NASA Science Chief, Former Astronaut John Grunsfeld Retiring

NASA science chief John Grunsfeld, a former astronaut who famously helped repair and upgrade the Hubble Space Telescope in orbit, will end his nearly quarter-century of service to the agency at the end of the month.

Grunsfeld — who has headed NASA's Science Mission Directorate since January 2012, and has therefore been in charge for big moments such as the Mars rover Curiosity's Red Planet touchdown in August 2012 and the New Horizons spacecraft's epic flyby of Pluto in July 2015 — will retire effective April 30, agency officials announced today (April 5).

"After exploring strange new worlds and seeking out new life in the universe, I can now boldly go where I've rarely gone before — home," Grunsfeld said in a statement. "I'm grateful to have had this extraordinary opportunity to lead NASA science, and know that the agency is well-positioned to make the next giant leaps in exploration and discovery." [NASA's 10 Greatest Science Missions]

Grunsfeld, 57, was selected as a NASA astronaut in 1992 and flew on five space shuttle missions between 1995 and 2009, logging more than 58 days in orbit. His final three flights — in 1999, 2002 and 2009 — serviced the Hubble Space Telescope, which had launched in April 1990 with a slight flaw in its primary mirror.

Grunsfeld performed a total of eight spacewalks to help maintain, upgrade and repair Hubble, and he remains the last person ever to touch the iconic observatory.

"John leaves an extraordinary legacy of success that will forever remain a part of our nation's historic science and exploration achievements," NASA Administrator and former space shuttle commander Charlie Bolden said in the same statement. 

"Widely known as the Hubble Repairman, it was an honor to serve with him in the astronaut corps and watch him lead NASA's science portfolio during a time of remarkable discovery," Bolden added. "These are discoveries that have rewritten science textbooks and inspired the next generation of space explorers."

Science Mission Directorate deputy chief Geoff Yoder will serve as acting administrator until a replacement for Grunsfeld is found, NASA officials said.

Grunsfeld's retirement comes closely on the heels of NASA astronaut Scott Kelly, who has logged more total days in space (520) than any other American. Kelly left NASA on Friday (April 1), about one month after the unprecedented yearlong mission to the International Space Station he and cosmonaut Mikhail Kornienko conducted came to an end.

Follow Mike Wall on Twitter @michaeldwall and Google+. Follow us @Spacedotcom, Facebook or Google+. Originally published on Space.com.