On March 18, 1965, Russian cosmonaut Alexei Leonov stepped out of the Voskhod 2 spacecraft and into the unknown.
Leonov floats through space at the end of a lifeline during the first spacewalk. (AP Photo)
The Soviets had practiced the entire operation countless times on Earth. But actually opening the hatch hundreds of miles above our planet's surface must have been unimaginably nerve-wracking for Leonov. The astronaut told TIME photographer Marco Grob that one facet of the first spacewalk still stands out sharply in his memory.
"I remember the sound, this remarkable silence," he said. "You can hear your heart beat and you can hear yourself breathe. Nothing else can accurately represent what it sounds like when a human being is in the middle of this abyss."
Below, stunning images of other astronauts who, in the five decades since Leonov opened the Voskhod hatch, have ventured into that same silent chasm in the name of science.
June 3, 1965: Edward H. White completes the first U.S. spacewalk during the Gemini 4 mission. | (NASA/Roger Ressmeyer/CORBIS)
August 6, 1973: Jack Lousma, Skylab 3 pilot, deploys the twin-pole solar shield to help shade the Orbital Workshop. | (NASA)
February 9, 1984: Bruce McCandless fires the nitrogen-gas jets from his Manned Maneuvering Unit to venture some 300 feet from the Challenger shuttle without a tether. | (AP Photo/NASA)
September 3, 2009: John "Danny" Olivas smiles for the camera while he works on construction and maintenance of the International Space Station. | (NASA/Reuters/Corbis)
April 6, 1984: Two astronauts work on a satellite in the cargo bay of the Challenger space shuttle. | (NASA/Roger Ressmeyer/CORBIS)
October 22, 1993: Kathryn Thornton hovers over equipment on the Hubble Space Telescope, guided by the Remote Manipulator System. | (CORBIS)
December 2006: Astronauts Robert L. Curbeam and Christer Fugelsang work to attach a new truss segment to the ISS and upgrade the power grid. | (STS-116 Shuttle Crew/NASA.gov)
November 2007: Scott Parazynski assesses his repair work during a 7-hour, 19-minute spacewalk. | (NASA)
November 15, 2010: Oleg Skripochka helps install a multipurpose workstation on the ISS. | (NASA)
November 9, 2013: Oleg Kotov smiles while working on the ISS. | (NASA)
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