How will ESA’s Hera mission find its way through deep space and then navigate around the Didymos binary asteroid system? The spacecraft will navigate itself in three different modes, dependent on its distance from the asteroids. From far away the asteroid pair will appear as one bright point among many, but will gradually move compared to the still background stars. The second mode will see Hera from 30 km to 8 km distance, centring Didymos in its field of view by looking for the contrast between the edge of the roughly-circular asteroid and the cold dark of space. When the spacecraft grows closer than about 8 km then Didymos will fill its field of view. So Hera will switch to identifying surface features such as boulders and craters and tracking their motion to determine the spacecraft’s own relative position.

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