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Space is an exceedingly random place. Everything in the known universe may be governed by some pretty hard laws of physics, but so are BBs in a jar when you shake them up and down. That doesn’t stop things from getting very chaotic inside. The same extreme arbitrariness is worth keeping in mind as we contemplate our planet’s close brush with an asteroid this week.

(FROM THE ARCHIVES: Asteroids: Whew!)

As TIME reports in this week’s issue (available to subscribers here), astronomers have known for the better part of a year that asteroid 2012 DA14—a medium sized, 150 ft (50 m) rock weighing 143,000 tons—was closing in on us. They knew that it would miss us too, by 17,200 miles (27,700 km). That seems like a big number, but in a solar system measured in billions of miles and a universe measured in billions of light years, it vanishes to inconsequence. The fact is, the odds of our getting clobbered by the rogue rock were in some ways the same as its missing us—at least when you fold into the equation how little it would have taken to change both its course and its impact. So what would those x-factors have been that would have turned a near miss into a true disaster, and what would the nastiness that resulted have looked like?

Let’s start by making 2012 DA14 bigger—though it hardly needs the extra bulk. At its current size, it would produce a blast equivalent to 2.4 megatons, or 180 Hiroshimas, after it entered our atmosphere. A significantly bigger asteroid would produce a significantly bigger blast and there’s no shortage of those cosmic missiles out there. Astronomers estimate there are 2,400 objects in the vicinity of Earth that are at least 0.5 km (0.3 mi) across and 860 of those are a full 1 km (.62 mi.). A 0.5 km rock would produce a 5,000 megaton blast—not to mention a 7.1 Richter-scale shock. Let’s split the difference then, but err on the size of conservatism: Our death rock would be a comparatively modest 100 m, or 330 ft., across.

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The speed of the object would make a difference too. Over the course of the past several months, 2012 DA14 has been clipping along at a brisk 17,450 mph (28,000 k/h), and that’s what packs it so full of energy. Just like a baseball that’s tossed lightly at your chest doesn’t hurt you but a fastball thrown by a pro player would crack your sternum, so too does velocity turn a nuisance rock into a unguided missile. But the speed of 2012 DA14, not to mention its angle of flight, saved our hides too. A difference of a few centimeters per second—faster or slower, depending on when and where the force was applied—would have been enough to shift the asteroid’s trajectory and put us in its crosshairs. A random bump from a smaller bit of space rubble could have done that trick, as could a gravity tug from a larger object the asteroid passed. So could the light of the sun.

Solar energy becomes heat energy when it strikes an asteroid, and that can exert a physical force all its own. A rock with a bright surface reflects more light away and feels less of an effect. A darker surface absorbs more and gets nudged more. And solar flares—which occur unpredictably—can mix things up further, exerting a sudden pressure that compounds the sun’s the slow, steady one.

“Heat can push these bodies around,” says Paul Chodas, a research scientist in the Near-Earth Object (NEO) Program Office at NASA‘s Jet Propulsion Laboratory in Pasadena. “We can determine future trajectories out to about 100 years, but there are uncertainties like this that make it difficult to forecast as reliably for the smaller objects.” Asteroids that have traces of water—and in a cosmos as heavily hydrated as ours that’s not uncommon—can outgas tiny amounts of vapor, which act as small, natural thrusters that perturbs the flight path further. Given all of those x-factors, it’s hardly unreasonable to say that 2012 DA14 might have easily gotten nudged our way.

The first word of the incoming missile would likely come from one of three main observatories NASA has assigned to scan the sky for cosmic ordnance all day, every day. But amateurs and academic observatories around the world could spot it too (it was an amateur who discovered 2012 DA14, in fact). We also might not see it at all. Like fighter planes taking advantage of natural blind spots in the sky, asteroids can approach from the direction of the sun, making them impossible to see until it’s way too late. “The smaller ones aren’t even visible until they’re on the way back out,” says Don Yeomans, head of the NEO office.

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But imagine we saw our 100-meter asteroid when it was, say, eight months away. NASA would first send out word to the rest of the world’s astronomers, who would begin training their telescopes on it too, something they do already even for benign rocks. They measure light output, rotation rate and trajectory, conducting their own plight-path calculations to help confirm what the NASA software is spitting out. Radio-telescopes at Goldstone Observatory in the Mojave Desert and the Aricebo Observatory in Puerto Rico would swing into position as well, bouncing signals off the rock to determine distance, velocity and doppler shift—or the stretching or compressing of the signals reflecting back to determine the degree to which the object is moving toward or away from the observer.

“There is a small army of worldwide professionals and amateurs doing this,” says Yeomans.

The good thing about so much crowd-sourcing is that when hundreds or thousands of astronomers reach a conclusion, you can be pretty sure it’s a solid one. The bad thing is that when that conclusion is one you don’t like, there’s no court of appeal. In this case, let’s imagine that the conclusion is a truly bad one—that our football-field sized rock would be targeting a populated area. And for the sake of doomsday pizzaz, let’s make that populated area New York City.

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The five boroughs that comprise the city have a total area of 469 sq. mi., including 164 sq. mi. of water, and are home to 8.3 million people. A rock barreling in from space would likely be made of some kind silicate—the overwhelming majority of asteroids are—which means it wouldn’t survive the blazing heat of entry intact and instead would explode in the sky. Trajectory analysts at NASA and elsewhere would be able to forecast the exact date, time, place and angle of entry, so imagine Central Park, at high noon, on a work day, at an angle of entry that maximizes the possible explosion.

As it happens, we have a very good idea of how things would unfold in a situation like that since an extremely similar scenario played out before, on June 30, 1908, near the Tunguska River in Central Russia. At 7:14 that morning, a massive blast from what is calculated to have been a 100-meter asteroid occurred somewhere from 3 to 6 mi. (5 to 10 km) above the surface. The region was heavily forested and lightly populated—which was a very good thing—but the devastation was nonetheless stunning. Roughly 80 million trees were leveled or incinerated in a footprint of destruction extending 830 sq. mi. (2,150 sq. km). The energy released by the blast is estimated to have been 30 megatons—or 1,000 Hiroshimas.

An 830 sq. mi blast ring has a radius of 14.4 mi. (23.2 km). Position that over New York City and you’d have destruction reaching deep into Queens in the east and Staten Island in the South; west to Paterson and Montclair, NJ; and north to Yonkers and New Rochelle, NY. Manhattan, the Bronx and Brooklyn would be swallowed whole. Evacuation in advance of the blast would be a massive challenge, since the array of bridges and tunnels that connect the boroughs are natural choke points. The many months of notice the residents would have before the big day arrived would make things a bit easier, but fleeing from an asteroid is very different from fleeing from other kinds of disasters. People evacuating in advance of, say, a hurricane can usually just load up their cars and go, since even after a superstorm like Katrina, most of them will simply be turning around and coming home. After a Tunguska-like blast, most people would not have any home left at all.

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As with a nuclear blast, the devastation would be greatest at the epicenter of the event and fade the farther away you moved, and while there would be no radiation to contend with, the immediate destruction would be pretty much the same. In a 6.5 mi. radius, all that would be left of most buildings would be the foundations, though some sturdier, reinforced structures like stocky old banks might survive. Out to 11 or so miles, multi-story buildings would be skeletonized—their curtain walls stripped away and only their frameworks left standing. Small, individual family homes would be destroyed completely. It would not be until about 20 miles away that most tall buildings would survive—windowless, to be sure—and some single-family dwellings would too. The economic damage—nationally and globally—would be incalculable.

Tunguska-type hits are rare—even smaller asteroids like 2012 DA14 strike the planet only once every 1,200 years or so. Bigger objects—on the order of 2 km (1.25 mi.)—hit only every 100,000 years or so. The stakes being what they are, though, that’s little comfort. When you’re dealing with something like asteroids, all it takes is one. That’s something the dinosaurs of 65 million years ago could tell you—if an asteroid hadn’t killed them all, of course.

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