Why is a NASA spacecraft crashing into an asteroid?

tech.hindustantimes.com:

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.

“This is stuff of science-fiction books and really corny episodes of “StarTrek” from when I was a kid, and now it's real,” NASA program scientist Tom Statler said Thursday.

Cameras and telescopes will watch the crash, but it will take days or even weeks to find out if it actually changed the orbit.

The $325 million planetary defense test began with Dart's launch last fall.

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's Applied Physics Laboratory, 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