Glenn died on December 8, 2016 at age 95.
The longtime senator is being honored in a ceremony at Arlington National Cemetery.
He worked for NASA from 1959 to 1964 before working as an executive for Royal Crown Cola and then launching a bid for the U.S. Senate.
On Dec. 8, 2016, John Herschel Glenn, Jr. a veteran of both the U.S. Navy and the U.S. Marine Corps, NASA astronaut, and a United States Senator from OH, died at the age of 95.
Glenn, who author Tom Wolfe once called "the last true national hero America has ever had", died four months ago in his home state of OH at the age of 95.
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Mrs Glenn, 97, wore a red dress and held a single red rose as she sat under a tent at the grave site, next to the couple's two children, John David and Carolyn.
Flags will be flown at half-staff across the state on Thursday in honor of OH senator and legendary NASA astronaut John Glenn.
The U.S. Marine Corps will live stream his service, starting around 9:20 a.m. The event also will air on NASA TV. On the 50th anniversary of that first orbit, Glenn told NPR's Audie Cornish that the backdrop of the Cold War put extra pressure on his launch - not to mention, the uncertainty of the technology itself.
When he was 77 and completing his fourth term in 1998, he returned to space as a crew member aboard the space shuttle Discovery. The small-town OH boy married his childhood sweetheart, served as a fighter pilot in two wars, a test pilot, an astronaut and a U.S. Senator. Their marriage persisted for 73 years; Annie attended Glenn's funeral a few months ago in a wheelchair.
Deemed too valuable an asset to the United States to fly in space again, Glenn left NASA shortly after his flight for a career in politics.