It’s a question of when, not if, a large asteroid or comet will be on course to collide with Earth and cause calamitous destruction. For almost two decades, NASA has monitored and measured these potentially hazardous objects and debated ways to mitigate the threat.

So much for watchful waiting.

In June, the space agency announced it will build scientists’ best idea for defense: a small spaceship they will deliberately smash into an asteroid at very high speed in order to bump the object into a new trajectory. Called a kinetic impactor, the Double Asteroid Redirection Test, or DART, is an international collaboration among NASA, the European Space Agency, Observatoire de la Cote d’Azur and the Johns Hopkins Applied Physics Laboratory.

If all goes as planned, DART will hurtle toward a non-threating near-Earth asteroid called Didymos – Greek for “twin” – an asteroid binary system that consists of two bodies. Didymos A is about one-half mile in size; Didymos B, just 530 feet in size, orbits its twin.

Sometime during the asteroid’s close pass by Earth in 2022, DART will strike the smaller of the two bodies, Didymos B. Scientists hope that the impact will change the speed and direction of the asteroid just a bit. A separate spacecraft observatory, the ESA-developed AIM, will perform the first-ever close-up examination of a binary asteroid, delivering high-resolution images and measurements. AIM and telescopes and radar on Earth will measure any change in the asteroid’s orbit around its twin.

If DART does the job, scientists figure they can develop a kinetic impactor to defend against an actual asteroid on a collision course with earth, making a slight change in its trajectory early enough to redirect it harmlessly off into space.

Watch an animated depiction of the refrigerator-sized DART, which will smash into Didymos 2 at a speed nine times faster than a bullet, approximately 3.7 miles per second:

“A binary asteroid is the perfect natural laboratory for this test,” said Tom Statler, program scientist for DART at NASA headquarters. “The fact that Didymos B is in orbit around Didymos A makes it easier to see the results of the impact, and ensures that the experiment doesn’t change the orbit of the pair around the sun.”

Small asteroids hit Earth regularly, but break apart in the upper atmosphere. Objects large enough to do damage on the surface are rare.

NASA’s search for potentially hazardous objects has focused on objects bigger than .6 miles in diameter, sufficiently large to do cause global damage, with orbits that bring them near Earth. NASA estimates that it has identified and is tracking 93 percent of threatening objects of this size.

DART will test technology for deflecting objects in the intermediate size range. These are large enough to do regional damage here on Earth and there are many more that have not been observed and could someday hit earth.

A large object that unexpectedly entered the atmosphere in 2013 in Chelyabinsk, Russia, injured more than 1,500 people and caused extensive damage.

To address and develop technologies to counter the threat, NASA established its Planetary Defense Coordination Office in 2016 to find, track and characterize potentially hazardous asteroids and comets, and issue warnings about possible impacts.

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