In the first-of-its kind, save-the-world experiment, NASA is about to clobber a small, harmless asteroid millions of miles away. A spacecraft named Dart will zero in on the asteroid Monday, intent on slamming it head-on at 14,000 mph (22,500 kph). The impact should be just enough to nudge the asteroid into a slightly tighter orbit around its companion space rock — demonstrating that if a killer asteroid ever heads our way, we’d stand a fighting chance of diverting it. Cameras and telescopes will watch the crash, but it will take months to find out if it actually changed the orbit. The $325 million planetary defense test began with Dart’s launch last fall. ASTEROID TARGET The asteroid with the bull’s-eye on it is Dimorphos, about 7 million miles (9.6 million kilometers) from Earth. It is actually the puny sidekick of a 2,500-foot (780-meter) asteroid named Didymos, Greek for twin. Discovered in 1996, Didymos is spinning so fast that scientists believe it flung off material that eventually formed a moonlet. Dimorphos — roughly 525 feet (160 meters) across — orbits its parent body at a distance of less than a mile (1.2 kilometers). “This really is about asteroid deflection, not disruption,” said Nancy Chabot, a planetary scientist and mission team leader at Johns Hopkins University, which is managing the effort. “This isn’t going to blow up the asteroid. It isn’t going to put it into lots of pieces.” Rather, the impact will dig out a crater tens of yards (meters) in size and hurl some 2 million pounds (1 million kilograms) of rocks and dirt into space. NASA insists there’s a zero chance either asteroid will threaten Earth — now or in the future. That’s why the pair was picked. DART, THE IMPACTOR Johns Hopkins took a minimalist approach in developing Dart — short for Double Asteroid Redirection Test — given that it’s essentially a battering ram and faces sure destruction. It has a single instrument: a camera used for navigating, targeting and chronicling the final action. Believed to be essentially a rubble pile, Dimorphos will emerge as a point of light an hour before impact, looming larger and larger in the camera images beamed back to Earth. Managers are confident Dart won’t smash into the larger Didymos by mistake. The spacecraft’s navigation is designed to distinguish between the two asteroids and, in the final 50 minutes, target the smaller one. The size of a small vending machine at 1,260 pounds (570 kilograms), the spacecraft will slam into roughly 11 billion pounds (5 billion kilograms) of asteroid. “Sometimes we describe it as running a golf cart into a Great Pyramid,” said Chabot. Unless Dart misses — NASA puts the odds of that happening at less than 10% — it will be the end of the road for Dart. If it goes screaming past both space rocks, it will encounter them again in a couple years for Take 2. SAVING EARTH Little Dimorphos completes a lap around big Didymos every 11 hours and 55 minutes. The impact by Dart should shave about 10 minutes off that. Although the strike itself should be immediately apparent, it will take months to verify the moonlet’s tweaked orbit. Cameras on Dart and a mini tagalong satellite will capture the collision up close. Telescopes on all seven continents, […]

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