Editor's note: Buzz Aldrin, whose new book is "Magnificent Desolation," was one of the two American astronauts who were the first people to set foot on the moon.
[cnn-photo-caption image= http://i2.cdn.turner.com/cnn/2009/images/06/23/art.buzz.aldrin.moon.jpg caption="Buzz Aldrin walks on the moon in a photo taken by Neil Armstrong, his colleague on the 1969 mission. "]
By Buzz Aldrin
Special to CNN
(CNN) - Four decades have passed since the summer of 1969, when Neil Armstrong, Mike Collins and I flew America's first lunar landing mission.
The passage of time has not faded either the memory of that summer or the importance of what we achieved, for our mission was about more than just exploring the moon.
On July 20, 1969, Neil and I were peering out the window of our lunar lander, the Eagle, as it descended toward the lunar surface. The ship's computer was steering us toward a field of boulders the size of cars.
That same computer - with less power than today's pocket calculators - was signaling that it was overworked and dangerously overloaded. Our single tank of fuel was nearly empty as we approached the surface, invisible to us, cloaked in a cloud of swirling dust.
Neil took manual control and flew us toward a smoother terrain. Then, as the shadow of our landing gear appeared etched onto the surface in the gloom, a light on our console flashed that contact had been made.
"Houston, Tranquility Base Here," Neil radioed to the listening world back home. "The Eagle has landed!"
For more than two hours, we explored the moon's dusty surface. We could easily tell that the planet we had landed on was very different from our home. The horizon visibly curved away in the distance, a sign of the moon's much smaller landscape.