Be afraid – but not too afraid. It’s true that at some point, if we don’t take steps to prevent it, a big dumb object is going to smash into the Earth so hard that civilization as we know it will be seriously inconvenienced, or even wiped out (see? No reason for too much fear). But the odds are that such a cataclysm lies way, way in the future.
Still, you never know…
The chunks of cosmic debris we need to keep a keen eye on are so-called near-Earth objects (NEOs) – asteroids and (a few) comets whose orbits can sometimes bring them close to the Earth. Around 9,000 NEOs have been discovered to date, the vast majority of them asteroids. Of these, about 1,350 are considered potentially hazardous asteroids (PHAs).
A PHA is defined as an asteroid that can approach the Earth closer than 0.05 astronomical unit (7.5 million kilometers) and has a diameter of at least 100 to 150 meters. As one might imagine, an object of that size is big enough to cause a lot of damage. The latest estimate, based on a survey known as NEOWISE, was carried out using NASA’s WISE (Wide-field Infrared Survey Explorer) satellite. To date, it is estimated that we’ve found between 20 and 30 percent of all these threatening objects.
The good news is that, because of monitoring programs like NEOWISE and the ground-based Lincoln Near-Earth Asteroid Research (LINEAR) project and Catalina Sky Survey, we’re not going to be caught unawares by anything really big. Nothing as large as the 10-kilometer-wide rocky missile that was, in part, responsible for wiping out the dinosaurs and vast swathes of other life some 65 million years ago is going to hit us without many decades warning.
The bad news is that, thanks to all the extra scrutiny, we’ve become aware of just how much stuff there is out there that poses a danger to us. So-called extinction-level impacts may be very rare, but even an object as small as 100 meters across could cause regional devastation if it struck on land, and huge tsunami if it came down in the ocean. And the fact is, there could be many objects of this size or larger, which come uncomfortably close to the Earth, that still lurk undiscovered.
To measure the threat level of NEOs, astronomers have devised two main scales. The simpler one, known as the Torino scale, goes from 0 (no threat) to 10 (certain impact causing global climatic change and threatening the future, or very existence, of civilization). Only two asteroids at present have non-zero Torino ratings. These are 2007 VK184, which has a 1 in 1820 chance of hitting us in June 2048, and 2011 AG5, which is currently given a 1 in 500 chance of impact in February 2040. Both objects are in the 130 to 140-meter size range.
The highest Torino ranking ever (4) was briefly ascribed to the 350-meter-wide Apophis, and triggered some tabloid scare stories. But improved orbit calculations have seen Apophis’ Torino score drop to 0.
The next step beyond detection, argue some scientists and engineers, is to put schemes in place to divert any asteroid that looks like it might be on a collision course. In the meantime? Sleep tight – and watch the skies…