In terms of size, Didymoon is about as big as the Great Pyramid in Giza, making it the smallest asteroid to ever be visited by a probe. However, if an asteroid that big were to rocket into the Earth at a typical speed of nearly 19 miles per second, it could potentially wreck an entire region of the globe, according to the the ESA. Of course, knocking a truly mammoth Earth-threatening asteroid off course -- one that's thousands of times the size of Didymoon -- would require some serious power. But, hey, doing the test is the first step.
As for when this is all going to happen, NASA plans to launch its probe between 2020 and 2021, and expects it to smack into Didymoon in October 2022. After that, an ESA craft known as Hera will launch toward Didymos and Didymoon and investigate the crater created by DART a few years later to determine its "momentum transfer" and precisely how effective the deflection method worked. NASA unveiled its latest DART mission plans in 2018, but the ESA just announced it will be teaming up to do the critical follow-up research.