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

An image shot by NASA's Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the Aqua satellite on Wednesday. It shows multiple wildfires in Calif. trailing smoke into the Pacific Ocean. Lynn Jenner/NASA/Goddard hide caption

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An artist's rendering of Kepler-186f, the first validated Earth-size planet to orbit in the habitable zone of a distant star. T. Pyle/NASA/SETI Institute/JPL-Caltech hide caption

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Japan Aerospace Exploration Agency astronaut Aki Hoshide makes a space walk outside the International Space Station in 2012. NASA/Getty Images hide caption

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For helping to find asteroids, NASA has set up a contest with cash awards. In 2012, the agency said that "more potentially hazardous asteroids, or PHAs, are closely aligned with the plane of our solar system than previous models suggested." NASA hide caption

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People view the Northern Lights over Bamburgh Castle Beach Thursday in Northumberland, England. A powerful solar flare caused the aurora borealis to be visible farther south than usual. Josh Maidwell/Barcroft Media/Landov hide caption

itoggle caption Josh Maidwell/Barcroft Media/Landov