British-born astronaut and climate scientist Piers Sellers has died of pancreatic cancer at the age of 61.
Dr Sellers, who flew on three space shuttle missions between 2002 and 2010, died on Friday morning in Houston.
The Nasa climate scientist, originally from Crowborough in East Sussex, gained American citizenship to fulfil a childhood dream of flying into space.
In a statement Nasa administrator Charles Bolden led the tributes to Dr Sellers, calling him "a tremendous public servant who was dedicated to Nasa, the nation and the world".
Mr Bolden added: "He was a strident defender and eloquent spokesperson for our home planet, Earth.
"Spacewalker and scientist, free thinker and friend to our planet, and all who seek new knowledge, to say he will be missed would be a gross understatement."
In July Dr Sellers was awarded Nasa's Distinguished Service Medal, the agency's highest honour.
Dr Sellers logged 35 days in space during the three Nasa shuttle missions, and was appointed an OBE in the New Year's Honours List in 2011.