NASA will provide the collider, the Double Asteroid Impact Test (DART). It should launch in summer 2021 and will smack into the smaller of the two Didymos asteroids at about 14,764MPH. An Italian cubesat, LICIACube, will study the moment of impact. After that, the ESA will launch a Hera probe in October 2024 to study the target asteroid, including the impact crater, mass and a radar probe (the first ever for an asteroid). Hera will take roughly two years to arrive.

Scientists chose the asteroid pair precisely because it was easy to test. The smaller rock orbits slowly enough that it should be realistic to change its orbit in a noticeable way. That wouldn't be an option with an asteroid flying solo in a solar orbit, the ESA said.

Of course, there's a big difference between a test like this and a true crisis. While the target asteroid could do serious damage at about 525 feet across, it's still small and slow. A future deflection system may have to knock far larger and faster asteroids out of their collision courses. This is an important step toward a proper defense system, though, and might be enough to protect humanity against many potential impacts.