SAFER backpack test. Astronaut Mark Lee seen testing the Simplified Aid for EVA Rescue (SAFER) system during Shuttle Mission STS-64. Behind Lee, 240 kilometres below, is a large bank of clouds over the Earth. SAFER is a scaled-down version of the Manned Manoeuvring Unit (MMU) used in previous Shuttle space walks, and is intended to provide emergency free-flying mobility or a self-rescue capability to astronauts during extravehicular activity (EVA). SAFER clips around the normal life-support backpack, and is powered by 24 fixed nitrogen thrusters. The nitrogen supply may be replenished during an EVA from the payload bay. STS-64 flew on 9-20 September 1994.