Humans live in a vast solar system where 2,000 feet seems a razor-thin distance.
Yet it's just wide enough to trigger concerns that an asteroid due to buzz Earth on April 13, 2029 may shift its orbit enough to return and strike the planet seven years later.
The concern: Within the object's range of possible fly-by distances lie a handful of gravitational "sweet spots," areas some 2,000 feet across that are also known as keyholes.
The physics may sound complex, but the potential ramifications are plain enough. If the asteroid passes through the most probable keyhole, its new orbit would send it slamming into Earth in 2036. It's unclear to some experts whether ground-based observatories alone will be able to provide enough accurate information in time to mount a mission to divert the asteroid, if that becomes necessary.
So NASA researchers have begun considering whether the US needs to tag the asteroid, known as 99942 Apophis, with a radio beacon before 2013.
Timing is everything, astronomers say. If officials attempt to divert the asteroid before 2029, they need to nudge the space rock's position by roughly half a mile - something well within the range of existing technology. After 2029, they would need to shove the asteroid by a distance as least as large as Earth's diameter. That feat would tax humanity's current capabilities.
NASA's review of the issue was triggered by a letter from the B612 Foundation. The foundation's handful of specialists hope to demonstrate controlled asteroid-diversion techniques by 2015.
Last Wednesday, representatives from the foundation met with colleagues at NASA's Jet Propulsion Laboratory (JPL) to review the issue. The foundation's letter marks the first time specialists in the asteroid-hazard field have called for a scouting mission to assess such a threat.
"We understand the risk from this object, and while it's small, it's not zero," says David Morrison, the senior scientist at NASA's Astrobiology Institute at the Ames Research Center at Moffett Field, Calif.
The call for a reconnaissance mission also illustrates how far the field of asteroid-hazard assessment has come.
"Ten years ago, we would have been blissfully ignorant," says Donald Yeomans, who heads NASA's near-Earth object project at JPL. Today, at least five programs worldwide are hunting down near-Earth objects. NASA is well on its way toward achieving its goal of cataloging 90 percent of the near-Earth objects larger than 0.6 miles across by 2008. And it is devising ways to ensure that information about potential hazards reaches top decisionmakers throughout the government.
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.