NASA will provide the collision, the dual asteroid impact test (DART). It is due to launch in the summer of 2021 and will enter the smaller of the two Didymos asteroids by about 14.764MPH. The Italian LICIACube cubicle will study the moment of impact. Then, in October 2024, ESA will launch a Hera probe to investigate the target asteroid, including the impact crater, the mass, and the radar probe (the first for an asteroid).
Scientists chose the asteroid pair precisely because it was easy to test. The smaller scale orbits slowly enough to make it realistic to change its orbit in a noticeable way. This would not be an option with an asteroid flying solo in solar orbit, ESA said.
Of course, there is a big difference between a test like this and a real crisis. Although the target asteroid can do serious damage at about 525 feet, it is still small and slow. A future deflection system may have to kill far larger and faster asteroids than its colliding courses. This is an important step towards a proper defense system and may be sufficient to protect humanity from many potential impacts.