An asteroid, headed our way

Based on available data, astronomers give Apophis - a 1,000-foot wide chunk of space debris - a 1-in-15,000 chance of a 2036 strike. Yet if the asteroid hits, they add, damage to infrastructure alone could exceed $400 billion. When the possibility of the asteroid passing through two other keyholes is taken into account, the combined chance of the asteroid hitting the planet shifts to 1 in 10,000, notes Clark Chapman, a senior scientist with the Southwest Research Institute in Boulder, Colo.

"A frequent flier probably would not want to board an airliner if there's a 1-in-10,000 chance it's going to crash," he says.

The asteroid in question was discovered last June. Initially, it looked as though it might strike Earth in 2029. But additional observations eliminated that possibility. Instead the asteroid will come within 22,600 miles of Earth - just inside the altitude where major communications satellites orbit. The asteroid will be visible to the naked eye in the night skies over Europe and western Africa, where it will appear a bit dimmer than the North Star.

But this estimated distance carries an uncertainty that spans several thousand miles either side of its expected path - a region of space that includes three gravitational keyholes.

JPL's analysis will look at several factors. One involves estimating whether additional ground observations will be sufficient to resolve the question of whether the asteroid will pass through one of the keyholes. The asteroid belongs to a class known as Atens, which orbit the sun in less than a year and pass through Earth's orbit. Because Atens spend so much of their time in the direction of the Sun, observations from Earth are difficult. After next year, the next opportunity to gather data on the asteroid from the ground will come in 2012-2013.

In addition, questions remain over how long a tagging mission - and if necessary a deflection mission - would take to plan and execute. If missions can be mounted in six years or less, NASA could postpone a decision to tag the asteroid until 2014. This would give astronomers time to incorporate their latest observations as they refine calculations of Apophis's orbit. But if a tagging mission took seven to eight years and a diversion mission took another 12 years, the case grows for launching the tagging mission sooner rather than later.

Dr. Yeomans, the head of the near-Earth-object program at JPL, says the next step is to examine whether additional ground-based observations are likely to solve the collision riddle in a timely fashion.

"I can't stress this enough: The overwhelming most-likely scenario is that radar and optical data this year and next or in 2012 and 2013 will completely remove the impact probabilities," he says.

"If this is the case, why are we worried now? If it's a 1-in-15,000 shot and we come up a loser," there's still time to mount a tagging and a deflection mission, he says.

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