NEWSBig moments in space explorationOctober 4, 1957: Sputnik 1 was the first satellite in space. Launched by the Soviet Union, the satellite's success triggered the Space Race, an integral part of the Cold War.NASAApril 12, 1961: This undated photo shows cosmonaut Maj. Yuri Gagarin in his space suit. It was the Soviet Union's own giant leap for mankind, one that would spur a humiliated America to race for the moon. It happened on Tuesday, April 12, 1961 when the air force pilot became the first human in space.APMay 5, 1961: Alan Shepard sits in his Freedom 7 Mercury capsule. Shepard was the second person ever to go to space and the first American.NASADecember 21, 1968: This famous photo, taken by the crew of Apollo 8, shows the Earthrise. The crew of Apollo 8 were the first humans to go to the moon and back.NASAJuly 20, 1969: Apollo 11 astronaut Buzz Aldrin walks on the moon. The members of Apollo 11 were the first humans on the moon.NASAApril 11, 1970: Apollo 13 commander James A. Lovell Jr., foreground, speaks during a news conference in Cape Kennedy, Fla. before the spacecraft launched on its ill-fated journey to the moon. The unsuccessful journey was made famous by the movie, "Apollo 13"APMay 14, 1973: Skylab, the first United States manned space station, was launched in 1973. Here it is shown in orbit at the end of its mission in 1979 when it crashed back to Earth. The orbiting lab was designed by engineers at the Marshall Space Flight Center in Huntsville, Ala.NASA, Associated PressJuly 17, 1975: Astronaut Thomas P. Stafford (left) and cosmonaut Aleksei A. Leonov shake hands after a successful Apollo-Soyuz linkup in space. The U.S.-Soviet mission was the first multinational manned mission.NASAApril 12, 1981: Space Shuttle Columbia launched into orbit from NASA's Kennedy Space Center in Florida. The shuttle was the first reusable spacecraft.NASAJune 18, 1983: This NASA file photo shows America's first woman astronaut Sally Ride, as she communicates with ground controllers from the flight deck during the six-day space mission of the Challenger. Ride first launched into space in 1983, on the seventh US space shuttle mission.AFP/Getty ImagesJanuary 28, 1986: The space shuttle Challenger explodes shortly after lifting off from Kennedy Space Center. All seven crew members died in the explosion, which was blamed on faulty O-rings in the shuttle's booster rockets. The disaster shattered NASA's image and the belief that flying on a spacecraft could become as routine as flying on an airplane.BRUCE WEAVER, APApril 24, 1990: The Hubble Space Telescope, the first of NASA's Great Observatories to reach orbit, launched. It still remains in operation today.NASADecember 4, 1996: Mars Pathfinder was designed to show the development of "faster, better and cheaper" spacecraft was possible. It was the first operational rover on another planet.NASANovember 20, 1998: The International Space Station is launched. It becomes the first multinational space station and the largest man-made object built in space. Astronaut Jerry Ross adjusts his tools from his perch at the end of the space shuttle's robot arm as he works on the International Space Station module Unity Monday, Dec. 7, 1998, in this image from NASA television.NASA TV, Associated PressNovember 26, 2011: "Curiosity" is a robotic rover exploring Gale Crater on Mars. This file photo released on June 23, 2014 by NASA, shows NASA's Curiosity Mars rover self-portrait. NASA announced Thursday, Sept. 11, 2014, that the rover has reached the base of Mount Sharp, its long-term science destination since landing two years ago.Uncredited, APMay 25, 2012: SpaceX becomes the first private company to send a spacecraft to the International Space Station. In this handout image released by NASA, the SpaceX Falcon 9 rocket and Dragon spacecraft lift off on May 22, 2012 from the Cape Canaveral Air Force Station launch complex 40 in Cape Canaveral, Florida.NASA/HANDOUT, EPAJune 28, 2015: Space X's Falcon 9 rocket as it lifts off from space launch complex 40 at Cape Canaveral, Florida June 28, 2015 with a Dragon CRS7 spacecraft. The unmanned SpaceX Falcon 9 rocket exploded minutes after liftoff from Cape Canaveral, Florida, following what was meant to be a routine cargo mission to the International Space Station.BRUCE WEAVER, AFP/Getty ImagesJuly 14, 2015: Pluto nearly fills the frame in this image from NASA's New Horizons spacecraft, taken on July 13, 2015. This is the last and most detailed image sent to Earth before the spacecraft's closest approach to Pluto on July 14.NASA, Getty ImagesJuly 23, 2015: This artist's rendering made available by NASA shows a comparison between the Earth, left, and the planet Kepler-452b. It is the first near-Earth-size planet orbiting in the habitable zone of a sun-like star, found using data from NASA's Kepler mission.T. Pyle, APThese dark, narrow, 100-meter-long streaks called recurring slope lineae flowing downhill on Mars are inferred to have been formed by contemporary flowing water. Recently, planetary scientists detected hydrated salts on these slopes at Horowitz crater, corroborating their original hypothesis that the streaks are indeed formed by liquid water.NASA/JPL/University Of Arizona