(CNN) -- Blasting off from Earth and hurtling through space at thousands of miles an hour, it takes astronauts three days to reach the moon -- a tiny distance in a universe measured in light years, but a fantastic voyage on a human scale. Buzz Aldrin walks on the moon on July 20, 1969. NASA is preparing to send astronauts back and build a lunar base. Now plans are under way to go back, even as the future of U.S. human space exploration is under close scrutiny and pressure is growing on NASA to aim for another alien world. It's a crossroads Neil Armstrong could have hardly imagined on July 20, 1969, when he became the first human to set foot on the moon and uttered his now famous phrase: "That's one small step for man, one giant leap for mankind."
@highlight
NASA plans to return astronauts to the moon by 2020, build outpost by 2025
@highlight
Constellation Program's new fleet of space vehicles will replace the space shuttles
@highlight
Critics say NASA should skip the moon and set its sights directly on Mars
@highlight
Independent panel is reviewing plans and looking at "potential alternatives"
"We won the moon race; now it's time for us to live and work on @placeholder," Aldrin wrote in a commentary for CNN.