Astronaut Ed White floats in the microgravity of space fifty years ago during the first U.S. spacewalk outside the Gemini IV spacecraft June 3, 1965. He is secured to the spacecraft by a 25-foot umbilical line and tether wrapped in gold tape. REUTERS/NASA/Jim McDivitt/Handout

The first American spacewalk, conducted in 1965 by Gemini 4 astronaut Ed White, is seen during a screening of archival footage on Capitol Hill in Washington May 6, 2008. REUTERS/Larry Downing

Astronaut Ed White floats in the microgravity of space fifty years ago during the first U.S. spacewalk outside the Gemini IV spacecraft June 3, 1965. In his left hand is a Hand-Held Self-Maneuvering Unit with which he controls his movements in space. REUTERS/NASA/Jim McDivitt/Handout

Astronaut Sory Musgrave, anchored on the Remote Manipulator System arm, prepares to install protective covers on the magnetometers of the Hubble Space Telescope December 9, 1993. Astronaut Jeffrey Hoffman (bottom of frame) assists. REUTERS/NASA/Handout

Space shuttle Discovery astronaut Mark Lee teams with Steven Smith (out of frame but reflected in helmet visor of Lee) during their spacewalking service mission on the Hubble Space Telescope February 16, 1997. REUTERS/NASA/Handout

Astronauts Richard J. Hieb, Thomas D. Akers and Pierre J. Thuot (L-R) attach a specially designed grapple bar underneath the 4.5 ton Intelsat VI satellite at the cargo bay of the space shuttle Endeavour May 13, 1992. REUTERS/NASA/Handout

Astronaut Mark Lee floats free in space around the earth while testing a new jetpack rescue device (SAFER) September 16, 1994. Lee and his colleague Col. Carl Meade from the Space Shuttle Discovery tested the device in the first untethered spacewalk by NASA astronauts in ten years. REUTERS/NASA/Handout

Mir space station commander Anatoly Solovyov cuts through the insulation cover of the Spektr module with a special hammer-like instrument during six hours of work September 6, 1997. Solovyov and NASA researcher Michael Foale went on a risky spacewalk trying to repair damage caused by a cargo tug that collided with the Mir station. REUTERS/Handout

Astronaut Nicole Stott, STS-128 mission specialist, waves as she pauses during a spacewalk, with the solar panels of International Space Station as a backdrop, as construction and maintenance continue on the International Space Station September 1, 2009. REUTERS/NASA/Handout

The Space Shuttle Discovery, docked to the Destiny laboratory of International Space Station, is seen during a spacewalk August 3, 2005. Astronaut Soichi Noguchi, a mission specialist representing Japan Aerospace Exploration Agency (JAXA), is partially visible between payload bay of Discovery and Destiny. The blackness of space and horizon of Earth formed the backdrop for the image. REUTERS/NASA/Handout

Astronaut Robert L. Satcher Jr. uses a digital still camera to take a picture of the reflection in his helmet visor as he works outside the International Space Station as construction and maintenance continue on the International Space Station November 19, 2009. REUTERS/NASA/ Handout

Spacewalker Ron Garan rides on the robotic arm of the International Space Station as he transfers a failed pump module to the cargo bay of space shuttle Atlantis July 12, 2011. REUTERS/NASA/Handout

Astronaut Steve Bowen participates in a spacewalk as construction and maintenance continue on the International Space Station March 2, 2011. REUTERS/NASA/Handout

Spacewalker James Reilly works with the horizon of Earth and the blackness of space in the background June 11, 2007. REUTERS/NASA/Handout

Spacewalker Robert Behnken works outside the International Space Station February 14, 2010. REUTERS/NASA/Handout

NASA astronaut Terry Virts, Flight Engineer of Expedition 42, works to complete a cable routing task while the sun begins to peak over the horizon of Earth on the International Space Station, February 21, 2015. REUTERS/NASA/Handout

NASA Expedition 35 Flight Engineers Chris Cassidy (pictured) and Tom Marshburn (out of frame) conduct a spacewalk to inspect and replace a pump controller box on the far port truss of International Space Station (P6) leaking ammonia coolant May 11, 2013. REUTERS/NASA/Handout

NASA astronaut Ron Garan checks out his pistol grip tool in the Quest airlock of International Space Station prior to his spacewalk July 12, 2011. REUTERS/NASA/Handout

Astronaut Robert L. Satcher Jr. works outside the International Space Station as construction and maintenance continue on the International Space Station November 19, 2009. REUTERS/NASA/Handout

Carl Meade floats untethered and Mark Lee is anchored to the robot arm of Space Shuttle Discovery while testing new jetpack rescue system (SAFER) of NASA September 28, 1994. REUTERS/NASA/Handout

Astronaut Kathryn Thornton, on the end of the Remote Manipulator System of the Shuttle Endeaovour, hovers over servicing equipment for the Hubble Space Telescope December 6, 1993. REUTERS/NASA/Handout

NASA astronaut Sunita Williams, Expedition 32 flight engineer, takes part in a spacewalk to install a Main Bus Switching Unit outside the International Space Station September 5, 2012. REUTERS/NASA/Handout

Astronaut Dave Wolf, attached to the robot arm of International Space Station, works on the station July 20, 2009. REUTERS/NASA/Handout

Japan Aerospace Exploration Agency astronaut Aki Hoshide, Expedition 32 flight engineer, works on the installation of a Main Bus Switching Unit and a camera on the Canadarm 2 on the International Space Station September 5, 2012. REUTERS/NASA/Handout

NASA astronaut Reid Wiseman and European Space Agency astronaut Alexander Gerst (not shown) work outside the Quest airlock of International Space Station October 8, 2014. REUTERS/Alexander Gerst/NASA/ESA/Handout

Astronaut Dale Gardner holds up a "For Sale" sign referring to the two satellites, Palapa B-2 and Westar 6 that they retrieved from orbit after their Payload Assist Modules (PAM) failed to fire, aboard Space Shuttle Discovery November 14, 1984. Astronaut Joseph P. Allen IV, who also participated in the two EVAs, is reflected in the helmet visor of Gardner. A portion of each of two recovered satellites is in the lower right corner, with Westar 6 nearer Discovery aft. REUTERS/NASA/Handout

Dwarfed by the International Space Station, NASA astronauts Andrew Feustel (R) and Greg Chamitoff (L) work during their spacewalk in support of construction and maintenance of the ISS May 20, 2011. REUTERS/NASA/Handout

NASA astronaut Barry Butch Wilmore, Commander of Expedition 42, catches a selfie with his reflection on the International Space Station, February 25, 2015. REUTERS/NASA/Handout

Space shuttle Endeavour astronaut Andrew Feustel is seen inside the hatch of the Quest airlock on the International Space Station May 20, 2011. REUTERS/NASA/Ron Garan/Handout

Astronaut Soichi Noguchi, STS-114 mission specialist representing Japan Aerospace Exploration Agency, acts as observer and communication relay station between fellow spacewalker Stephen K. Robinson (out of frame) and astronaut Andrew S. W. Thomas aboard Space Shuttle Discovery, while perched on a Space Station truss August 3, 2005. A portion of the thermal protection tiles on the underside of Discovery is visible at lower left. REUTERS/NASA/Handout