"Never forget." A statement embedded in the hearts of Americans 20 years after the harrowing terrorist attacks on the World Trade Center. September 11 has always been so traumatic yet memorable not only for ordinary Americans, but also for the sole NASA astronaut on the International Space Station during the time of the attacks.

Expedition 3 Commander Astronaut Frank Culbertson was on board the ISS on that fateful day, and the only American in the crew. When he learned of the attacks, Culbertson immediately documented the event in a series of photographs from space as the ISS traversed New York City at exactly the same time the planes hit the Twin Towers.

NASA Astronaut Captures 9-11 Images from Space: 'How Horrible'

Culbertson took dramatic images in the minutes and hours following the terrorist attack. As he captured those images, Culbertson shared his thoughts on the terrible events that are unfolding beneath him. "Te world changed today," Culbertson said in his public letter published on NASA's website, adding that what he says or does is "very minor compared to the significance" of what happened to America during the attacks.

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In the images he captured, Culbertson said it was "horrible to see smoke pouring from wounds" in his country that is being attacked "from such a fantastic vantage point." He said that being in a spacecraft dedicated to improving life on Earth and watching life being destroyed by such terrorist acts is "jolting to the psyche , no matter who you are."

The photographs have been published on the NASA website to commemorate the 20^th anniversary of the attacks.

The smoke, Culbertson said, seemed to have "an odd bloom to it at the base of the column that was streaming south of the city." He said he had been reading news articles and looked at New York as the South Tower collapsed. Seeing the event, Culbertson said, "how horrible."

As the U.S. mark the devastating attacks, ISS astronauts are paying their tribute from space.

ISS Astronauts Honor 9/11 Victims, Survivors, First Responders

NASA astronaut Shane Kimbrough, one of seven ISS crewmembers from the U.S., Russia, Japan, and France, beamed a video message to Earth to honor the people who lost their lives and were affected by the attacks, Space.com reported.

As an American flag floated in the background as he spoke from the ISS' s Japanese Kibo laboratory, Kimbrough said to all the 9/11 victims, survivors and first responders, "We remember."

Those terrifying images of that day "are still present in so many of our minds," Kimbrough said.

Kimbrough added that while many struggled to understand the consequences of the attacks, it led to a wave of patriotism among Americans in support of those affected. He also thanked the military and other entities who continue to work tirelessly to prevent such attacks in the future.

Despite the unfortunate event, it brought out the "strength and resilience of our nation," which saw the "incredible support from people around the world." People, like him, who lived through the event had the opportunity to share what was learned and what can be accomplished as a nation, Kimbrough added.

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