Who it is: Bruce McCandless II, a NASA astronaut, testing the Manned Maneuvering Unit (MMU) during space shuttle mission STS-41-B

When it was taken: Feb. 7, 1984

Where it is: Low Earth orbit

Why it's so special: This week is the 40th anniversary of the first untethered spacewalk. On Feb. 7, 1984, NASA astronaut Bruce McCandless II strapped on a hand-controlled backpack and, propelled by its nitrogen gas thrusters, flew out of the space shuttle Challenger payload bay. He got to about 320 feet (98 meters) from the spacecraft, becoming the first human to attempt a spacewalk without a safety tether. McCandless orbited Earth — as the first human satellite — for 1 hour, 22 minutes.

The moment was immortalized by Robert "Hoot" Gibson, Challenger's pilot. He used a Hasselblad camera to capture McCandless flying solo high above Earth. The images became arguably the most iconic of the entire space shuttle program.

Related: 15 jaw-dropping spacewalk images

Although the images were unplanned, Gibson knew instantly that they would be iconic, so he took three light-meter readings and checked the focus four times for each photograph, according to NASA. He even tilted the camera to make sure the horizon was level in the images.

"It may have been one small step for Neil," McCandless said as he orbited Earth solo, "but it's a heck of a big leap for me." (His quip, of course, referenced Neil Armstrong's famous line, "That's one small step for a man, one giant leap for mankind," when taking the first human step on the moon 15 years earlier.)

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Fittingly, McCandless served as the voice of NASA's Mission Control during 1969's Apollo 11 and 1971's Apollo 14 missions. His second and final spaceflight was in April 1990 on the STS-31 mission to deploy the Hubble Space Telescope into Earth's orbit from the space shuttle Discovery cargo bay. He died in 2017, at age 80. His backpack, the MMU, is on display at the National Air and Space Museum's Steven F. Udvar-Hazy Center in Chantilly, Virginia. The MMU was used during three missions in 1984 before being retired for safety reasons.