American space legend John Glenn dead at 95

John Glenn, who made history twice as the first American to orbit the Earth and the first senior citizen to venture into space, has died at the age of 95, the Ohio college bearing his name said Thursday.

John Glenn Source: AAP

The US space agency NASA was among the first to pay tribute to the legendary astronaut who later served in the Senate for more than two decades, calling him a "hero."

"We are saddened by the loss of Sen. John Glenn, the first American to orbit Earth. A true American hero. Godspeed, John Glenn. Ad astra," NASA tweeted.

Glenn died on Thursday in Columbus, Ohio, the John Glenn College of Public Affairs announced on its website, paying tribute to "an American icon."

Glenn had been in declining health, undergoing heart-valve replacement surgery in 2014, reportedly suffering a stroke, and was admitted more than a week ago to a cancer ward, according to the college.

The college at Ohio State University said Glenn's "belief in civic duty and public service led him to serve his country as a Marine combat pilot, a pioneer in space travel and an United States senator from Ohio."

"Though he soared deep into space and to the heights of Capitol Hill, his heart never strayed from his steadfast Ohio roots," said Ohio Governor John Kasich.

"Godspeed, John Glenn!"

Glenn served in both World War II and the Korean War, and in 1957 he made the first nonstop supersonic flight from Los Angeles to New York.

In 1962, Glenn was the first US astronaut to orbit the Earth in a flight lasting just under five hours, and returned to space decades later at the age of 77 -- becoming the oldest astronaut in space. 

After his 23-year career in the US military and space program., Glenn entered the Senate as a Democrat.

US President Barack Obama on Thursday paid tribute to legendary astronaut John Glenn, calling the first American to orbit the Earth -- who died at the age of 95 -- an icon and a friend.

"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," Obama said in a statement.

"With John's passing, our nation has lost an icon and Michelle and I have lost a friend," the president said. "On behalf of a grateful nation, Godspeed, John Glenn."

In 2012, Obama awarded Glenn the Presidential Medal of Freedom, the country's highest civilian honor.

"On the morning that John Glenn blasted off into space, America stood still," Obama said during the medal ceremony. 

"The first American to orbit the Earth, John Glenn became a hero in every sense of the word."

In this 2012 file photo, President Barack Obama awards the Medal of Freedom to former astronaut John Glenn during a ceremony in the White House (AAP) Source: AAP