Scientists at Arizona's Kitt Peak National Observatory first spotted the Apophis asteroid in June 2004. (Photo: Bryan Allen)
Friday the 13th of April 2029 could be a very unlucky day for planet Earth. At 4:36 am Greenwich Mean Time, a 25-million-ton, 820-ft.-wide asteroid called 99942 Apophis will slice across the orbit of the moon and barrel toward Earth at more than 28,000 mph. The huge pockmarked rock, two-thirds the size of Devils Tower in Wyoming, will pack the energy of 65,000 Hiroshima bombs—enough to wipe out a small country or kick up an 800-ft. tsunami.
On this day, however, Apophis is not expected to live up to its namesake, the ancient Egyptian god of darkness and destruction. Scientists are 99.7 percent certain it will pass at a distance of 18,800 to 20,800 miles. In astronomical terms, 20,000 miles is a mere stone's throw, shorter than a round-trip flight from New York to Melbourne, Australia, and well inside the orbits of Earth's many geosynchronous communications satellites. For a couple of hours after dusk, people in Europe, Africa and western Asia will see what looks like a medium-bright star creeping westward through the constellation of Cancer, making Apophis the first asteroid in human history to be clearly visible to the naked eye. And then it will be gone, having vanished into the dark vastness of space. We will have dodged a cosmic bullet.
Maybe. Scientists calculate that if Apophis passes at a distance of exactly 18,893 miles, it will go through a "gravitational keyhole." This small region in space—only about a half mile wide, or twice the diameter of the asteroid itself—is where Earth's gravity would perturb Apophis in just the wrong way, causing it to enter an orbit seven-sixths as long as Earth's. In other words, the planet will be squarely in the crosshairs for a potentially catastrophic asteroid impact precisely seven years later, on April 13, 2036.
Radar and optical tracking during Apophis's fly-by last summer put the odds of the asteroid passing through the keyhole at about 45,000-to-1. "People have a hard time reasoning with low-probability/high-consequence risks," says Michael DeKay of the Center for Risk Perception and Communication at Carnegie Mellon University. "Some people say, 'Why bother, it's not really going to happen.' But others say that when the potential consequences are so serious, even a tiny risk is unacceptable."
Former astronaut Rusty Schweickart, now 71, knows a thing or two about objects flying through space, having been one himself during a spacewalk on the Apollo 9 mission in 1969. Through the B612 Foundation, which he co-founded in 2001, Schweickart has been prodding NASA to do something about Apophis—and soon. "We need to act," he says. "If we blow this, it'll be criminal."
If the dice do land the wrong way in 2029, Apophis would have to be deflected by some 5000 miles to miss the Earth in 2036. Hollywood notwithstanding, that's a feat far beyond any current human technology. The fanciful mission in the 1998 movie Armageddon—to drill a hole more than 800 ft. into an asteroid and detonate a nuclear bomb inside it—is about as technically feasible as time travel. In reality, after April 13, 2029, there would be little we could do but plot the precise impact point and start evacuating people.
Diagram: How to Off An Asteroid
Click to enlarge
Fortunately, Apophis needs to be nudged only about a mile to avoid a gravitational "keyhole" in space—a region that would send the asteroid on a collision course with Earth. Otherwise, it would have to be diverted 5000 miles for it to miss our planet. This reduces the energy required to deflect Apophis by a factor of about 10,000—making it theoretically possible using current technology. A number of methods have been proposed to do the job.
According to projections, an Apophis impact would occur somewhere along a curving 30-mile-wide swath stretching across Russia, the Pacific Ocean, Central America and on into the Atlantic. Managua, Nicaragua; San José, Costa Rica; and Caracas, Venezuela, all would be in line for near-direct hits and complete destruction. The most likely target, though, is several thousand miles off the West Coast, where Apophis would create a 5-mile-wide, 9000-ft.-deep "crater" in the water. The collapse of that transient water crater would trigger tsunamis that would hammer California with an hour-long fusillade of 50-ft. waves.
BUT DON'T EVACUATE
just yet. Although we can't force Apophis to miss the Earth after 2029, we have the technology to nudge it slightly off course well before then, causing it to miss the keyhole in the first place. According to NASA, a simple 1-ton "kinetic energy impactor" spacecraft thumping into Apophis at 5000 mph would do the trick. We already have a template for such a mission: NASA's Deep Impact space probe—named after another 1998 cosmic-collision movie—slammed into the comet Tempel 1 in 2005 to gather data about the composition of its surface. Alternatively, an ion-drive-powered "gravity tractor" spacecraft could hover above Apophis and use its own tiny gravity to gently pull the asteroid off course.
In 2005, Schweickart urged NASA administrator Michael Griffin to start planning a mission to land a radio transponder on Apophis. Tracking data from the device would almost certainly confirm that the asteroid won't hit the keyhole in 2029, allowing everyone on Earth to breathe a collective sigh of relief. But if it didn't, there still would be time to design and launch a deflection mission, a project that Schweickart estimates could take as long as 12 years. It would need to be completed by about 2026 to allow enough time for a spacecraft's tiny nudge to take effect.
NASA, however, is taking a wait-and-see attitude. An analysis by Steven Chesley of the Near Earth Object program at the Jet Propulsion Laboratory (JPL) in Pasadena, Calif., concludes that we can safely sit tight until 2013. That's when Apophis swings by Earth in prime position for tracking by the 1000-ft.-dia. radio telescope in Arecibo, Puerto Rico. This data could also rule out a keyhole hit in 2029. But if it doesn't, the transponder mission and, if necessary, a last-resort deflection mission could still be launched in time, according to Chesley. "There's no rush right now," he says. "But if it's still serious by 2014, we need to start designing real missions."