Buzz Aldrin said Glenn, who has died aged 95, was a "pioneer of space flight"
President Obama has led tributes to John Glenn, the first US astronaut to orbit the Earth, who has died aged 95.
Glenn first traveled to space in the Friendship 7 spacecraft in 1962, and orbited the Earth three times.
As well as his career as an astronaut, Glenn served 24 years as a US senator for Ohio.
In 1984, he failed to secure the Democratic presidential nomination.
He returned to space in the shuttle Discovery at age 77 in 1998, and as a result holds the record for the oldest person in space.
In a statement, President Obama said: "With John's passing, our nation has lost an icon and Michelle and I have lost a friend. John spent his life breaking barriers, from defending our freedom as a decorated Marine Corps fighter pilot in World War II and Korea, to setting a transcontinental speed record, to becoming, at age 77, the oldest human to touch the stars.
"John always had the right stuff, inspiring generations of scientists, engineers and astronauts who will take us to Mars and beyond - not just to visit, but to stay [...] The last of America's first astronauts has left us, but propelled by their example we know that our future here on Earth compels us to keep reaching for the heavens.
"On behalf of a grateful nation, Godspeed, John Glenn," he added.
Apollo 11 pilot Buzz Aldrin said he heard of his friend's death while recovering from an illness in hospital.
Aldrin observed: "With the news today I’m saddened again to hear that we have lost the pioneer of space flight for the United States, second only to Yuri Gagarin, and he will always go down in history as certainly one of the most influential officers in the Marine Corps and of course as one of the original Mercury 7 astronauts.
"I am very sorry that he has departed us with his wisdom. I join that crowd of people and the entire nation and the world in paying homage to his service."
NASA Administrator Charles Bolden, meanwhile, stated: “Senator Glenn's legacy is one of risk and accomplishment, of history created and duty to country carried out under great pressure with the whole world watching. The entire NASA Family will be forever grateful for his outstanding service, commitment and friendship."