NASA astronauts Mark Vande Hei, left, and Joe Acaba, center, listen as Sergeant Craig Hudson explains the tradition of the Honor Guard at the Tomb of the Unknowns, Friday, June 15, 2018 at the Arlington National Cemetery in Arlington, Va. Photo Credit: (NASA/Aubrey Gemignani)

NASA astronauts Mark Vande Hei, left, and Joe Acaba, center, listen as Sergeant Craig Hudson explains the tradition of the Honor Guard at the Tomb of the Unknowns, Friday, June 15, 2018 at the Arlington National Cemetery in Arlington, Va. Photo Credit: (NASA/Aubrey Gemignani)

NASA astronauts Mark Vande Hei, left, and Joe Acaba, center, listen as Sergeant Craig Hudson explains the tradition of the Honor Guard at the Tomb of the Unknowns, Friday, June 15, 2018 at the Arlington National Cemetery in Arlington, Va. Photo Credit: (NASA/Aubrey Gemignani)

NASA astronauts Mark Vande Hei, third from left, and Joe Acaba, third from right, meet with the Honor Guard at the Tomb of the Unknowns, Friday, June 15, 2018 at the Arlington National Cemetery in Arlington, Va. Photo Credit: (NASA/Aubrey Gemignani)