On the 23rd anniversary of the September 11 attacks, NASA released a letter from the only American astronaut in space at the time, Frank Culbertson, who was reflecting on witnessing the tragedy from the International Space Station (ISS).

In the letter, dated September 12, 2001, Culbertson recollects the moment he learned of the attacks while being aboard the International Space Station (ISS). Describing his shock and horror upon hearing about the events, he wrote, "Well, obviously the world changed today. 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. Hard to know at whom to direct our anger and fear..."

Culbertson, who was serving as the commander of Expedition Three at that time, captured images of the smoke plume rising from lower Manhattan shortly after the collapse of the Twin Towers. His letter vividly narrates his experience of seeing his country under one of the biggest terrorist attacks of all time that killed 3000 people.

"It's difficult to describe how it feels to be the only American completely off the planet at a time such as this. The feeling that I should be there with all of you, dealing with this, helping in some way, is overwhelming," he wrote.

Culbertson witnessed the smoke rising from New York City as they passed overhead. He captured video footage of the smoke, which he described as having an "odd bloom" at its base. He later learned that the pilot of the plane that hit the Pentagon was his classmate, Captain Charles Burlingame.

Today, 23 years after the attack, a memorial exists where the Twin Towers once stood to remember those who lost their lives in the terror attack. Pew Research in its study said, "It is difficult to think of an event that so profoundly transformed US public opinion across so many dimensions as the 9/11 attacks." Culbertson's photograph and his letter serve as a lasting record of that fateful day in American history, noting that things would "never be the same again" after that.