One of the most sensational photos in the history of space travel shows astronaut Bruce McCandless, completely detached in his space suit, floating above the blue planet – 98 meters away from Space Shuttle Challenger and 300 kilometers above Earth. This apparently limitless freedom was made possible by the Manned Maneuvering Unit (MMU) – a kind of space backpack that turned the astronaut suit into a spacecraft.
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As early as the 1960s, NASA and the US military were working on the first concepts to give astronauts their own freedom of movement outside the spacecraft using a rocket backpack. The Astronaut Maneuvering Unit (AMU) was one of the precursor projects to the MMU. In Gemini 9 it should be tested in the field. The test had to be aborted due to various problems during donning.
With the Skylab space station, however, internal tests were successful. Bruce McCandless was also involved in the further development of the MMU. He joined the astronaut corps in 1966 and was placed in the fifth group of astronauts, the first astronaut with a primarily scientific education. He had a special job on Apollo 11: When Neil Armstrong and Buzz Aldrin entered the moon on July 20, 1969, McCandless, as Capcom spokesman, kept a direct line to the astronauts.
Fully developed for the space shuttle program
When the Space Shuttle first flew into space in 1981, the MMU was still under development. The realization got a boost when the solar observation satellite of the Solar Maximum Mission (SMM), which was launched into space in 1980, blew three fuses of the attitude control and the satellite, which cost millions, could no longer be aligned to the sun after almost a year of operation. Now there was a specific task for the MMU: The satellite was to be brought into the space shuttle’s loading bay, repaired and released back into space.
The planners set the planning for the maiden flight and the test of the MMU system for spring 1984. The new mission numbering scheme was curious: Instead of STS-10 as the successor to STS-9, it was called STS-41-B: STS stood for Space Transportation System, the 4 for the US budget year 1984, the 1 for the launch site (Kennedy Space Center) and B for the second scheduled mission of 1984.
The Space Shuttle Challenger lifted off on February 3, 1984 at 1:00 p.m. sharp from Kennedy Space Center’s launch pad LC-39A. And the mission was initially under a bad star:
The two satellites Palapa B2 and Western Union’s Westar VI were deployed and were supposed to reach a higher orbit independently with a rocket stage. However, the PAM-D upper stage failed in both cases, so that they were placed in useless orbits that were too low. Also, an experiment with a balloon failed: it burst about 30 meters from the space shuttle instead of being available for target practice. It threatened to be an embarrassing flight.
Jet backpack for outdoor movements
Bruce McCandless and Robert Stewart tested the MMU on February 7: Both put on the suit and prepared for the spacewalk (extravehicular activity, EVA). The backpack was 1.24 meters high, 81 centimeters wide and had a mass of 136 kilograms. The astronauts clipped the backpack to the space suit using two snap fasteners.
The controls were located in front of the armrests: the left hand control provided forward and backward, up and down, and left and right thrust. With the right control, rotational movements around yaw, roll and pitch were possible. 24 compressed nitrogen gas nozzles, each with 7.36 N, were responsible for the precise thrust. They were fed via two Kevlar-coated tanks that carried 11.8 kilograms (each 5.9 kg per tank). Two gauges were for the gas tanks, and lights were used to show the astronauts which jet was currently active. The filling quantity was sufficient for a service life of about 6 hours and 100 meters away from the shuttle was recommended as the maximum range. The highest achievable speed difference was about 25 meters per second.
NASA paid close attention to safety. Technically, the MMU was designed redundantly, so that the astronaut still had another system available if one system failed. There were always two astronauts with MMUs on duty in order to be able to provide help quickly in the event of an emergency. In addition, the cargo bay was largely empty. The space shuttle could have carefully caught a damaged astronaut again.
McCandless couldn’t take his eyes off Challenger
In addition, Bruce McCandless was instructed to keep his eyes off the space shuttle at all times, as NASA feared that he might otherwise lose his bearings when looking at the endless space or at Earth. For the legendary shot, they waited until the space shuttle flew into the sunrise. Astronaut Hoot Gibson took several photos with his Hasselblad camera and one of them has become what is arguably the most used NASA photo today. The only thing that annoyed Gibson was that his name never appeared as a source, only: NASA.
After 5:50 hours, the first test with both astronauts was completed and the MMUs proved themselves: they worked exactly as they should. Nasa was particularly pleased, as the numerous mishaps took a back seat. The following day they informed the astronauts via radio: “We had fun watching you yesterday. And you are the talk of the earth today.”
In the second test on February 9, 1984, which lasted 6 hours and 17 minutes, the astronauts test activities such as repairing and refueling a satellite. The German pallet satellite SPAS-01 was originally intended for further testing, but a defect in the shuttle’s gripping arm prevented that part of the exercise.
End after only three missions
The MMU was thus ready for the repair of Solarmax, which then took place in April 1984 with the subsequent Space Shuttle mission STS 41-C. Ironically, the Manned Manouvering Unit’s third and final mission was in November 1984 with STS-51-A: Palapa B2 and Western Union’s Westar VI captured the two satellites that STS 41-B had launched into false orbits.
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At the latest after the Challenger disaster on January 28, 1986, the two MMU units remained on the ground. On the one hand, because Nasa no longer transported commercial satellites with the Space Shuttle, on the other hand, because it was too dangerous for Nasa. They were also too bulky for the construction of the later ISS space station. The successor is SAFER (Simplified Aid for EVA Rescue), a small system to prevent the astronaut from drifting.
The image of the free-floating astronaut remains in people’s minds to this day – completely detached from Earth.