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Spacewalk. European Space Agency (ESA) astronauts perform an extravehicular activity (EVA or spacewalk). They are servicing the Hubble Space Telescope (HST) during the space shuttle Discovery's mission STS-103 (19-27 December 1999). The astronauts Michael Foale (left) and Claude Nicollier (right) are replacing one of Hubble's three fine guidance sensors (FGS). These devices allow the telescope to find and view its target. Nicollier is strapped to the shuttle's robotic arm, known as the remote manipulator system (RMS). This mission was the third to service the HST since its launch in 1990. Photographed from Discovery's cargo bay. (KEYSTONE/SCIENCE PHOTO LIBRARY/NASA)