What astronaut propulsion backpack enabled untethered shuttle spacewalks in 1984?

The Manned Maneuvering Unit enabled untethered shuttle spacewalks in 1984.

The MMU was a self-contained, astronaut-controlled propulsion backpack designed to let spacewalkers move independently of the shuttle. It used nitrogen gas thrusters operated through hand controllers, allowing an astronaut to translate, rotate, fly away from the orbiter, and return without a safety tether.

NASA first tested the MMU operationally during mission 41-B, launched by Challenger on February 3, 1984. Bruce McCandless II and Robert L. Stewart conducted demonstration flights in the payload bay and nearby space. The system was developed for tasks such as satellite servicing and retrieval, where a tether could restrict an astronaut’s reach.

The MMU is often confused with the Extravehicular Mobility Unit, which was the pressurized spacesuit worn during shuttle spacewalks. The Orbital Maneuvering System moved the entire shuttle, not an individual astronaut. SAFER, introduced later, was a smaller emergency self-rescue unit worn by spacewalkers rather than the large operational backpack used in 1984.