It’s pretty obvious that the first untethered spacewalk would be on the list of one of the most iconic moments in human space history. This image and the one below, shot by astronaut Robert Gibson, rank among NASA’s top ten most-requested images. It was captured during mission STS-41B on 11 February 1984.

Let us take a look at the image and understand what’s happening here. Astronaut and mission specialist Bruce McCandless is about 300 feet away from the Challenger. He is executing an untethered spacewalk or Extra Vehicular Activity. He is using a nitrogen-propelled, hand controlled Manned Maneuvering Unit (MMU), to move around. A technology devised by him and a team of engineers at NASA.

McCandless, who also enjoys the distinction of being the first “human satellite,” was cruising through the empty expanse at the same speed as that of his space shuttle. That’s a staggering speed of 17,500 miles an hour (28,000 km/h). That is crazy fast, but padded by vacuum, he didn’t sense the motion.

Why won’t there be any more untethered spacewalks or EVAs?

The MMUs were designed to allow astronauts to repair faulty satellites or bring them back to the payload bays for servicing or stowage for a return to earth. But with the advent of precise robotic arms, the need for a physical presence to carry out every extravehicular task was somewhat minimized. Also, if the necessity of an EVA arrives, it just seems safer to be tethered. Especially when you rocketing at 17500 miles an hour. The MMUs were retired in the same year of their first use, 1984, after a safety check conducted following the unfortunate Challenger disaster.

Another propulsion device replaced the MMU in 1994, aptly named SAFER. The Simplified Aid For EVA Rescue, as the name suggests, is a simple and small jetpack worn during every EVA. It is supposed to be used only during an emergency.