A small asteroid passed extremely close to the Earth on January 9, missing us by only 192,500 kilometers (119,500 miles), roughly half the distance between the Earth and the Moon. Because of its small size, the asteroid, called 2017 AG13, wasn’t spotted until two days before its approach.
Thankfully, 2017 AG13 isn’t a civilization-ending apocalypse rock, although it had the potential to cause a fair amount of damage: while it is moving fairly quickly in relation to us, at 15.7 kilometers (9.8 miles) per second — double the speed of the International Space Station — it is rather small, being between 16 and 37 meters (53 to 121 feet) across, and is being described as being similar in size to the meteor that exploded over Chelyabinsk, Russia, in 2013.
In regards to addressing the issue of potential asteroid impacts, NASA’s OSIRIS-REx space probe, launched last September, is due to begin accelerating this month, to keep its rendezvous with the asteroid Bennu in August of 2018. Bennu is a 500-meter (1,640 foot) asteroid that has a fairly low probability of hitting Earth, but its dark surface is a cause for concern for scientists, and it is this aspect of Bennu that OSIRIS-REx is being sent to investigate.
While the orbits of asteroids can be plotted accurately, ones with dark surfaces like Bennu are subject to an effect called the Yarkovksy effect: in sunlight, the dark surface can heat up dramatically, but when the asteroid rotates and the heated side turns to face away from the sun, it cools off. The resulting temperature difference between the two sides of the asteroid can create a subtle imbalance in the object’s orbit, an imbalance that can reduce the accuracy of our plotting of its orbit.
And these orbital projections are key to dealing with a potential impact hazard: regardless of how we plan to deal with it — attaching a rocket motor, nudging it with a nuke, or spray-painting it to alter its Yarkovksy effect — a spacecraft would have to be designed, built and sent on a possibly years-long journey to intercept the asteroid, meaning accurate orbital calculations would be required, hence the importance of OSIRIS-REx’s mission.
Increased awareness of the potential danger that asteroids present has also increased funding for asteroid spotting and surveying programs. "There was a time where we didn’t have a program to look for objects, and it was done privately. You had guys like [astronomer Eugene] Shoemaker driving out every month to Palomar Observatory to look for them," explains Southwest Research Institute astronomer Bill Bottke. "Now we have $50 million annually to look for them. Now we’re getting serious science missions to look at these."