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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). Hubble's solar arrays can be clearly seen at centre left & right. Astronauts Steven Smith and John Grunsfeld are replacing the gyroscopes which control HST's pointing mechanism. The astronauts are strapped to the shuttle's robo- tic arm, known as the remote manipulator system (RMS). This image was taken from the Discovery shuttle's cargo bay during mission STS-103 (19-27 December 1999). Replacing the HST's computer was also part of the servicing. This mission was the 3rd to service the HST since its launch in 1990. (KEYSTONE/SCIENCE PHOTO LIBRARY/NASA)