Unsettling clues that underscore our lack of knowledge about the asteroid threat can be found as far as 500 million miles away and a few centuries back in time.
On the night of July 19, 2009 Australian amateur astronomer Anthony Wesley photographed a mysterious dark spot that appeared low in Jupiter's southern hemisphere.
To science fiction fans it had an uncanny resemblance to the pitch black alien monolith in the Arthur C. Clarke novel "2010: Odyssey Two."
In reality, something probably a few hundred feet across –- either a wayward comet or asteroid - had plunged into Jupiter's atmosphere and exploded. Nobody saw the interplanetary missile, but the blacken residue was strong circumstantial evidence. The blast beneath the cloud tops was equal to a few thousand standard nuclear bombs exploding (1 megaton of TNT yield per bomb). The mega-blast shot black debris up into the atmosphere through the object's entry tube like a mischievous kid blowing soda out of a straw.
Wesley wasn't the first person since the invention of the telescope to ever seen a dark splotch on Jupiter. Be he was able to immediately recognized what he was looking at, thanks to a cosmic event that happened 15 years earlier - almost to the day.
The only way this could be deduced was that from July 16 to 22, 1994 astronomers witnessed a string of comet pieces carpet bomb Jupiter. The doomed comet, called Shoemaker Levy 9 (SL9), was possibly snagged 30 years earlier by Jupiter's gravity and broken apart into at least two dozen pieces. It was tracked for more than a year before it made the fateful plunge into Jupiter's cloud tops.
A friend of mine, NASA astronomer Kelly Fast, was so fascinated by this unexpected new intruder in 2009, that she made a music video to rock performer Prince's "1999." I might have been tempted to do a musical parody on the 1983 song "King of Pain" by The Police: "There's a little black spot on the sun (er, Jupiter) today. . ."
What's eerie is the possibility that veteran astronomers may have seen dark spots caused by other Jupiter impacts long before the two most recent events. After SL9, Thomas Hockey of the University of Northern Iowa surveyed astronomical reports predating 1878 and look at several hundred drawings of Jupiter. He found four instances where Jupiter might have been whacked.