Cassini within 175 kilometers (109 miles) of the moon, showing that the landscape near its south pole is studded with ice boulders the size of houses, while impact craters in the region are almost entirely absent. Some of the ice blocks are up to 100 meters (328 feet) across, and they appear in an area that lacks the fine-grained frost found elsewhere on Enceladus. All that...

Of the many things demonstrated by the Deep Impact mission to comet Tempel 1, the evolution of astronomy is not the least significant. Gone are the days of the isolated mountain-top observer painstakingly examining photographic plates whose findings might be corroborated only weeks or months later by other astronomers. During the Deep Impact mission, the European Southern Observatory used every modern communications tool in the book as part of a collaboration between all major observatories worldwide. The result: round the clock data through a wide variety of instruments both Earth and space-based, making Deep Impact a hugely successful example of distributed astronomy -- 'distributed' as in 'distributed computing,' where powerful resources share their capabilities to produce a result far greater than any one of them could achieve. From all this, we know that Tempel 1 was not significantly transformed by the Deep Impact collision. In fact, the impactor evidently did not create a...

Each new world we visit offers a different perspective on how planets and their moons form. Consider Saturn's moon Hyperion, the density of which now appears to be only about 60 percent that of solid water ice. What that means is that much of the moon's interior -- 40 percent or more -- is made up of empty space, so that Hyperion is not so much a solid body as a conglomeration of icy rubble. The Hubble images acquired between June 9 and 11 confirm this estimate, showing an object that looks almost sponge-like, bearing the imprint of countless craters which seem relatively recent. What we can gather from all this is that Hyperion is a moon that is pushing a critical limit beyond which the internal pressure of its gravity would start to crush weaker materials, closing up those porous spaces and establishing the more familiar spherical shape of larger bodies. Hyperion's diameter (adjusting for its irregular shape) is 360 x 280 x 225 km (223 x 174 x 140 miles). We'll have a much closer...

Tempel 1, the comet that slammed into the Deep Impact probe on July 4, is three miles wide by seven miles long, and evidently coated with a fine, powdery dust. That dust, says Deep Impact principal investigator Dr. Michael A'Hearn of the University of Maryland, is more like talcum powder than beach sand. "The major surprise was the opacity of the plume the impactor created and the light it gave off," said A'Hearn, adding "And the surface is definitely not what most people think of when they think of comets -- an ice cube." Meanwhile, the immense data recovery and analysis process continues. 4500 images were taken by the spacecraft's three cameras, with the memory of the impactor's plunge into the comet's nucleus still fresh. The final images from the impactor showed surface detail down to the level of four-meter objects, a factor of ten better than any previous comet mission. And we now know that the impactor hit the nucleus at a 25 degree oblique angle relative to the cometary...

Scientists now believe that Deep Impact's 820-pound impactor vaporized deep inside comet Tempel 1's surface when the collision took place on July 4. Moving at 23,000 miles per hour, the impact would have been severe, generating temperatures of several thousand degrees Kelvin, and creating what Dr. Pete Schultz of Brown University calls "...our own incandescent photo flash." Even 50 minutes after Tempel 1 and the impactor collided, the visual effects were still striking, as shown in this spectacular photo from the flyby probe. In the second photograph, taken from the impactor probe, the comet is seen in startling detail. Note the topographical features, which include what may be ancient impact craters and a variety of ridge lines. Clearly, Deep Impact wasn't the first object to hit this comet! Image credit for both photos: NASA/JPL-Caltech/UMD. Meanwhile, x-rays from Tempel 1 have been confirmed by the XMM-Newton space observatory. Previous observations of comets had suggested that...

The Hubble images below (click to enlarge) give an idea of the size of the Deep Impact event. At left is the comet Tempel 1 before the collision. In the middle image, taken 15 minutes after impact, the comet is four times brighter than before and shows a substantially increased cloud of dust and gas. The image at far right was taken over an hour after the collision. Note the fan shape of the ejecta -- the debris, according to this Hubble news release, is pushing outward at about 1800 kilometers per hour. Image: The potato-shaped comet is 14 kilometres wide and 4 kilometres long. Tempel 1's nucleus is too small even for the Hubble telescope to resolve. These visible-light images were taken by the Advanced Camera for Surveys' High Resolution Camera. Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA). Data from Deep Impact continue to accumulate. Images taken by ESA's XMM-Newton observatory (an x-ray observatory based in space) show the emissions of hydroxyl ions, the direct...

Deep Impact gave mission scientists what they bargained for -- and more-- when its impactor collided with comet Tempel 1 last night. The collision occurred at 0152 EDT (0652 GMT), with the first image returning at 0157. Calling the impact 'spectacular,' principal investigator Dr. Michael A'Hearn of the University of Maryland, College Park added, ""With this much data we have a long night ahead of us, but that is what we were hoping for. There is so much here it is difficult to know where to begin." Note the use of autonomous navigation aboard the impactor, which was released from the mothership at 0207 EDT (0607 GMT) on the 3rd. On its own, the impactor needed to make three targeting maneuvers as it closed on the comet, the first of these 90 minutes before impact, followed by two more at 35 and 12.5 minutes respectively. The result: a 10 kilometer per second collision with the comet, while the mothership monitored events from nearby, and legions of telescopes followed the action from...