On Friday, an asteroid about half the size of a football field will zip past Earth.
And it will be close, according to experts. Then again, in outer space, close is a relative term.
In this case, it's about 17,200 miles.
The asteroid, assigned the name 2012 DA14, will pass closer to Earth than many of the orbiting satellites we launch.
"It's the closest approach for an object this size or larger," said Don Yeomans, the manager of NASA's Near Earth Object Program in Pasadena, Calif.
Yeomans, however, is talking about recent times. Earth is pockmarked with the scars
of ancient collisions. Take Meteor Crater in Arizona. A 40-foot-wide asteroid struck near Winslow about 50,000 years ago and left a crater that measures 1 mile across and more than 550 feet deep.
More recently, a similar-size asteroid exploded above the Podkamennaya Tunguska River in Siberia in 1908. The shock wave leveled 800 square miles of forest. That event is one reason Yeomans and other professional and amateur astronomers spend their time scanning the galaxy for signs that the sky is falling.Asteroids are the rocky and metallic leftovers of our solar system's formation. Most orbit the sun in a zone between Mars and Jupiter.
But millions of others follow different paths. Those asteroids that come closest to Earth are the ones Yeomans tries to find and monitor.Years of searching have identified nearly every really big rock -- those with diameters of at least 6 miles -- with a near-Earth orbit. There are four: Ganymed, Eros, Sisyphus and Eric. Eric is the most recent, having been discovered in 1990.
Yeomans said they've found about 94 percent of the near-Earth asteroids larger than 0.6 mile in diameter. There are 907 identified asteroids kept in lists maintained by the Minor Planet Center in Cambridge, Mass. An impact with anything that large would affect all of Earth.
A super-size asteroid would mean even more bad news. For example, an asteroid at least 6 miles across is believed to have hit Earth near Mexico's Yucatan Peninsula about 65.5 million years ago. That one helped wipe out dinosaurs.The search for "potentially hazardous asteroids" now focuses on rocks that are at least 459 feet in diameter. These are roughly three times the size of 2012 DA14.So far, Yeomans said, his team has identified about 40 percent of those asteroids.
Though an asteroid of that size wouldn't cause mass extinctions, it would have "devastating local and regional impacts," said Robert McMillan, an astronomer with the University of Arizona's Spacewatch Program.
An ocean impact, for example could create a tsunami. That would be good compared to the alternative.
"The land impacts are much more scary," McMillan said "(One) could wipe out an entire metropolitan area."McMillan's Spacewatch program is part of a network of astronomers that helps NASA identify the orbits of newly discovered asteroids.
NASA has four land-based telescopes that scan the sky for new asteroids. The telescopes snap images of sections of the sky in 10-minute intervals. A computer program then picks the asteroids out against the background of stars.
Each asteroid then is tracked for about an hour, enough time to estimate its size and direction." They need to be relatively close to Earth and on the opposite side of the sun," said Pasquale Tricarico, an astronomer with the Planetary Research Institute in Tucson, Ariz. Astronomers estimate an asteroid's size by measuring the amount of light it reflects back to Earth.
Any asteroid that might have a near-Earth orbit is named and its vitals are sent to the Minor Planet Center, where staff members post the data on its website. Then astronomers pick up the trail and track them.McMillan said he uses as many as four telescopes at the Kitt Peak National Observatory near Tucson to follow up on newly discovered asteroids and track their trajectories. He sends as many as 3,000 of these observations, called tracklets, each year to the Minor Planet Center.
These are strung together and used by Yeomans' office to help calculate the asteroids' orbits. Asteroids that are deemed near Earth object s are further studied to analyze the potential for Earth collisions. Those that threaten to strike Earth are further scrutinized to weigh the risk. On Jan. 10, Yeoman's office announced that an asteroid named Apophis won't hit Earth in 2036. It will come within 19,400 miles of Earth, setting a record for the largest asteroid of its size to come that close.
As of last week, NASA was tracking 1,372 potentially hazardous asteroids. The chance that any of those might hit Earth is remote, Yeomans said.
Apophis, for example, was originally assigned a 2.7 percent chance of hitting Earth in 2029 before its orbit was tracked more precisely.
Yeomans said he doesn't spend a lot of time tracking near misses. In 2008, his office warned the White House about 2008 PC3, an asteroid about the size of a truck that exploded without harm in the atmosphere over northern Sudan.
"There wasn't any damage," Yeomans said. "Although, if you are going to have an explosion in the atmosphere over that part of the world, you'd want to know that it was an asteroid."
The ultimate goal is to identify larger asteroids years before they pose a threat. That gives officials time to plan how to keep a rock from striking the planet. Doing so is relatively simple, Yeomans said. It involves firing a rocket at the asteroid to knock it off course.
Yeomans said NASA's Deep Impact mission, which fired a probe into the Tempel 1 comet in 2005, proves that the system would work."It's tracking an object in space and sending something up to smack it," he said.
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