 He was Chief of the Astroparticle Physics Laboratory at NASA's Goddard Space Flight Center from 1995 until his death, and was best known for his work developing the field from early balloon instruments to today's space observatories such as the NASA Swift mission, for which he was the Principal Investigator.He was leading the WFIRST (now called the Nancy Grace Roman Space Telescope) wide-field infrared telescope forward toward a launch in the mid-2020s.He was a member of the National Academy of Sciences and the American Academy of Arts and Sciences.