Astronaut propulsion unit

[2] NASA chief astronaut Deke Slayton later speculated in his autobiography that the AMU may have been developed for the MOL program because the Air Force "thought they might have the chance to inspect somebody else's satellites.

The Skylab AMU was the closest to the Shuttle MMU, but was not used outside the spacecraft because the EVAs were conducted with the astronauts attached to life support umbilicals, and to prevent damage to the delicate solar arrays on the Apollo Telescope Mount.

The Manned Maneuvering Unit (MMU) is a propulsion backpack which was used by NASA astronauts on three space shuttle missions in 1984.

The SPK (or UMK, UPMK[6]) was larger than the Space Shuttle MMU, contained oxygen instead of nitrogen and was attached to a safety tether.

It was automatically stabilized, used 6 degrees of freedom, weighed less than 180 kg, had a delta-v of 30 m/s, practical speed of 1 m/s, and an emergency mode that allows for rotational acceleration of 8°/s^2.

[7] The Simplified Aid For EVA Rescue (SAFER) is a smaller backpack propulsion system intended as a safety device during space walks.

However SAFER is less complex, less expensive and simpler to use than the MMU, and the limited delta-v is sufficient for the intended rescue-only task.