Incoming near earth asteroid under close watch*

Astronomers don't yet know if second visit will be a rendezvous or collision

When it was discovered in 2004, the asteroid Apophis (circled here) was
briefly estimated to have a 2.7 percent chance of impacting the Earth in
2029. Additional measurements later showed there was no impact risk at
that time. Now astronomers predict the chance of an impact with Apophis
in 20 years is about 1 in 45,000 — but the asteroid will be back.
Analysis of the asteroid's orbit show it will return to Earth seven
years later.

By Irene Klotz
updated 1:25 p.m. PT., April 14, 2009

Exactly 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.

The chance of an impact are extremely remote — only about 1 in 45,000 —
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.
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"It can't even be said for certain what side of the sun (the asteroid)
will be on in," said Jon Giorgini, a senior analyst with the Solar
System Dynamics group at NASA's Jet Propulsion Laboratory in Pasadena,
Calif.

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.

Giorgini 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.

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."