Astronaut Frank Culbertson had perhaps the most unique perspective of any American who witnessed 9/11.

He saw it from space.

Culbertson was the only American aboard the International Space Station when planes first hit the World Trade Center in New York, 11 years ago today.

Soon after Mission Control told him the news, he realized the station would soon be passing over New York. He peered out the station's window and watched helplessly as he floated first over Canada and then just above the East Coast.

Even 200 miles above the surface of the Earth, the geography of New York City was crystal clear. Rising from the tip of lower Manhattan was a cloud of dark gray smoke.

He grabbed his camera and snapped.

Astronauts often talk about how the experience of seeing the Earth from space changes people, but for Culbertson the effect was particularly sobering. The "isolation" of being so far away, magnified by the feeling of helplessness.

Culbertson wrote a letter from orbit and transmitted it to the ground.

"Obviously the world changed today," Culbertson wrote. "What I say or do is very minor compared to the significance of what happened to our country today when it was attacked by …. by whom? Terrorists is all we know, I guess."

"I was flabbergasted, then horrified," Culbertson wrote. "My first thought was that this wasn't a real conversation, that I was still listening to one of my Tom Clancy tapes. It just didn't seem possible on this scale in our country. I couldn't even imagine the particulars, even before the news of further destruction began coming in."

"Other than the emotional impact of our country being attacked and thousands of our citizens and maybe some friends being killed," he wrote, "the most overwhelming feeling being where I am is one of isolation."