In the early morning hours of March 3, 2009, a previously undetected asteroid hurtled past Earth at a mere 48,800 miles (60,000 km) above the surface of the Southern Pacific Ocean. While this distance may seem too large to cause concern, in astronomical terms it is a mere hair’s breadth. To put it in perspective, the asteroid’s path brought it nearly 7 times closer to Earth than the moon, and only 2 times as high as some telecommunication satellites currently in orbit.
The asteroid was first spotted by astronomer Robert H. McNaught at the Siding Spring observatory in Australia only 24 hours before it passed by Earth. Given this amount of advance warning, the only option would have been for people to evacuate as quickly as possible if it had been on a collision course with a highly populated area. McNaught surmised that if the asteroid had landed in the ocean, it likely would have caused a tsunami.
The asteroid, called 2009 DD45, measured an estimated 69 to 154 feet (30 to 50 meters) in width, and is the largest object so far observed to pass this close to Earth. The closest near-miss for an asteroid happened in 2004, when a 5-10 meter wide space rock whizzed by at a height of just 6,500 km.
What would happen if an asteroid of this size collided with Earth? A similarly-sized asteroid hit Tunguska, Siberia in 1908 with the force of a nuclear explosion. The Tunguska asteroid is believed to have exploded in the air before striking, with a blast equivalent to 5-30 megatons of TNT, or up to 1,000 times the energy of the bomb dropped on Hiroshima. The impact flattened 800 square miles (2,000 square km) of forest, felling 80 million trees, and the shock wave was felt hundreds of miles away.
While a collision with a large asteroid is of greatest concern (a one kilometer wide asteroid would cause global devastation), computer-simulated models of airbursts indicate that even small asteroids are capable of massive destruction if they explode in mid-air as happened in the Tunguska event. This finding is a concern, since asteroids smaller than 140 meters in diameter are difficult to detect.
The Near-Earth Object Program of NASA’s Jet Propulsion Laboratory is involved in tracking the orbits of asteroids and that pass within range of Earth. 2009 DD45 is believed to orbit the sun once every 1.5 years, and scientists will be paying close attention to its orbit to determine whether it will pose a hazard in the future.