Asteroid 87 Sylvia, known since 2001 to be part of a double system, has just gotten more interesting. A team of astronomers at the University of California at Berkeley and the Observatoire de Paris have now established that the asteroid is actually part of a triple system, the first ever discovered. Using data from the European Southern Observatory's Very Large Telescope array in Chile, the team found two small moons in nearly circular orbits. Astronomical names are always a challenge, but in this case naming comes easy. 87 Sylvia was named for Rhea Sylvia, the mother of the founders of Rome. Thus it made perfect sense for Berkeley's Franck Marchis to propose Romulus and Remus as the names of the moons. The suggestion has already been approved by the International Astronomical Union. Image: Artist's conception shows twin moonlets, Romulus and Remus, orbiting the large main-belt asteroid 87 Sylvia. Credit: European Southern Observatory. Calling them 'moons' should not obscure the fact...

The recent images from Cassini's flyby of Mimas remind us how violent the history of the early Solar System was. Now a study at the Australian National University shows that three huge asteroids -- between 20 and 50 kilometers across and traveling as a cluster -- collided with the Earth some 3.2 billion years ago. The result: volcanic eruptions, the creation of major fault lines, earthquakes and a variety of disruptive effects deep inside Earth's crust. This work grows out of evidence of extraterrestrial impact deposits from this era discovered in South Africa and extends the effects of those impacts to the Pilbara region of Western Australia. According to an ANU news release, the materials studied in the eastern Transvaal had indicated that impact craters several hundreds of kilometers in diameter had been created in oceanic parts of the Earth. In the Australian studies, the impacts coincide with the formation of fault escarpments and fault troughs and a major volcanic episode....

Can any other surface in the Solar System be this battered? The twin views below show Saturn's moon Mimas as imaged by Cassini on August 2. Note how the false-color brings out the variation across this tortured surface -- at left is an enhanced clear-filter image, at right a color composite of ultraviolet, infrared and clear-filter images. The most prominent feature in both is the 140-kilometer wide Herschel Crater, an ancient strike that is today filled with landslide material. Close study reveals numerous other craters and long grooves similar to those found on asteroids. Are these grooves related to the enormous impact that created the Herschel crater? No one knows, but study of this moon's turbulent history may help scientists understand how many impactors have moved through the Saturn system, a reminder of how dangerous a place the Solar System can be when you're in the crosshairs of an approaching piece of space debris. Mimas is a tiny place, measuring just 397 kilometers...

Over the weekend, more news came in about 2003 UB313, the soon to be renamed new planet discovered at Palomar Observatory. As discussed in astronomer Mike Brown's page on the discovery, 2003 UB313 is the largest object found orbiting the Sun since Neptune and its moon Triton in 1846. There seems to be no doubt that it is larger than Pluto and, like that planet, a member of the Kuiper Belt, a vast band of icy objects beyond Neptune. Remarkably, the 2003 UB313 team has found 80 bright Kuiper Belt objects since beginning its survey of the outer Solar System in 2001, but none so newsworthy as this one. Artist2003 UB313 takes twice as long to orbit the Sun as Pluto, and it is currently more than three times more distant, although it moves in an elliptical orbit that can bring it within Pluto's orbit and close to that of Neptune. Size limits can now be set ranging from 2210 to 3550 kilometers, depending on the nature of the surface and how it reflects light. Hubble measurements, already...

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...