Testing the Hera asteroid mission’s autonomous vision-based navigation at GMV in Spain. The spacecraft takes advantage of the roughly-spherical shape of the main Didymos asteroid to fit it within a circle and estimate the line-of-sight distance between the spacecraft and the asteroid ‘centroid’. Once it moves less than 8 km from Didymain, the asteroid fills its field of view and this navigation mode is unavailable. Then comes the most ambitious navigation mode of all, based on autonomous feature tracking with no absolute reference.This involves imaging the same surface features – such as boulders and craters – in different pictures to gain a sense of how Hera is moving with respect to the surface, combined in turn with other information including onboard accelerometers for dead reckoning and the thermal infrared camera for overflying the asteroid’s night side.The same technique can also be used for navigating around the smaller Didymos moonlet asteroid.