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What if the Sky Is Falling? One doesn’t have to be paranoid to be- lieve the sky is falling: asteroids and comets do rain down on Earth. These rocky or icy chunks usually orbit the sun between Mars and Jupiter or beyond Neptune, but gravity and collisions occasionally redirect them inward, sometimes on a collision course with Earth. What happens next depends on the size of the asteroids. Some are too small to survive the passage through the atmo- sphere, where they burn up. Larger objects, however, do sometimes crash into Earth and create craters or worse—and they don’t need to be terribly large to be devastating. The rock that likely exploded over Siberia in 1908 and knocked down trees for hundreds of miles in all directions from its shockwave alone, equal to 10–15 megatons (million tons) of TNT, is thought to have been less than 60 meters wide. An object only a few times that size could not only cause tremen- dous regional damage on land but could also spawn deadly tsunamis from an impact at sea—potentially reaching shorelines far 22 from the impact. Moreover, the vapor and debris produced by a substantially larger collision could block out the Sun, resulting in massive climate change and numerous extinctions. Near-Earth objects (NEOs) are asteroids and comets in orbits that allow them to enter Earth’s neighborhood as they orbit the Sun. NEOs whiz through Earth’s vicinity on a regular basis, leading the U.S. government to launch a neighborhood watch. NASA de- tects, tracks, and characterizes NEOs using ground- and space-based telescopes. Ac- cording to NASA, 8,871 near-Earth objects have been discovered, of which 1,300 have been classified as potentially hazardous. If the ship-sized asteroid that passed by Earth last year (closer than the Moon) had slightly altered course, its impact would have been equivalent to 10 million of the earliest atomic bombs. This threat has led some to consider using a nuclear energy source to disrupt an NEO in space before it can unleash that level of destruction down here. Los Alamos astrophysicist Robert Weav- er has already demonstrated this is pos- sible. To simulate the phenomenon in 3-D, 1663 los alamos science and technology magazine june 2012 How could the world be saved from a killer asteroid? Weaver ran a detailed simulation on Cielo, one of the Lab’s supercomputers, to reveal how a potentially hazardous object could be disrupted. In January, Weaver released a video (available online at com/user/LosAlamosNationalLab) revealing how a one-megaton nuclear energy source might affect a half-kilometer-long asteroid capable of destroying a continent. The simulation shows how a single explosion directed at the asteroid would send a shock wave through it, impacting the individual granite chunks comprising it and shatter- ing the entire object. Weaver pronounced the hazard “fully mitigated.” Score one for happy humans, zero for killer asteroid. “Prior to my calculations, it was merely speculation that a nuclear source might be a good option to deter an asteroid. We provided the physics and hydrodynam- ics—definitive scientific results about how a nuclear burst could do the job,” says Weaver. The simulation revealed that the high velocities of the asteroid debris frag- ments prevented them from reassembling afterward and, in fact, caused them to disperse well beyond their original Earth- crossing trajectory. “It’s highly unlikely for them to pose a secondary threat.” Nonnuclear alternatives have been explored previously, including conventional explosives and some other creative ways to steer an Earth-crossing object off course, but according to a NASA study delivered to Congress in 2007, a nuclear device remains the most effective option. Nonetheless,