In an image provided by NASA, astronaut Randy Bresnik prepares to enter Boeing's CST-100 spacecraft for an evaluation at the company's Houston Product Support Center. NASA awarded Boeing with a $4.2 billion contract Tuesday. AP hide caption

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During the Apollo 12 mission, astronaut Alan Bean holds a container of lunar soil. The astronaut Charles "Pete" Conrad, who took the photograph, is reflected in Bean's faceshield. Bean says he used to think that in his lifetime, we'd build a base on the moon and start preparing to travel to Mars. Hulton Archive/Getty Images hide caption

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An artist's concept shows the New Horizons spacecraft as it approaches Pluto and its largest moon, Charon, in July 2015. Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute hide caption

itoggle caption Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

Early days: NASA's International Sun-Earth Explorer C (also known as ISEE-3 and ICE) was undergoing testing and evaluation inside the Goddard Space Flight Center's dynamic test chamber when this photo was snapped in 1976. NASA hide caption

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With a comet fly-by and a solar orbit behind it, ISEE-3 has now revved its engines for a swing past the moon. Mark Maxwell/Courtesy ISEE-3 Reboot Project hide caption

itoggle caption Mark Maxwell/Courtesy ISEE-3 Reboot Project

Artist concept of New Horizons spacecraft. The Hubble Space Telescope is being pressed into service to help scientists look for a post-Pluto target for the space probe. Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute hide caption

itoggle caption Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

Whatever name it sailed under — International Sun-Earth Explorer 3, and International Cometary Explorer, among others — this spacecraft has scored a number of firsts over the years, including the first comet flyby. NASA Goddard Spaceflight Center hide caption

itoggle caption NASA Goddard Spaceflight Center