SAFER backpack test. Astronaut Mark C. Lee testing the simplified aid for EVA rescue (SAFER) during shuttle mission STS-64. He is over 200 kilometres above Earth (bottom). SAFER is a backpack propulsion unit designed to provide emergency free-flying mobility and self-rescue to astronauts during extravehicular activity (EVA). It is a scaled-down version of the manned manoeuvering (MMU) unit used in space walks, with small nitrogen thrusters that are controlled by hand. STS-62 flew on 9-20 September 1994.