Mar. 14

PARIS (AFP) - Astronomers have pieced together the remnants of a mighty collision that smashed apart a planet-sized rock in the Kuiper Belt, on the far-flung fringes of the Solar System.

First identified in the 1950s by Dutch-US skygazer Gerard Kuiper, the disk-shaped belt is believed to be populated by tens of thousands of icy bodies, encircling the Sun beyond the orbit of Neptune.

A team led by Michael Brown of the California Institute of Technology (Caltech) took a close look at the belt's third largest object, 2003 EL61.

Nicknamed "Santa" because the team spotted it at Christmas-time, 2003 EL61 is a grey rock so big that it is a contender for the new category of a Solar System "dwarf planet."

Brown describes Santa as "one of the strangest objects in the Solar System," likening its shape to a cigar or an American football that has too little air in it and been stepped on.

Flanked by two moonlets, 2003 EL61 measures some 1,500 kilometers (950 miles) across, tumbling over and over at a prodigious rate and pursuing a weird egg-shaped orbit inclined at nearly 30 degrees to the plane at which almost all of the Solar System's objects travel.

Brown's team found five other rocks, measuring between 10 and 400 kilometers (six and 250 miles) across, that they believe were smashed away from 2003 EL61 in the distant past.

The cluster shares the same colour and the light they reflect has a signature that suggests they are covered with surface water ice. The paper is published on Thursday in the weekly British journal Nature.

The Kuiper Belt and the asteroid belt, which is located in the gap between Mars and Jupiter, are believed to be rubble left over from the building of the Solar System.

Astronomers have already identified 35 "collisional families" -- clusters formed when their parent rock is whacked -- in the asteroid belt.

But this is the first time something similar has been spotted in the Kuiper Belt, and the discovery could shed light on the dynamics that shaped the early Solar System and the surface of Kuiper Belt objects themselves.

The chance of the collision occurring that formed the Santa family "is only about 1 in 2,000," estimates Alessandro Morbidelli of France's Observatoire de la Cote d'Azur in a commentary, also published by Nature.

"We have many indications that the Kuiper Belt was initially much more massive than it is today," says Morbidelli.

"Presumably, therefore, the collision must have happened in the distant past, when the belt was far more crowded with large objects."

Brown is one of the leading authorities on the Kuiper Belt, a region also inhabited by Pluto.

His discovery of a big Kuiper Belt object, 2003 UB303 (since renamed 2003 Eris), sparked a fierce debate about the status of planets, leading to Pluto's relegation last year to the status of "dwarf planet" by International Astronomical Union (IAU).


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Good find, thank you.

Mar. 17

Caltech astronomers have detected a family of rocky objects in the Kuiper Belt that were formed by something hitting an object larger than Pluto.

Such groups of objects, called collisional families, are common in the asteroid belt between Mars and Jupiter, but the new family is the first to be found in the Kuiper Belt, which is beyond Neptune, more than 3 billion miles from the sun.

The parent object is a large, football-shaped rock called EL61, discovered in 2003 by Caltech astronomer Mike Brown and colleagues. The same team reported in Thursday's issue of the journal Nature that EL61 has two moons and many smaller fragments that are circling the sun in the same orbit.

EL61 is an anomaly in the belt. It is composed primarily of rock, although its surface is covered with pure ice; other objects in the belt are mostly ice. And it spins unusually fast, completing a full revolution in less than four hours.

The newly discovered objects are also ice-covered rocks, Brown said — strong evidence that they split off from the parent.

The team concluded that the original object was spherical and about the size of Pluto until it was hit by a smaller body about 4.5 billion years ago.

"The impact made a tremendous fireball, and large icy chunks of the big object split off and went flying into space, leaving behind a huge ice-covered rock spinning end-over-end every four hours," Brown said.

It spins so fast that it has deformed into the shape of a football, he said, "but one that's a bit deflated and stepped on."

Brown predicts that EL61 will become a massive comet, about 6,000 times brighter than Hale-Bopp, which last appeared in 1997. But, he added, that won't occur for another billion years.

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I have merged two threads on the same subject and renamed the thread so thet the subject matter is clearer.