What did NASA's DART mission show when it crashed into an asteroid?

DART showed that humans can change the motion of an asteroid: in September 2022 NASA's Double Asteroid Redirection Test hit Dimorphos, a 160-metre moonlet of the asteroid Didymos, and shortened its orbit around Didymos by 32 minutes. The success threshold had been only 73 seconds. Neither asteroid poses an impact threat to Earth; they were chosen as a safe, well-suited target.

The surprise was where the push came from. The deflection resulted from debris thrown off by the impact, whose recoil was substantially larger than the impact itself. The momentum change was amplified by a factor of 2.2 to 4.9, depending on the mass of Dimorphos. That means a smaller impactor or shorter lead times could produce a given deflection than previously expected.

The test was watched by many eyes. An Italian CubeSat called LICIACube, which separated 15 days before impact, photographed the aftermath, and the Hubble and James Webb space telescopes and the ATLAS observatory detected the plume of ejecta. The final pictures showed an egg-shaped body covered with boulders, which suggests a rubble pile.

It also slightly changed the orbit of the pair around the Sun, the first time human action measurably did that to a celestial body. The change is tiny, but a small change in velocity accumulates into a large change in position over years, so it could help only if applied early enough.