Tributes poured in from around the world to Neil Armstrong, the first man to walk on the Moon, who has died at the age of 82.
The former Nasa astronaut had suffered complications from heart surgery he underwent earlier this month, his family said.
He famously uttered the quote moments after setting foot on the lunar surface: "That's one small step for man, one giant leap for mankind."
US president Barack Obama hailed Armstrong as one of America's greatest heroes.
In a statement issued by the White House, Mr Obama said the crew of Apollo 11 carried with them the aspirations of an entire nation when they set out for the moon in 1969.
He said: "Neil was among the greatest of American heroes - not just of his time, but of all time. When he and his fellow crew members lifted off aboard Apollo 11 in 1969, they carried with them the aspirations of an entire nation.
They set out to show the world that the American spirit can see beyond what seems unimaginable - that with enough drive and ingenuity, anything is possible. And when Neil stepped foot on the surface of the moon for the first time, he delivered a moment of human achievement that will never be forgotten.
"Today, Neil's spirit of discovery lives on in all the men and women who have devoted their lives to exploring the unknown - including those who are ensuring that we reach higher and go further in space. That legacy will endure - sparked by a man who taught us the enormous power of one small step."
Astronomer Sir Patrick Moore said: "As the first man on the Moon, he broke all records. I knew him well. He was a man who had all the courage in the world."
Physicist Professor Brian Cox tweeted: "Sad to hear about death of Neil Armstrong. I do think Apollo was the greatest of human achievements. For once, we reached beyond our grasp."
Armstrong commanded the Apollo 11 spacecraft that landed on the Moon on July 20 1969. He and fellow astronaut Edwin "Buzz" Aldrin spent nearly three hours walking on the Moon, collecting samples, conducting experiments and taking photographs.
Mike Cruise, professor of astrophysics and space research at the University of Birmingham, said: "The people at the front of the race always have to tread on new ground. He led the whole world into a space era of greater proportion than has been achieved by satellites.
"You wonder when his first steps will be followed up. It must have been very awe-inspiring to step on to, essentially, a new planet."
Jamie Burgess, from the UK National Space Centre, in Leicester, said: "I think it is safe to say (he was) a very brave man, a very courageous man. To be able to have that bravery is an incredible quality in a person. He is a fantastic character and will be sorely missed."
An estimated 600 million people - a fifth of the world's population - watched and listened to the first moon landing, the largest audience for any single event in history.
The moonwalk marked America's victory in the Cold War space race that began on October 4 1957 with the launch of the Soviet Union's satellite Sputnik 1.
Armstrong and then wife Janet would later meet Queen Elizabeth and Prince Andrew at Buckingham Palace at a reception following the moon-landing in 1969 during his 22-nation 38-day world tour.
Announcing his death, Mr Armstrong's family said in a statement: "We are heartbroken to share the news that Neil Armstrong has passed away following complications resulting from cardiovascular procedures.
"Neil was our loving husband, father, grandfather, brother and friend. Neil Armstrong was also a reluctant American hero who always believed he was just doing his job."
It continued: "For those who may ask what they can do to honour Neil, we have a simple request. Honour his example of service, accomplishment and modesty, and the next time you walk outside on a clear night and see the Moon smiling down at you, think of Neil Armstrong and give him a wink."
Space agency Nasa tweeted: "Nasa offers its condolences on today's passing of Neil Armstrong, former test pilot, astronaut & the 1st man on the moon. Neil was 82."
Meanwhile, Apollo 11 command module pilot Michael Collins, who was part of the historic moon-landing mission, paid tribute to Armstrong, his former colleague.
He said: "He was the best, and I will miss him terribly."
Former NASA astronaut Harrison Schmitt, who also set foot on the moon, described Armstrong as a "remarkable individual".
He told BBC Breakfast "I remember him as a professional at all times, very mature but also a very warm individual. He did not seek the limelight as some may have."
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