Complex sentence: The STS-82 mission was the second in a series of planned servicing missions to the orbiting Hubble Space Telescope ("HST "), which had been placed in orbit on 24 April 1990 by"Discovery" during STS-31.
Split into simpler sentences: The STS-82 mission was the second in a series of planned servicing missions to the orbiting Hubble Space Telescope (HST). HST was placed in orbit on 24 April 1990 by the Space Shuttle Discovery on STS-31.