NASA presents a video tribute to the astronauts of the Apollo 1, Challenger and Columbia tragedies.

By Alan Boyle, Science Editor, NBC News

This should be the saddest week of the year for NASA which is marking the anniversaries of three fatal tragedies, including the 10th anniversary of the shuttle Columbia's catastrophic breakup. But the way NASA Administrator Charles Bolden sees it, this week is not just about mourning 17 dead astronauts.

"I think this is not a memorial. It's a celebration, because of what they made possible," he told NBC News this month during a visit to Seattle. "We're commemorating them, and we're thanking them by continuing to move forward and not dropping back and dwelling on the pain. They'd be pretty angry, I think, if they saw that."

The week of celebration and, yes, of commemoration begins on Sunday with the 46th anniversary of the 1967 Apollo 1 launch-pad fire. The 27th anniversary of the 1986 Challenger explosion follows on Monday. This year, NASA is focusing the most on Friday, the 10th anniversary of the Columbia tragedy, which has been set aside as the agency's "Day of Remembrance" for all of its fallen astronauts.

Ever since the loss of Columbia and its crew of seven, NASA has organized solemn commemorations during the last week of January.

"We honor the memory of all three crews that were lost over the history of human spaceflight," Bolden explained. "We thought it was fitting that it be somewhere around the dates of those three losses. We think about this every day, to be quite honest. But we take these particular times and set them aside, when we can let everyone else around the world join us and help celebrate."

There's that word again.

"I use the term 'celebrate' because we have to remember that, yeah, we lost some valiant people but what their sacrifice brought is what we should really be thinking about: the fact that they dared to challenge and do things differently," Bolden said. "Because of what they did, we're well on the cusp of going deeper into space than we've ever gone before."

Each tragedytook a terribletoll and in each case, NASA learned from its mistakes:

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NASA celebrates its fallen astronauts