A heartbreaking video shows the moment an astronaut watched the tragic 9/11 attacks unfold from space.
As we remember those who died in the September 11 attacks 23 years ago, many of us will also be remembering where we were when the devastating events were unfolding.
But while many of us were watching from home or with family, naval aviator and astronaut Frank Culbertson was the only American in space at the time.
Culbertson was the only person from the US onboard the International Space Station (ISS) as it was in low orbit on the day of 9/11.
After taking off a month earlier from the Kennedy Space Center, his time on the ISS was supposed to be memorable due to the fact that it was his first (and only) time in command of the station. However, his time onboard will forever be remembered in history due to the fact he was the only American off-planet at the time of the attacks.
In the documentary Astronauts: Houston We Have a Problem, Culbertson recalls: "The weather was perfectly clear that day. I could easily see New York City - a big black column of smoke coming out of the city, and as I zoomed in with the video camera, I could see this big grey blob enveloping southern Manhattan.
The astronaut continued: "What we were seeing was the second tower coming down."
Culbertson can also be heard reporting on what he could see to Mission Control at the time. "We can see New York City, and the smoke from the fires," he describes.
"Our prayers and thoughts go out to all the people there and everywhere else. Here I'm looking up and down the East Coast to see if I can see anything else," the astronaut adds.
In a video shared to NASA's YouTube channel, Culbertson recalls: "I didn't know exactly what was happening but I knew it was really bad because there was a big cloud of debris covering Manhattan."
After being informed of what was happening back on Earth, he says: "That's when it really became painful, because it was like seeing a wound in the side of your country... of your family and friends."
In an interview with NASA back in 2015, Culbertson revealed that he heard the news of the attacks from flight surgeon Dr. Steve Hart back on Earth, who had had called to relay some test results.
"Steve said, 'We're not having a very good day down here on Earth,'" Culbertson said. "He began to describe to me what was happening in New York. ... As we were talking, he said we just lost another airplane somewhere in Pennsylvania, we don't know where or what's happening. They told us all they could, which wasn't very much."
As the ISS moved over New England, Culbertson grabbed a video camera and started recording.
"I could clearly see the city," he said. "It was a perfect weather day all over the United States.
"The only activity I could see was this big black column of smoke coming out of New York City.
"When I zoomed in with the video camera, I saw this big gray blob basically enveloping the southern part of Manhattan. What I was seeing was the second tower coming down."
Tragically, the "blob" he was describing was the second tower of the World Trade Center collapsing.
"It was horrible to see my country under attack," he recalled.
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