In February of 2013, skywatchers around the world turned their attention toward asteroid 2012 DA14, a cosmic rock about 150 feet (50 meters) in diameter that was going to fly closer to Earth than the spacecraft that bring us satellite TV.
Little did they realize as they prepared for the once-in-several-decades event that another bit of celestial debris was hurtling toward Earth, with a more direct heading. On Feb. 15, 2013, the Chelyabinsk meteor, a roughly 62-foot (19 meter)-diameter asteroid exploded over the city of Chelyabinsk, Russia, as it entered Earth's atmosphere at a shallow angle. The blast shattered windows and damaged buildings, and nearly two thousand people were hurt, though thankfully no one died.
"It turned out that two completely independent asteroids were coming by that day," said Philip Lubin, UC Santa Barbara professor of physics, and one of the many scientists anticipating 2012 DA14's near-Earth rendezvous. "One of them we knew was going to miss the Earth. The other one, we didn't even know it was coming."
For Lubin and scientists like him, incidents like these underline the importance of robust planetary defense - the detection, tracking, characterization and ultimately defense against potentially dangerous asteroids and comets. City-threatening events like Chelyabinsk are rare, happening about once every 50 to 100 years, but they are potentially devastating. The most recent of these occurrences was the Tunguska Event, an airburst over eastern Siberia in 1908, which flattened hundreds of square miles of forest. Rarer still, but nevertheless possible, are objects that threaten mass-extinction, such as the Chicxulub impactor, which wiped out the dinosaurs some 66 million years ago, or the more recent (12,800 years ago) airburst that caused widespread burning and the onset of an "impact winter" called the Younger Dryas.
However, one cannot discount the possibility of larger objects coming uncomfortably close to Earth in the near future: Apophis, with its 1,214-foot (370 meter) diameter, is due to make a close pass on Friday the 13th in April 2029, while Bennu, at 1,608 feet (490 m) in diameter, is expected to perform a similar pass in 2036. Though they are not anticipated to hit Earth, even relatively small changes in their orbit could cause them to enter gravitational pockets called "keyholes" that can place them on a more direct trajectory toward Earth.
"If it goes through the gravitational keyhole, it will generally hit Earth on the next round," Lubin said.
Strategies for planetary defense have progressed from research into better methods for understanding the threats, to efforts to deflect potential hazards and change their orbits, including a strategy developed by Lubin's group, which proposed the use of lasers to push threatening objects out of Earth's way.
