Reposted from the NYTimes:
http://www.nytimes.com/2007/03/20/science/space/20kuip.html?ref=science



Sometime, probably several billion years ago, a rocky iceball almost as large as Pluto crashed into another almost Pluto-size iceball.

Amazingly, the pieces that flew apart from that long ago smash-up are still easily identifiable in the night sky, astronomers reported last week.

"We have pieces we can put back together again," said Michael E. Brown, a professor of planetary astronomy at the California Institute of Technology and the lead author of an article describing the discovery in the current issue of the journal Nature.

Dr. Brown turned up the first piece in December 2004 during a systematic search for large bodies hiding beyond Neptune in a ring of debris known as the Kuiper Belt. Given the time of year, he gave that Kuiper Belt object the nickname Santa. (A couple of weeks later, the same search turned up a Kuiper Belt object larger than Pluto, precipitating the events that led to the demotion of Pluto to dwarf planet last year.)

Santa, with a mass about one-third that of Pluto, is an oddball. It is shaped like a football, stretched out because it spins end over end once every four hours. It is made mostly of rock, but has an outer layer of shiny water ice. It has two moons, nicknamed "Rudolph" and "Blitzen."

The rapid spinning and the two moons were hints of a past collision.

Meanwhile, Kristina M. Barkume, one of Dr. Brown's graduate students, was measuring the reflectivity of about 30 Kuiper Belt objects to infrared light. Six were noticeably different, absorbing a specific wavelength corresponding to the presence of ice.

One of them was Santa, which is also known by the more formal designation 2003 EL61.

The scientists looked up the orbits of the other five Kuiper Belt objects on the list. "It was instantaneously obvious," Dr. Brown said. " 'Oh my God, these things all have the same orbit, and they have the exact same orbit as Santa.'"

The astronomers have since found two additional Kuiper Belt objects that are shards from the same impact.

Calculations suggest that the pre-impact Santa was about 1,000 miles wide, or about two-thirds the width of Pluto. It collided with another object perhaps 700 miles wide at a speed of perhaps 7,000 miles per hour. That impact pulverized most of Santa's outer layers of ice, leaving the rocky core, Dr. Brown said.

"It's telling us something very important about the early evolution of the solar system, and we're still trying to figure that out," said Harold F. Levison, a planetary scientist at the Southwest Research Institute's department of space studies in Boulder, Colo. "It was a surprise, no doubt about it. When I first heard about it, I didn't believe it, but the paper convinced me."

The Kuiper Belt is believed to have once been much denser and originally closer to the Sun, but was later pushed outward by the migration of the giant planets like Neptune. However, those changes would have pushed apart the orbits of Santa and its brethren, so the observation that all are still in the same orbit means the collision that formed them must have occurred after the Kuiper Belt settled in its current location, Dr. Levison said.

That in turn brings up the question of how seemingly unlikely it was that the collision occurred at all. Only a handful of objects in the current Kuiper Belt are larger than 500 miles wide, and it is highly unlikely that two would ever cross paths.

Dr. Levison said he had worked out a possible solution. In the past, a higher number of Kuiper Belt objects traveled in elliptical orbits, and two of those must have been the ones that collided.

Santa will not be around forever. It is in a resonant orbit with Neptune, traveling around the Sun 7 times for each 12 orbits of Neptune. That resonance is unstable, and in about a billion years, Neptune will give Santa a gravitational push into the inner solar system.

If Santa can make it past Jupiter, which deflects many such interlopers from the outer solar system, it will turn into a giant comet. "It'll be most spectacular thing that anyone has ever seen — if anyone is around to see it," Dr. Brown said.

Other fragments must have also been kicked inward by Neptune, which raises the possibility that at least some observed comets are also pieces of Santa.

"That is pure speculation," Dr. Brown said. But he added, "That's one of the things I like to think about that might be really true."