David L. Chandler in New Scientist:
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.