For nearly 17 years, NASA's Solar Terrestrial Relations Observatory-A spacecraft drifted through space on a lonely mission. It traveled around the sun far ahead of Earth, conducting groundbreaking research on the solar system's star.
Like many NASA spacecraft, STEREO-A outlived its mission life span of two years. Instead, it traveled farther and farther away from Earth on a journey that became fraught with uncertainty as it passed behind the sun in 2015, temporarily severing contact with NASA. The same year, the agency lost contact with STEREO-A's sibling vessel, STEREO-B, which was traveling a similar path.