Astronomers Watch The Skies For Threat Of Deadly Impact

Shortly after sunset Friday, April 13, 2029, if the sky is clear enough, people across Europe and North Africa will see an asteroid appear as a bright point of light flying 19,400 miles overhead before it disappears silently below the western horizon.

A short time later, if astronomers' worst fears are realized, the asteroid will pass through a region of space less than 2,000 feet across. At that place, the gravitational pull of Earth will yank the asteroid into a new orbit around the sun - and on a collision course with Earth seven years later.

It all sounds like the premise of "Armageddon," "Deep Impact" or some other blockbuster Hollywood film. But the asteroid, named 99942 Apophis, is science fact, not science fiction. In December 2004, astronomers caused a brief stir when their calculations estimated that the newly discovered asteroid - named after the ancient Egyptian god, Apep, the Destroyer - might collide with Earth in 2029.

Additional tracking data quickly ruled out the possibility of a 2029 collision. But the potential for a strike in 2036, should the asteroid enter that crucial gravitational space, places it at the top of NASA's list of 3,800 near-Earth asteroids the agency has identified.

Based on the latest information, the asteroid, which is nearly twice the size of a typical football stadium, has a 1-in-6,250 chance of colliding with Earth on April 13, 2036.

We're very concerned that people put this in perspective," said Russell Schweickart, a former Apollo astronaut and head of a foundation that focuses public attention on the threat from asteroids and comets.
"This is not something to lose sleep over, (but) it is something the government needs to attend to."

Right now, NASA is doing little more than looking for asteroids and keeping track of them, Schweickart said. Plans to deflect Apophis, if it becomes necessary, exist only in the pages of a few academic papers.
Last year, Schweickart's group, the B612 Foundation - named for the asteroid in the book, "The Little Prince" - corresponded with NASA officials about the threat of Apophis.

"It would have devastating consequences if it hit," Schweickart wrote. "There is the serious question of whether, if it is headed toward impact, we will know enough to make a timely decision."

Schweickart and other scientists urged NASA to place a data-tracking radio transponder on the asteroid's surface by 2014.

A transponder would help nail down orbital alterations caused by a phenomenon called the Yarkovsky effect. This is produced when an asteroid absorbs energy from the sun and re-radiates it back into space as heat. With one side of the asteroid lit and the other in darkness, the imbalance in thermal radiation produces a tiny acceleration. A transponder would help scientists understand how the Yarkovsky effect is influencing the asteroid's orbit.

NASA responded to the urging with a wait-and-see proposal. "We conclude a space mission based solely on any perceived collision hazard is not warranted at this time," wrote Mary L. Cleave, associate administrator for NASA's science mission directorate.

The agency believes continued optical and radio telescope observations will rule out Apophis as a threat. If not, NASA would launch a mission to the asteroid by 2018. A radio transponder, placed either in orbit or on its surface, would determine the asteroid's position in 2029 down to a few hundred feet, according to NASA.

If an impact seems probable, a rocket would be launched to deflect the asteroid. The design phase would have to be completed by 2020 in order to launch by 2024, NASA noted.

Schweickart said he doesn't necessarily disagree with NASA's analysis, as long as the agency can design, build, launch and successfully complete such a mission before 2029. "The danger is being overly optimistic about how long it takes to do that."

If a deflection mission becomes necessary, scientists agree, it will need to be completed before 2029 when Apophis would commit itself to a future collision course. Due to the physics of gravity and orbital mechanics, delaying action would require much, much more energy to move the asteroid.

"That (will be) an impossible task, I'll tell you right now," Schweickart said.

If Apophis assumes a collision course, it would crash into Earth's atmosphere at about 28,000 miles per hour and explode with a force of an 870-megaton blast - an explosion 58,000 times more powerful than the bomb dropped on Hiroshima.
 
Apophis would likely hit along a narrow corridor in the Pacific Ocean, sending monstrous tsunami waves toward the West Coast, the B612 Foundation has estimated. Immediate damage in the U.S. - independent of deaths and subsequent economic fallout - could top $400 billion.
 
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