Why didn’t astronomers know that asteroid DD45 was about to get so close to the Earth? Some suggest that it would have devastated a forest like the Siberian hit in 1908. I thought it was assumed that the possibility of an asteroid crashing was negligible. So why are some astronomers provoking the alert once again?
This asteroid is established to have a diameter of 30-40 meters, slightly smaller than the 1908 Tunguska impactor. We are not sure whether an object this size would make it through the atmosphere to cause massive damage like Tunguska; it might have have exploded too high to cause too much ground damage. However, you are correct that an impact this size might cause substantial damage, and therefore we would very much like to be able to detect such objects before they hit. However, there are likely to be a million asteroids this size with Earth-crossing orbits. They are far too faint to be reliably detected with the current Spaceguard Survey, which is aiming for completeness for asteroids only larger than 1 km in diameter, which is about a thousand times larger (in mass and energy) than DD45 (see the NASA impact hazard website at http://impact.arc.nasa.gov). The asteroid had to come very close before it was within range of our survey telescopes. For perspective, nearly 100 asteroids come this close to Earth for every on e that hits, with one sailing by every few years. Astronomers are well aware of the impact threat from objects this size. If you do the arithmetic, the risk to any one person from such impacts is very small compared to other natural hazards. Still, this close flyby is a reminder (or perhaps a wake-up call) that we live in a cosmic shooting gallery and could be hit by a small asteroid at any time, with no warning at all. Look at the cratered face of the Moon to get an idea of Earth’s impact history.
NLSI Interim Director
March 5, 2009