It was nearly midnight here in Australia when a sneaky asteroid flew really close past our planet on the evening of 9 January.
The space rock zoomed by at only half the distance between us and the Moon—that’s roughly 192 thousand kilometres.
The asteroid, called 2017 AG13, is on the small side as far as space rocks go, measuring somewhere between 11-34 metres. Astronomers at Catalina Sky Survey first detected it just two days before the fly-by, and their initial observations show the asteroid has a more elliptical orbit than our planet. This means that in its course around the sun, it crosses the orbits of both Venus and Earth.
The good news is that an asteroid this size wouldn’t have obliterated us if it had crashed into our atmosphere. But it could have wreaked some havoc.
For comparison, the 2013 surprise bolide that streaked across the sky in Chelyabinsk was some 20 metres across. As it exploded, the shockwave produced significant ground damage, and more than a 1000 people sought treatment for various injuries, mostly lacerations from shattered window glass.
When a meteor struck near Chelyabinsk, Russia in 2013, the shockwave from the impact rocked the city.
Surprise asteroid fly-bys aren’t as rare as you think.
“It's remarkably frequent,” says Associate Professor Jonti Horner, astronomy expert at the University of Southern Queensland.
“We didn't see [the Chelyabinsk meteorite] at all, because it came out from the direction of the Sun, so it came out of the daylight sky. Part of the problem is that we see these things only by the light that they reflect—they're not self-luminous.”
According to Horner, it would be a real “stroke of luck” to spot an asteroid like AG13 before it had moved close enough, just because it’s so small and doesn’t glow, either.
“We're pushing the limits of technology to find these faint, moving objects against the background stars,” says Horner.
Once astronomers have spotted one of these asteroids, they can keep tabs on them, building a database of the things near our planet, especially the ones that could cross our orbit.
“We have greater capacity than ever before to monitor asteroids that have close approaches,” says Neil deGrasse Tyson in the February issue of National Geographic magazine. “For me the danger zone is, are you coming closer to Earth than the orbit of the moon?”
An asteroid can be on a collision course with Earth at any given time. But what was our closest encounter to date?
What he is referring to are ‘potentially hazardous asteroids’, a subclass of all the space rocks scattered around our solar system that could come in close enough to do some serious damage. Still, even most of those are likely to be harmless, says Horner.
“Even if this were to hit the Earth, the likelihood is that the airburst of the explosion would be so distant from any area of human habitation, that it wouldn't really pose a risk,” he says. “We'd have to be really unlucky for it to hit Sydney or Brisbane."
"The really scary things would be those that are a kilometre or so across, since they could do a lot more damage, but there are fewer of them."
Lastly, it’s good to remember that space rocks aren’t all dangerous—in fact, they can be a fascinating source of knowledge. In Australia we have the Desert Fireball Network, a project dedicated to understanding our solar system by studying meteorites. Just last year, researchers from Perth used it to track down a meteorite older than our own planet.
Header image: An artist’s impression of an asteroid near Earth. SHUTTERSTOCK ILLUSTRATION, IMAGE ELEMENTS BY NASA/JPL