If you’re the type to worry, don’t click here — especially if the type of worrying you do is about things that sound really, really scary and that you have absolutely no control over. Clicking on that site you really shouldn’t click on will take you to a page on NASA’s Near-Earth Object Program website, which lists every comet or asteroid of any size at all that will be passing through Earth’s orbital neighborhood in the upcoming days, months and years. The fact is, there are a whole lot more of them than you likely know: from Feb. 5 to May 5 of this year, no fewer than 77 space rocks that could, in theory have Earth’s name on them, will be whizzing by. On March 20 alone, when you may have been planning to celebrate the first day of spring, there will be seven.
Errant space rocks, of course, can do a lot of damage. Our moon is believed to have been created when a Mars-size planetesimal sideswiped the Earth 4.5 billion years ago. The dinosaurs were all but certainly wiped out by a direct hit by a far smaller projectile 65 million years ago. As recently as June 30, 1908, an asteroid measuring up to 330 ft. (100 m) across famously exploded in the skies over the Tunguska region in Siberia, unleashing a blast with the equivalent of 30 megatons of TNT and destroying 770 sq. mi. (2,000 sq km) of forest.
What has a lot of cosmic worriers glancing skyward this month is the announcement that on Feb. 15, a 148-ft.-long (45 m) asteroid known as 2012 DA 14 will pass just 17,200 miles (27,7000 km) above the Earth. And if 17,200 miles sounds like a lot, consider that it’s only one-thirteenth of the distance to the moon and actually below the 22,000 miles (35,800 km) altitude at which some of our satellites orbit. That leaves awfully little margin for error in NASA’s cosmic calculations. So there’s plenty of reason to worry, yes? Well, no, actually. But making that call for any one object — knowing which space bullets are likely to hit us and which ones we’re likely to dodge — can be a complicated business.
Near-Earth objects (NEOs) are very particular things. They’re bodies that at some point in their orbit through the solar system dip within 1.3 AUs (astronomical units) from the sun. A single AU is the sun-Earth distance, or 93 million miles (150 million km), so 1.3 AU would be 121 million miles. There are untold thousands of NEOs at large — 9,668 of which have so far been counted and cataloged by NASA and other space agencies and observatories around the world. Most are comparatively small, but 861 have a diameter of at least 1 km (.62 miles), which could pack quite a wallop.
Still, just being within 1.3 AUs is only part of the danger equation. Some NEOs circle the sun either entirely within Earth’s orbit or entirely outside it, and while they may pass close by, they pose little hazard, since a collision is likely only when the orbits of two bodies cross. Others do cross Earth’s orbit with varying degrees of proximity. Of all the known objects, 1,376 are classified Potentially Hazardous Asteroids, based both on their size and on an approach distance that brings them within .05 AU, or 4.65 million miles (7.5 million km), of us.
The amount of damage an asteroid could do if it clobbered us is determined largely by its mass. Anything that’s up to 40 m (131 ft.) across or, as NASA puts it on its website, “smaller than a modest office building,” would be incinerated by the atmosphere before it hit the ground. At most, its remains would produce a blast equivalent to three megatons — very bad news for anyone in the vicinity but not the kind of Earth-cracking disaster they make bad action movies out of. Asteroids from 40 m to 1 km could do “tremendous damage on a local scale,” NASA says. A hit by a 2-km or larger asteroid would cause a million-megaton blast and “produce severe environmental damage on a global scale. The probable consequence would be an ‘impact winter’ with loss of crops worldwide and subsequent starvation and disease.”
The overall risk any one asteroid poses is calculated on what’s known as the Torino Scale, a grid with the probability of impact — from effectively zero to effectively 100% — on its x axis and the size of the object on the y. After objects are given a Torino score, they are ranked on a five-color chart — think of the department of Homeland Security’s now defunct terrorism-threat level — going from white (no hazard) through yellow (“meeting attention of astronomers”) to red (certain collision, capable of causing at least regional devastation of a kind seen only once every 10,000 to 100,000 years). And what is the Torino rating for 2012 DA 14, which will whiz by on Feb. 15? A reassuring white — or no hazard at all.
(MORE: The Great, Wet Cosmic Rock)
None of this means that the bright red rocks aren’t out there, and the world’s space agencies have a lot of smart minds working on ways to deflect or destroy them. Advanced tracking and trajectory-modeling computers allow us to know sometimes centuries in advance just where any object will be at any time. And NASA has already proved itself adept at rendezvousing with asteroids: the Dawn probe is in the midst of a multiyear asteroid pas de deux, having orbited Vesta — the second largest object in our solar system’s asteroid belt — from 2011 to 2012, then peeled off to visit Ceres, the largest, with an arrival date set for 2015. In 2001, the NEAR Shoemaker spacecraft actually touched down on the asteroid Eros.
There will be no such attention paid to 2012 DA 14 when it passes, though stargazers with telescopes in Australia, Eastern Europe, Asia and especially Indonesia will be able to see it, moving south to north at an apparent speed of 1 degree per minute. The little sky show belies the dangers 2012 DA 14’s bigger, deadlier cousins pose, something that keeps NASA scientists lying awake at night — partly so that you don’t have to.