[meteorite-list] Comet Put on List of Potential Earth Impactors

[Ron Baalke]

http://www.newscientistspace.com/article/dn7449.html

Comet put on list of potential Earth impactors
David L Chandler
New Scientist
June 1, 2005

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.

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.

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.

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.

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.

Collision course?

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.

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.

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.

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.

Forceful outgassing

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."

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.

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.

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.