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.
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.
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.
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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