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NASA has outlined what it could do, and in what time frame, in case a quarter-mile-wide asteroid named Apophis is on a course to slam into Earth in the year 2036. The timetable was released by the B612 Foundation, a group that is pressing NASA and other government agencies to do more to head off threats from near-Earth objects.

The plan runs like this: Eight years from now, if there's still a chance of a collision in 2036, NASA would start drawing up plans to put a probe on the space rock or in orbit around it in 2019. Measurements sent back from the probe would characterize Apophis' course to an accuracy of mere yards (meters) by the year 2020.

If those readings still could not rule out a strike in 2036, NASA would try to deflect the asteroid into a non-threatening course in the 2024-2028 time frame by firing an impactor at it — using this year's Deep Impact comet-blasting probe as a model. Experts would start planning for the "Son of Deep Impact" mission even before they knew whether or not it was needed.

The plan is described in a letter attributed to Mary Cleave, NASA's associate administrator for the science mission directorate, as well as a scientific paper by Steve Chesley, an asteroid specialist at NASA's Jet Propulsion Laboratory. The letter was addressed to the B612 Foundation, and B612 made the letter and the paper public on Friday evening.

Although Cleave declined to comment on the plan in an e-mail exchange with MSNBC.com on Sunday, NASA confirmed on Monday that the letter distributed by the B612 Foundation was authentic. The outlines of the plan match reports that emerged from an August scientific conference in Brazil, where Chesley presented his paper.

‘Thorough and thoughtful’ analysis
The B612 Foundation said it was grateful for NASA's "thorough and thoughtful" analysis, which came in response to the foundation's call for a near-term mission to the asteroid, back in June. Former astronaut Rusty Schweickart, the chairman of the California-based foundation's board, said the plan would pose challenges for NASA officials.

"It's certainly a tight schedule," he told MSNBC.com Sunday, "but on the other hand, they're the experts."

Schweickart also noted that Apophis was an unusual case among potentially threatening asteroids, in that it would take a relatively small deflection to eliminate the possibility of a catastrophic collision. "In the typical case, that isn't going to do the job," he said.

Flurry of concern
Apophis, also known as 2004 MN4, stirred up a flurry of concern last December when the risk of collision was raised temporarily to as high as 1 out of 40 for the year 2029. With an estimated diameter of 1,300 feet (400 meters), the asteroid could destroy a city if it hit the wrong place on land, or raise a deadly tsunami if it plunged into the ocean.

Fortunately, more precise plotting ruled out a collision in 2029. However, Apophis will still make an extremely close pass — missing Earth by mere tens of thousands of miles. At that distance, Earth's gravitational pull could perturb Apophis' orbit enough to put it on a track to hit during another pass in 2036. Experts say that could happen if, during the 2029 close encounter, the asteroid passes through an outer-space "keyhole" that measures about 2,000 feet (600 meters) across. - gravitacija radi posao

In statistical terms, the risk of an impact is now set at 1 in 5,560, based on the uncertainties surrounding Apophis' orbit.

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