On June 18, 1983, Sally Ride became the first American woman to fly in space when the space shuttle Challenger launched on mission STS-7 from Pad 39A, Kennedy Space Center. One of her jobs was to call out "Roll program" seven seconds after launch. "I'll guarantee that those were the hardest words I ever had to get out of my mouth," she said later.

This galaxy, known as NGC 6503, has found itself in a lonely position, at the edge of a strangely empty patch of space called the Local Void.

Like most moons in the solar system, Tethys is covered by impact craters. Some craters bear witness to incredibly violent events, such as the crater Odysseus (seen here at the right of the image). The image was taken in visible light with the Cassini spacecraft narrow-angle camera on April 11, 2015.

For 50 years, NASA has been "suiting up" for spacewalking. In this Feb. 7, 1984 photograph of the first untethered spacewalk, NASA astronaut Bruce McCandless is in the midst of the first "field" tryout of a nitrogen-propelled backpack device called the Manned Maneuvering Unit (MMU).

This parachute testing for NASA's InSight mission to Mars was conducted inside the world's largest wind tunnel, at NASA Ames Research Center, Moffett Field, California, in February 2015.

On May 13, 1992, following the successful capture of the Intelsat VI satellite, three astronauts continue moving the 4.5 ton communications satellite into the space shuttle Endeavour's cargo bay. The sections of Earth which form the backdrop for the scene are blanketed with thousands of square miles of clouds.

Engineers are using carbon dioxide snow to clean James Webb Space Telescope's mirrors without scratching them.

On April 29, 1990, the Space Shuttle Discovery approaches for landing on a concrete runway at Edwards Air Force Base to complete a highly successful five-day mission during which the Hubble Space Telescope was released into orbit. On board are astronauts Loren Shriver, Charles Bolden, Bruce McCandless II, Kathryn Sullivan and Steven Hawley.

The Soyuz TMA-16M spacecraft is rolled out by train to the launch pad at the Baikonur Cosmodrome, Kazakhstan, Wednesday, March 25, 2015.

The first crewed Gemini flight, Gemini III, lifted off Launch Pad 19 at 9:24 a.m. EST on March 23, 1965. The spacecraft "Molly Brown" carried astronauts Virgil I. "Gus" Grissom, command pilot, and John W. Young, pilot, on three orbits of Earth.

Apollo-Saturn 201 (AS-201), the first Saturn IB launch vehicle developed by NASA's Marshall Space Flight Center (MSFC), lifts off from Cape Canaveral, Florida, at 11:12 a.m. on Feb. 26, 1966.

NASA's Magnetospheric Multiscale (MMS) observatories are processed for launch in a clean room at the Astrotech Space Operations facility in Titusville, Florida

In the center of this image, taken with the NASA/ESA Hubble Space Telescope, is the galaxy cluster SDSS J1038+4849 — and it seems to be smiling.

On Feb. 5. 1971, the Apollo 14 crew landed on the moon. The crew members were Captain Alan Bartlett Shepard, Jr. (USN), commander; Major Stuart Allen Roosa (USAF), command module pilot; and Commander Edgar Dean Mitchell (USN), lunar module pilot. In this photo, Shepard stands by the Modular Equipment Transporter (MET).

This image of the interior view from the International Space Station's Cupola module was taken on Jan. 4, 2015.