His four space shuttle flights include docking with the Russian space station Mir aboard Atlantis in January 1997, flying on the Discovery mission in December 1999 to upgrade the Hubble Space Telescope, and returning on March 2002 on Columbia to Hubble, taking three space walks to install a new camera and upgrade other equipment. He has logged 45 days in space, with five space walks totalling more than 37 hours. He went on to be chief scientist at NASA's headquarters in Washington, DC, and was operating as associate administrator of NASA’s Science Mission Directorate. He helped to oversee several important missions, including the New Horizons fly-by of Pluto and Curiosity’s landing on Mars before he retired from NASA in April 2016. Grunsfeld was a senior research fellow at the California Institute of Technology, involved in X-ray and gamma-ray astronomy, before his selection as an astronaut in 1992.