John Glenn, a war hero who became the first American to orbit the Earth and later served four terms in the U.S. Senate, has been hospitalized in his home state of Ohio with an unexplained condition, a source close to his family confirmed to NBC News.

The 95-year-old former Marine suffered a stroke two years ago after having heart valve replacement surgery, but it was not clear what caused him to be taken to the James Cancer Hospital at Ohio State.

Glenn was admitted to the hospital more than a week ago, a spokesman for the John Glenn College of Public Affairs said in a statement. The spokesman said he did not know Glenn's "condition or illness or prognosis" and cautioned that Glenn did not necessarily have cancer.

Although he served as a Democratic senator for 25 years and was a decorated combat pilot in World War II and the Korean War, Glenn will always be known primarily for his 1962 ride in a space capsule dubbed Friendship 7 that circled the Earth, an accomplishment that put the United States on equal footing with Russia in the space race.

Glenn had joined the Mercury 7, America's first class of astronauts, after setting the transcontinental speed record as a test pilot. He aimed to be the first man in space, but was relegated to a backup role behind Alan Shepard. A Russian cosmonaut beat them to it, and Glenn got the Americans' lead role on February 20, 1962, riding a Mercury-Atlas rocket from Cape Canaveral.

Glenn's response from seeing the earth from more than 100 miles above became famous: "Oh, that view is tremendous!."

He landed to a hero's welcome, including a ticker tape parade, and decided to enter politics. Glenn, who'd become friends with John F. Kennedy, failed in his first two attempts at the Senate. In 1974 he finally won a seat representing Ohio. A liberal Democrat, he served four terms and retired in 1999. He briefly ran for president in 1984.

Before he left politics, however, Glenn returned to space on the shuttle Discovery, becoming, at 77, the oldest person to make such a voyage.

In 2011 Glen received the Congressional Gold Medal, America's highest civilian honor.