This was because he was on a mission from NASA in space. He was also a retired Naval captain.
On September 11th, the astronaut took photos in space of the smoke that was seen in lower Manhattan, which turned out to be the Twin Towers being hit.
After learning that the Pentagon was also struck, he found out that a former Naval classmate, Charles Burlingame was the original pilot of the plane before he was hijacked.
Frank Lee Culbertson was on a 4 month long mission aboard the STS-105 Discovery and left earth on August 12, 2001. Despite the tragedy that happened in his homeland that September, the mission had to be completed. He returned home on December 17th, 2001 at the Kennedy Space center.
