Cornell University astronomers announced last week that they tracked a near-Earth object (NEO) named 2005 YU55 as it skimmed within 1.5 million miles of our planet last April 19. The giant radio antenna at Arecibo, Puerto Rico measured the intruder’s size at a planet-walloping 1,300 feet in diameter (pictured above).

The speedy visitor is on a wanted list of “potentially hazardous asteroids” maintained by the Minor Planet Center, of the Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass.

WATCH VIDEO: With thousands of asteroids, comets and other near-Earth object buzzing by our planet, find out how we can avoid the same fate as the dinosaurs.

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.

The discoverer of Saturn’s rings, Giovanni Cassini, reported a dark spot on Jupiter on May 29 and 31, 1686. The spot was approximately the size of the largest SL9 impact. In April 1835 Johann Madler reported the appearance of a pair of dark spots. At the same time another observer, George Airy, independently reported a dark feature in Jupiter’s southern belts that looked nearly four times as large as shadows cast on Jupiter by the Galilean moons. (Oh if they only had the Internet back then!) In 1865 Jean Chacornac reported a dark spot which he took to be a cloud of vapor from a Jovian volcano. (He thought sunspots were made the same way!)

What’s unsettling is that after SL9, astronomers predicted a Jupiter impact by an object about 1000 feet across should happen every few hundred years. The 2009 collision demonstrates that we are largely in the dark about the frequency of collisions from space debris.

This was underscored by a spectacular Hubble Space Telescope picture taken last January that caught the aftermath of a collision between two bodies in the asteroid belt. This made a lot of secondary particles that pollute the solar system with even more fragments. Astronomers caught this collision, but how many other smashups go unseen and seed interstellar space with more debris?

This means our civilization must forever be diligent in continuously tracking and cataloging asteroids. NASA’s new Wide-field Infrared Survey Explorer (WISE) is already beginning to pluck out uncharted asteroids.

Though 2005 YU55 is scheduled to swing closer the Earth than the Moon on Nov 8, 2011 (just 13 months before the Mayan Calendar ends –- oh my!), the latest observations rule out any collisions with Earth for another century, say Cornell astronomers.

But what else is lurking out there that we don’t know about? President Barack Obama has proposed that NASA’s NEO program be increased from $3.7 million in 2009 to $20.3 million in 2011. And, 63 percent of the American public feel that NASA’s most important task is to keep an eye on asteroids according to a recent opinion poll.

Artwork Credit: Don Davis. Images credit: NASA