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Hubble Telescope servicing. Astronauts perform an extravehicular activity (EVA or spacewalk) during a space shuttle mission to service the Hubble Space Telescope (HST). One of Hubble's solar arrays can be seen in the background. Astronaut John Grunsfeld (centre) is replacing a radio transmitter in one of the HST's electronics bays. His feet are strapped to the end of the shuttle's robotic arm, known as the remote manipulator sys- tem (RMS). Also seen at lower right is astronaut Steven Smith. This image was taken from the Discovery shuttle's cargo bay during mission STS- 103 (19-27 December 1999). This mission was the 3rd to service the HST since its launch in 1990. (KEYSTONE/SCIENCE PHOTO LIBRARY/NASA/SCIENCE PHOTO LIBRARY)