Neil Armstrong, the first man to walk on the moon, has had heart bypass surgery and is recovering in hospital.
The retired astronaut had the operation just days after his 82nd birthday.
Mr Armstrong commanded the Apollo 11 spacecraft that landed on the moon on July 20, 1969.
He radioed back to Earth the historic news of "one giant leap for mankind" and spend nearly three hours walking on the moon with Edwin "Buzz" Aldrin.
A message on Mr Aldrin's Twitter account wished his former crewmate well.
A Facebook statement from Nasa administrator Charles Bolden said: "Neil's pioneering spirit will surely serve him well in this challenging time and the entire Nasa family is holding the Armstrong family in our thoughts and prayers."
It is not clear where Mr Armstrong, who turned 82 on Sunday, received treatment.
He and his wife Carol married in 1999 and made their home in the Cincinnati suburb of Indian Hill, but he has largely stayed out of public view in recent years.
He spoke at Ohio State University in February at an event honouring fellow astronaut John Glenn and the 50th anniversary of Glenn becoming the first American to orbit the Earth.