A huge city sized meteor slams into the earth's atmosphere creating shock waves. Mass extinction ... [+]
NASA, the European Space Agency (ESA) and others have for decades aimed to discover and catalog all the near-Earth asteroids (NEAs) that pose an existential threat to humans (and other species).
Recent data from ESA shows this goal has nearly been achieved with over 95 percent of the expected number of so-called “planet-killer” asteroids more than a kilometer in diameter now known. But it’s a very different story for smaller and much more abundant space rocks that might not wipe out everything, but could still level a major city in a direct impact scenario.
“Many potential city-killer asteroids (~100 m) are still undiscovered in the NEA population,” Seth Jacobson, a professor of planetary science at Michigan State University, told me via email.
Astronomers expect there is something like 30,000 NEAs between 100 and 300 meters in diameter that could fall into this category. So far, about 7,000 asteroids this size have been found near Earth, meaning that over 75 percent have yet to be discovered as they orbit the sun, perhaps passing in and out of our planet’s path.
ESA also estimates that an additional over 600, even larger NEAs between 300 and 1,000 meters lurk undiscovered.
But the city killers and planet killers are just the biggest near Earth asteroids out there. They are far from the majority.
ESA’s latest figures suggest there could be a million nearby asteroids between 30 and 100 meters in diameter, and over 98 percent of them remain undiscovered.
We seem to be impacted by one of these asteroids about once a century. Most recently, a bolide exploded over Russia in 2013, blowing out thousands of windows and injuring hundreds.
A far more violent impact and corresponding airburst is believed to have leveled a massive section of Siberian forest near the Tunguska River in 1908. The area is extremely remote, but it’s thought it may have contributed to three deaths.
There’s also been some research recently that suggests our modern era has been one of relative quiet when it comes to NEA impacts, and that we should expect more such impacts as frequently as once every few decades.
More Eyes On The Sky
The Chelyabinsk bolide is thought to have been firmly in the 30 to 100 meter range — perhaps not big enough to destroy a large city, but a direct hit on an urban area could certainly do some damage and cause causalities.
So the key question you might be asking: Was the Chelyabinsk meteoroid among those that had already been discovered?
The answer is an emphatic no. In fact, it went completely undetected until the moment it was exploding and breaking up in the atmosphere over central Russia. Further, it came from the direction of the sun, which is a major blind spot in our NEA detection infrastructure at the moment.
A rendering of NASA's NEO Surveyor spacecraft.
NASA seeks to address this glaring hole in our approach to planetary protection by launching a spacecraft called NEO Surveyor in the later part of the decade. The mission has the goal of eliminating the risk from all unknown asteroids wider than 140 meters and most asteroids in the 100 meter range.
“Subsequent mission extensions or another spacecraft would be needed to create a complete picture of all NEAs down to the Chelyabinsk size range,” Jacobson explained.
Meanwhile, we have to take solace in statistics. Large asteroid impacts seem to occur roughly every century, or perhaps once a generation in the worst-case estimate. Fortunately, the vast majority of our planet’s surface is taken up by either ocean or wilderness. So, statistically, the risk of casualties from a medium-sized NEA remains quite low.
But it would sure be nice to have a better idea of everything that’s flying around our planet right now. I’m sure the dinosaurs would have agreed.
