ESA’s Hera asteroid mission for planetary defence will journey to the distant Didymos asteroid system with two shoebox-sized ‘CubeSats’. CubeSats are small, low-cost satellites built up from 10 cm boxes, traditionally employed for educational purposes but increasingly finding operational uses in Earth orbit. The Milani mineral prospector and Juventas radar surveyor will be the first ESA CubeSats to operate in deep space. They will spend Hera’s two-year cruise phase to Didymos inside a pair of ‘Deep Space Deployers’ which will keep them alive and healthy. The pair will be deployed one at a time, starting with a partial deployment to check each CubeSat’s functionality. Finally, they will be deployed at just a few centimetres per second – any faster and they would risk being lost in space within the prevailing low gravity.