Today marks the fourth anniversary of NASA’s Columbia accident and the loss of the shuttle’s STS-107 astronaut crew.

But for the first time since that accident, NASA’s shuttle fleet is back in full swing and construction of the International Space Station (ISS) is back underway.

NASA held an agency-wide Day of Remembrance Monday to commemorate the loss of Columbia and its seven-astronaut crew, as well as honor the memories of those lost in the 1986 Challenger accident and Apollo 1 fire among others. But Columbia remains fresh in NASA’s collective conscience, the space agency’s chief said recently.

“We lost Columbia, that was as tragic as it gets in this business,” Griffin told space agency employees in a recent update. “It called into question our ability to fly the shuttle safely, and it called into question our ability to complete the space station. And that’s a program which now has, you know, two decades of commitment behind it.”

But Griffin says that NASA’s three successful shuttle missions – STS-121, STS-115 and STS-116 – laid those questions to rest, and the last two shuttle flights in particular returned the U.S. space agency back on the road to fulfill its ISS construction commitments.

“We showed we could the big, hard and difficult things still,” Griffin said. “And [2007] is an opportunity to demonstrate that further.”

NASA plans to launch five shuttle flights to the ISS by the end of 2007, a year that will see the deliver of new solar arrays, a connector node, a European laboratory and a Japanese logistics module to support that country’s own ISS laboratory. No less than 23 spacewalks are planned throughout the year for shuttle and ISS astronauts to complete the assembly work. 

“We’re moving into really complex missions to the station,” Griffin said. “You have to wish that program well.”