Bill Nelson: new NASA head hails 'new day in space'
Nelson took the oath of office from Vice President Kamala Harris, with his hand on a Bible, as he officially assumed the position of NASA administrator.
"It's a new day in space," he said, after bringing a Moon rock to the event.
Nelson, 78, who first went into space in 1986, takes over the department, with the US planning to return astronauts to the Moon by 2024.
During the Apollo program in 1972, humans last set foot on the Moon.
NASA's Artemis program aims to create a long-term presence on the Moon, complete with a lunar space station, to test new technologies that will pave the way for a crewed mission to Mars.
In 2019, then-Vice President Mike Pence challenged NASA to land the first woman and next man on the Moon by 2024, and President Joe Biden's administration has supported the same timeline.
Former Administrator Charlie Bolden, who served under Barack Obama, attended the ceremony on Monday, while former Administrator Jim Bridenstine, who served under Donald Trump, joined via video call.
Nelson said their presence was "to show the continuity and the bipartisanship, with which you run the nation's space program, particularly NASA."
Nelson, a sitting congressman, was a crew member on the Space Shuttle Columbia during a six-day flight in space in 1986, and he has since served on several congressional space and science committees.
"NASA is critical to US national and economic security," Harris said on Twitter.
"With decades of experience and as a former astronaut, Bill Nelson will advance NASA's science, aviation, and technology missions as Administrator."