Generally speaking, there's a collective sigh of relief when any large hulk of space rock flies past Earth - except, perhaps, for the smaller asteroids that scatter precious meteorites over California's ‘Gold Country.' But when one of the estimated 4,700 potentially hazardous asteroids (PHAs) glides by, we really should count our lucky stars: Those things could kill you.
PHAs are asteroids larger than 100 meters (330 feet) wide that drift close enough to Earth's orbit to be considered a threat. They come within eight million kilometers (five million miles), or around 20 times the Earth-moon distance, of our planet and should they be on a collision course with Earth, they could survive the atmospheric burn and wipe out a city (or worse).
They are an extra-special subset of a larger family of near-Earth asteroids (NEAs), space rocks that give astronomers a reason for pause.
The new estimate of 4,700 (plus or minus 1,500) PHAs comes courtesy of observations by NASA's Wide-field Infrared Survey Explorer (WISE) - specifically, its near-Earth object-hunting mission: NEOWISE. There's no real big surprise with this number - it does, after all, fall in line with most previous estimates - but WISE sampled 107 known PHAs to give a more precise idea of the total population of dangerous asteroids in our neighborhood. But here's the kicker: Only 20-30 percent of these asteroids have been discovered thus far.
"The NEOWISE analysis shows us we've made a good start at finding those objects that truly represent an impact hazard to Earth," said Lindley Johnson, program executive for the Near-Earth Object Observation Program at NASA Headquarters in Washington. "But we've many more to find, and it will take a concerted effort during the next couple of decades to find all of them that could do serious damage or be a mission destination in the future."
The NEOWISE analysis has turned up something interesting about PHAs. Generally, they possess much lower inclination orbits than the wider population of NEAs. This means that they have orbits around the sun more likely to collide with Earth. But there's a flip-side to this doomsday coin; their low inclination orbits make PHAs easier for future robotic and possible manned missions to track, land on and deflect, if necessary.
Another oddity noted by NEOWISE is that PHAs are generally brighter, yet smaller, than the greater NEA population. This information provides clues as to the composition of these particular asteroids. They are most likely composed of granite-like rock or metals. These materials make PHAs hardier and more likely to survive atmospheric entry should they collide with Earth - flip that doomsday coin back over. But once again, there's a plus side (flip it again).
As recently announced by the start-up company Planetary Resources, it is hoped asteroids may be hiding trillions of dollars of precious metals. Asteroid mining could therefore be an industry of the future. Although I have misgivings about such a business venture in the near-term, this NEOWISE data would surely help companies such as Planetary Resources' prospects for high-value asteroid targets that are easily accessible.
How they propose to carry out the expensive business of refining and transporting the materials back to Earth is open to debate, but the technologies they'd need to develop in the pursuit of profit could ultimately be used to safeguard life on Earth. The ability to rendezvous with a PHA is the first step in asteroid deflection (unless we decide to nuke ‘em, instead), so the more information we can gather as to the nature of these dangerous asteroids, the better.
Image: 433 Eros is a large near-Earth asteroid, but what of its smaller, potentially hazardous cousins? Credit: NASA