Giant Asteroid Found Far
Out In Solar System
By Frank D. Roylance
Baltimore Sun Staff

European scientists say object near Pluto's orbit is biggest of its kind.
For 200 years the giant asteroid Ceres has held the title as the largest known "minor planet" in the solar system.
Ceres is a spherical space rock orbiting in the asteroid belt between Mars and Jupiter.
It is nearly 600 miles in diameter, roughly the distance from Baltimore to Chicago.
Now a team of European astronomers is claiming that Ceres has been eclipsed in size by a newly discovered object, found near the orbit of Pluto.
The new asteroid could be as big as 870 miles across, according to calculations by a team led by Gerhard Hahn of the German Aerospace Center in Berlin.
The team's news release called the data "decisive ... relegating [Ceres] to second place after holding the asteroid size record for two hundred years."
Not so fast, said Brian Marsden, director of the International Astronomical Union's Minor Planet Center.
He said an asteroid's size can't be precisely determined without first knowing both its distance and its brightness, or reflectivity - also called its "albedo."
The Europeans have securely fixed the object's orbit and distance, he said.
Too soon to tell
But "it's a little premature for them to boldly come along and give a size, when they're still assuming an albedo."
More precise observations are needed, he said.
The new asteroid was discovered in May by a team led by Robert L. Millis, director of the Lowell Observatory in Flagstaff, Ariz. It was temporarily dubbed 2001 KX76.
It was found in an orbit beyond Neptune, about 4 billion miles from the sun, in the inner regions of a vast, icy realm of the outer solar system called the Kuiper Belt.
The discovery team made a preliminary estimate of KX76's diameter of between 595 and 788 miles, or about half the size of the planet Pluto.
More precise calculation
Thursday, however, the European Space Agency Information Center announced a more precise orbital calculation for KX76 using a "virtual telescope" to digitally search for the asteroid on star photos taken years ago.
Coupled to the same assumptions about KX76's brightness, the new orbital data boosted the asteroid's presumed diameter to between 744 and 868 miles.
Both KX76's discoverers and the Europeans assumed that the asteroid's albedo lay somewhere between those of another Kuiper Belt asteroid, called 20,000 Varuna, and a typical comet nucleus from that region of the solar system.
David C. Jewitt, an astronomer at the University of Hawaii and co-discoverer last year of the Ceres-sized Varuna, said it makes no difference to science whether KX76 is bigger than Ceres.
"It's just a record-keeping thing," he said.
The real importance of KX76's size, he said, is that it "fits in with a pattern."
More than 400 Kuiper Belt objects of various sizes have been found since 1992.
"And that size distribution probably extends all the way up to Pluto [1,426 miles in diameter] and probably includes Pluto as one of those bodies," he said.
And, he said, "it's quite possible there are a few objects bigger than Pluto waiting to be found.


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