About 20 years from today, an asteroid about the size of a 25-story building will come closer to Earth than the networks of communications satellites orbiting the planet.
NASA says there's no chance of an impact -- at least not in 2029 -- but the asteroid, named Apophis, will be back. Analysis of the asteroid's orbit show it will return to Earth seven years later.
Astronomers don't yet know if Apophis' second visit will be a rendezvous or a collision, as its orbit will be bent by Earth's gravity during the 2029 flyby.
Most of the rocks whizzing around Earth are too small to do damage even if they were on collision paths.
"Things much below 30 meters in size don't pose much of a threat at all since the atmosphere protects us," said Nick Kaiser, lead scientist of a new University of Hawaii asteroid-hunting project known as the Panoramic Survey Telescope and Rapid Response System, or Pan-STARRS.
The telescope array, funded by the Air Force Research Laboratories, combines small mirrors with massive digital cameras to image the entire sky several times a month. Pan-STARRS supplements ongoing NASA-funded efforts to survey 90 percent of near-Earth objects bigger than 140 meters, or 459 feet.
"As things get bigger, the amount of devastation goes up dramatically," Kaiser said, but so too does the length of time between occurrences.
A half-mile diameter object impacting Earth likely would cause a global catastrophe, but these events happen every couple of million years or so, Kaiser said.
The asteroid that is believed to be responsible for wiping out the dinosaurs 65 million years ago was about 10 kilometers, or 6.2 miles, in size. More recently, a rock about 50 meters, or 164 feet, exploded over an unpopulated region of Russia known as Tunguska, devastating an area tens of miles across.
As of March 31, NASA's list of so-called near-Earth objects numbers 6,191. The catalog includes 773 objects one kilometer in diameter or larger and 1,042 objects -- including Apophis -- classified as "potentially hazardous" to Earth, according to the agency's Near-Earth Object Program web site.
Jon Giorgini, a senior analyst with the Solar System Dynamics group at NASA's Jet Propulsion Laboratory in Pasadena, Calif., and colleagues used the giant radar dish at the Arecibo Observatory to map Apophis in 2005 and 2006, resulting in a reprieve for Earth during the 2029 encounter.
Shortly after the discovery of the asteroid in 2004, scientists had pegged the chance of a collision at 2.7 percent. Giorgini argues in his findings, published in the journalIcarus, that that kind of precise prediction is not yet possible.
"The point of the paper was to demonstrate that the impact probability could not be reliably computed because important parameters of the physics are unmeasured," Giorgini told Discovery News.
During its 2029 rendezvous, Apophis will pass about 18,000 miles from Earth -- closer than the 22,300-mile-high orbits of geostationary communication satellites. The asteroid will be clearly visible in the night sky to the unaided eye -- even with city lights.
Scientists don't expect to have to wait 20 years to nail down Apophis' future orbits. In 2013 and 2021 the asteroid will be about 9 million miles from Earth, close enough for radar studies.
"The existing surveys are homing in on the goal of detecting about 90 percent of the objects bigger than 1 kilometer," Kaiser said.
Pan-STARRS will help complete this survey and extend the list to objects that are about 300 meters in diameter when the four-part telescope is complete in 2013.
"These 300-meter objects deliver about 1,000 megatons of TNT equivalent, so while they don't kill everyone they would certainly give you a very bad day," Kaiser said. "It's been estimated that they would devastate an area equal in size to France. These things happen about every 70,000 years, so there's a 1-in-700 chance that one of these will collide in the next 100 years. "
"If one of these is out there, then Pan-STARRS 4 will detect it and figure out exactly when it is going to hit. More likely, we will do our survey and be able to give Earth a clean bill of health -- for now at least," he added.