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.

By David L Chandler

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