On the second day of 2014, a small asteroid blew up high in Earth’s atmosphere. It was relatively harmless—the rock was only a couple of meters across, far too small to hit the ground or do any real damage—and it disintegrated over the Atlantic Ocean anyway.
What’s cool is that now we know for sure this is the case: Infrasound detectors designed to listen for nuclear bomb detonations actually heard the explosion from the impact and were able to pinpoint the location of the event to a few hundred kilometers east off the coast of Venezuela.
NASA put together a nice informative video explaining it:
A quick background: The asteroid, 2014 AA, was first seen just a day before impact and was actually the first asteroid discovered in 2014. It was detected by the Mount Lemmon Survey asteroid search program and was quickly determined to have a short life ahead of it: It was due to impact the Earth in less than a day. This was only the second time an asteroid was discovered before it hit us; the first was 2008 TC3, which came in over Sudan and rained down small rocks over the desert (I have to add that years later I was able to hold a piece of that rock in my hand, and it was an eerie feeling).
Careful measurements of its orbit predicted an impact over the Atlantic, but the addition of the infrasound detectors nailed it. The principle there is actually pretty nifty. The Comprehensive Nuclear Test Ban Treaty Organization has what are essentially microphones installed in various locations around the world. When an explosion occurs, it creates very low-frequency sound waves in the atmosphere, and those can travel all the way around the globe. When an infrasound detector hears such an event, the timing is carefully recorded as well. By comparing the time the explosion was heard by detectors in different locations, the position of the explosion can be placed to pretty decent accuracy.*
The infrasound array determined the asteroid blew up over the Atlantic, which matched what the observatories on the ground predicted as well.
Besides 2014 AA and 2008 TC3, every impact we’ve ever seen (like the one over Chelyabinsk, Russia, in 2013) was a surprise; the asteroid was not detected before impact. However, we’re getting better at this, getting better sky coverage with more sensitive telescopes. If we continue to take this threat seriously—and we must—then discovering these small impactors before they hit could very well become the norm rather than the exception.
*This same timing technique was once used to determine the locations of gamma-ray bursts—vast, distant cosmic explosions—in the sky. A few years back I wrote a classroom activity that demonstrates this using lightning and thunder as an example; see Activity 2 in this workbook for more.