Hurtling asteroids are in the news again. According to CNET, 2006 QV89, a lump of rock half the size of a football field, has a slight chance (one in 7,000) of hitting Earth this fall. Meanwhile, KQED reports that a larger asteroid named Apophis will make a very close pass in 2029, coming closer than the television and telephone satellites that orbit the Earth.
Planetary Defense: Not Just Science Fiction
Luckily, the chances of Apophis slamming directly into us have been reduced from an earlier estimate of 2.7% to “practically nil.” Even if Apophis misses us, there is a chance of an asteroid eventually hitting Earth. A really large asteroid could wipe us out, just as one wiped out the dinosaurs millions of years ago, says KQED.
Even an asteroid a mere thousand feet across — about the size of Apophis, notes KQED — could hit Earth with 1,000 times the energy of the atomic bomb dropped on Hiroshima, destroying any city in its path and devastating an entire region, says EarthSky.org.
In the golden age of science fiction, planetary defense usually meant guarding Earth against malicious aliens or the occasional fleet of rebel space colonists. Now, according to NASA‘s planetary defense web pages, it means guarding Earth against boulders and mountains left over from the creation of the planets.
Eyes on the Skies
Foresight is essential in preventing an asteroid hitting Earth. Until recently, there was no systematic effort to track asteroids that pass near Earth. According to KQED, an Apophis-sized asteroid passed us closer than the moon in 2001 — but one knew it until 2017, when it was tracked, and analysis of its orbit revealed the earlier near miss.
Now, as the NASA planetary defense FAQ reports, the detection and tracking elements of defense are coming together, making it likely that we will know of any collision threat years in advance.
How to Shoot Down an Asteroid
When we know an asteroid’s path in advance, we can deflect it. The goal of deflection, says Space.com, is to “nudge” the asteroid into a slightly different orbit, so that it misses the Earth instead of slamming into us. A very slight nudge is sufficient if done early enough before the predicted impact. A velocity change of one meter per second adds up to roughly 20,000 miles in a year — five times Earth’s radius, or the difference between impact and a clean miss. (Orbital dynamics can complicate this picture, but small changes in speed still add up over time.)
So, how do you nudge an asteroid? Hitting it with a nuclear bomb is a Hollywood favorite, but it might break it apart if hit it too hard. Then you just have several smaller asteroids to contend with — one or more of which may still be on a collision course with Earth.
For this reason, the favored technique is a kinetic impactor, a spacecraft that hits the asteroid hard enough to move it into a slightly different orbit, but not so hard as to break it into fragments. NASA is already in the planning stages of a mission to test the idea on an asteroid called Didymos B, due to pass conveniently (but not dangerously) close to Earth in 2022 and 2024.
Alas, for Hollywood, a kinetic impactor requires neither a nuclear warhead nor astronauts, since the impactor will hit the asteroid at several miles per second. Planetary defense will, therefore, be strictly a robotic mission.