On 26 September 2022 humans made our first measurable change to a Solar System object, as NASA’s DART spacecraft impacted the Dimorphos asteroid, shifting its orbit around its larger Didymos parent asteroid.

This grand experiment was a success, but many unknowns remain. What is the precise mass and structure of Dimorphos? How did the impact change the Great-Pyramid-sized asteroid? Has it been left with a giant crater, sent spinning wildly or perhaps reshaped entirely? Finding out will help make the ‘kinetic impactor’ method of planetary defence into a well-understood technique that could be deployed reliably if we ever need it for real.

That’s why ESA’s Hera mission is on its way to Dimorphos to perform a close-up crash scene investigation. Arriving in autumn 2026, the suite of instruments aboard the spacecraft’s topside ‘Asteroid deck’ will perform a detailed survey. Then Hera will release a pair of shoebox-sized ‘CubeSats’ to fly even closer and eventually attempt to land.