For the vigilant and largely unsung astronomers who scan the skies for asteroids that could threaten the Earth, there is some good news and some bad news about 2004MN4. This was the initial designation for an asteroid that caused a flurry of alarm among scientists when it was discovered last December apparently heading for a frightening rendezvous with our planet on April 13, 2029.
Astronomers figured that there was a one in 50 chance that MN4 would actually strike the Earth. Such an impact by an asteroid estimated to be as large as 1300 feet across could devastate a large region and perhaps, depending on where it hit, cause millions of casualties and untold billions in property damage.
No wonder then that MN4 has been named Apophis, the Greek name for the Egyptian god of evil, destruction and darkness. But days after the initial discovery of the asteroid's trajectory, when astronomers found earlier, overlooked photos of the intruder in their archives and used them to refine estimates of its orbit, they were able to issue an all-clear. Apophis, it turns out, will come within as little as 15,000 miles from of Earth and will be visible to the naked eye in Europe and Africa on the evening of that April date, but will zoom safely past. Good news indeed.
But there are still some reasons for concern. As it passes so close the asteroid, tugged by Earth's gravity, will change its orbital path. That could be very bad news. If the altered orbit results in Apophis passing through any of several "keyholes," specific regions of space only about 2,000 feet across, the asteroid would then return periodically to dangerously close encounters with Earth. Passage through the keyhole that astronomers think most likely to be the asteroid's target in 2029, for example, would bring it back to the near vicinity of Earth every seven years, beginning in 2036, posing a serious threat each time.
This news was grist for the mills of the B612 Foundation (named after the fictional asteroid home of "The Little Prince," in Saint-Exupery's novel). The astronomers and scientists who founded B612 did so to alert Congress and the public to the menace of an asteroid strike and to lobby for a demonstration mission by 2015 that could show the feasibility of a controlled deflection of an object threatening to strike the Earth.
In a letter sent last month to NASA Administrator Michael Griffin, former astronaut Rusty Schweickart, B612's chairman, called attention to the Apophis dilemma. He urged that a radio transponder, similar to those on commercial airliners, be landed on the asteroid so that astronomers might track its orbit precisely to determine if it will pass through a keyhole, and he requested that NASA quickly estimate the time required for both landing the transponder and a subsequent deflection mission that could alter the asteroid's orbit.
Why the rush? The Apophis deflection, should it become necessary, must take place before the 2029 close approach. Earlier than that, just a simple nudge, accomplished, say, by firing a heavy object at the asteroid, could change its course enough to miss the crucial but small keyhole. Any time after that approach, should Apophis pass through the keyhole, we could be in trouble. NASA scientist David Morrison explains: "After 2029, the deflection would have to be vigorous enough to miss not just a tiny keyhole but the much larger target of the Earth itself. And such a deflection is far beyond present technology for an asteroid this large."
Given that deadline, some 24 years from now, there's seemingly plenty of time to take action. But Schweikart, who admits he is not expert in mission planning, speculates that a transponder mission, from initial planning to implantation might take, say, eight years. And he thinks that a following deflection attempt, if it proves necessary, could require as long as 15 years to implement. That's cutting it a little close, and, says Schweikart, all the more reason that NASA quickly calculate some realistic mission times. "It may turn out," he says, "that we have to begin planning those missions right now."