- European scientists say object near Pluto's orbit is
biggest of its kind.
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- For 200 years the giant asteroid Ceres has held the title
as the largest known "minor planet" in the solar system.
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- Ceres is a spherical space rock orbiting in the asteroid
belt between Mars and Jupiter.
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- It is nearly 600 miles in diameter, roughly the distance
from Baltimore to Chicago.
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- 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.
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- 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.
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- The team's news release called the data "decisive
... relegating [Ceres] to second place after holding the asteroid size
record for two hundred years."
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- Not so fast, said Brian Marsden, director of the International
Astronomical Union's Minor Planet Center.
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- 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."
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- The Europeans have securely fixed the object's orbit
and distance, he said.
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- Too soon to tell
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- But "it's a little premature for them to boldly
come along and give a size, when they're still assuming an albedo."
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- More precise observations are needed, he said.
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- 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.
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- 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.
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- 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.
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- More precise calculation
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- 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.
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- 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.
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- 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.
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- 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.
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- "It's just a record-keeping thing," he said.
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- The real importance of KX76's size, he said, is that
it "fits in with a pattern."
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- More than 400 Kuiper Belt objects of various sizes have
been found since 1992.
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- "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.
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- And, he said, "it's quite possible there are a few
objects bigger than Pluto waiting to be found.
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