- A comet has been added to the list of potentially threatening
near-Earth objects maintained by NASA's Jet Propulsion Laboratory. Comet
Catalina 2005 JQ5 is the largest - and therefore most potentially devastating
- of the 70 objects now being tracked. However, the chances of a collision
are very low.
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- The listing of Comet Catalina underscores the uncertainty
in the knowledge of whether comets or asteroids pose a greater threat to
Earth. Previous estimates of the proportion of the impact risk posed by
comets have varied widely, from 1% to 50%, with most recent estimates at
the lower end.
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- But comets are larger and faster-moving, on average,
so their impacts could be a significant part of the overall risk to human
life. And, unlike asteroids, they lie on randomly-oriented and usually
highly elongated orbits. This makes them much more likely to remain undiscovered
until they are very close to Earth.
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- Comet Catalina was found by the Catalina Sky Survey,
one of the six current, large-scale and automated search programmes for
near-Earth asteroids. It was initially designated as an asteroid when first
spotted on May 6. But was reclassified as a comet when observers saw characteristic
fuzziness in the image, indicating ice and dust streaming off.
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- Its size is estimated at 980 metres, but Steve Chesley
of JPL told New Scientist that the size determination is based on the assumption
it is a dark-bodied asteroid, and so the bright coma of a comet would cause
the estimate to be high. "It's really an upper limit," he said.
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- Collision course?
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- On 26 May, JPL's unique orbital calculation software
determined that Comet Catalina was on what could possibly be a collision
course with Earth, though the odds of such an impact were small: just 1
chance in 300,000 of a strike on June 11, 2085. Based on the 980-metre
size estimate, that would produce a 6-gigaton impact - equivalent to 6
billion tonnes of TNT.
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- Astronomers expected the addition of further observations
to the calculations to rule out any possibility of a collision, as happens
with most newly-seen objects.
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- But that did not quite happen. The comet's predicted
pathway actually drew even closer to making a perfect bull's-eye with the
Earth - its predicted path passes within 1000 kilometres of the where the
centre of our 12,700-km-diameter planet will be around that time.
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- However, uncertainty in the exact timing of the comet's
pass through the line of Earth's orbit dropped the odds of an impact to
about 1 in 120 million. That is very low, but the observations so far cannot
categorically rule a collision out.
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- Forceful outgassing
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- Chesley adds that even the slim 1 in 120 million odds
are an overestimate, because comets, unlike asteroids, can move in unpredictable
ways because of the forceful outgassing that creates their dusty comas
and tails. "The uncertainty is much larger than we're modelling,"
he said. "But I haven't come up with a good way of dealing with this."
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- The only other comet placed on the JPL list of near-Earth
objects with possible collision paths was added in 2003. But additional
observations ruled out a possible impact - that comet was removed from
the list after less than a week.
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- Just one other comet, Swift-Tuttle, has been recorded
with a non-zero possibility of impact. It was rediscovered in 1992 - after
more than a century's absence - before the JPL list was created.
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- Additional observations during Swift-Tuttle's passage,
thanks to the publicity surrounding the possible impact, made it possible
to rule out the possibility of an Earth impact anytime in this millennium.
However, Swift-Tuttle is on an orbit that will almost certainly cause it
to crash into the Earth or the moon eventually.
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- http://www.newscientistspace.com/article/dn7449.html
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