Pictures of space travel are now as vivid as our memories of them.

"When We Left Earth: The NASA Missions" marks a half-century of space exploration with original mission footage, some of it previously unreleased, upgraded to high-definition viewing standards.

The six-hour Discovery Channel series, to air over the next three Sundays (with two different one-hour episodes at 6 p.m. and 8 p.m. Hawaii time), follows the U.S. space program from its beginnings, including John Glenn's famed Mercury flight, the Apollo moon landing and the space shuttle missions.

"This is living American history that's been preserved through this great documentary," says narrator Gary Sinise, who narrated an earlier documentary about the shuttle and played an astronaut in "Apollo 13" and "Mission to Mars."

Sinise, who watched a shuttle launch in person in the 1990s, says he's impressed by the scope of the film. "It's fascinating stuff, to watch how far this program has come, and the challenges and risks and triumphs and tragedies that have come along the way."

Producers reviewed 500 hours of archived film and transferred 150 hours of the footage to high definition. The preservation project was underwritten by Discovery, which considers space a core genre.

In addition to newly released video and audio, the restoration reveals "a lot more information on those films than whatever was seen in standard definition," Discovery executive producer Bill Howard says.

The six one-hour episodes follow the space program from NASA's founding in 1958, weaving in interviews with astronauts, administrators and others who were there. One goal is to tell the story to people who were too young to remember the moon landings, Howard says.

"People under 40 don't know this," he says. "When you find these original films and put faces with missions, you've got real protagonists in an action movie. We wanted people who did not live through it to understand just how big of an adventure it was."

The series includes a rare interview with Neil Armstrong, the first man to walk on the moon. Armstrong also talks about the risks he faced on the lesser-known Gemini 8 mission in 1966, in which the world's first orbital docking maneuver went frighteningly awry.

The film can help maintain public interest and support as the space agency makes the transition to new projects, says astronaut Buzz Aldrin, who walked on the moon with Armstrong: "It's a good thing to remind the public of the steps NASA took toward exploration back in the early days. Fortunately, a few of us are still around who remember those days."

The project also details tragedy, including the deaths of astronauts on Apollo 1 and the Challenger and Columbia shuttles. They are reminders that the public cannot take space exploration for granted, says shuttle astronaut Jeffrey Hoffman, now a professor at MIT.

"One of the things we suffer from is that they make things look so easy that people get blasÈ," says Hoffman, whose participation in a Hubble telescope repair mission is detailed in the film. "One thing you learn (as an astronaut) is that you're operating in a very hazardous and unforgiving environment."

Space missions don't get prime-time treatment anymore, but people can stay in close touch. Cable networks are covering this week's spacewalks outside the International Space Station, and millions have followed the exploration of Mars on the Web.

Technological advancements, such as those that allowed the restoration of the "NASA "Missions" films, can make the public feel more involved, Hoffman says.

"People are getting access. The challenge is: How can we get a good enough experience?" he says. "Ultimately, what I look forward to is creating a virtual reality, so that people can experience what it's like to be on the moon, at least the visual and oral experience."

For now, "The NASA "Missions" tells the story. Says Aldrin: "These are wonderful reminiscences."