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 tragedy took a terrible toll — and in each case, NASA learned from its mistakes:

Apollo 1's three astronauts were Gus Grissom, one of the Mercury 7 pioneers; Ed White, the first American to do a spacewalk; and rookie spaceflier Roger Chaffee. They died during a pre-launch test at the launch pad when bad wiring sparked a blaze in the pure-oxygen environment inside their sealed capsule. After the fire, engineers overhauled the wiring system, switched over to a less flammable oxygen-nitrogen mix and redesigned the hatch to open outward instead of inward. Years later, Apollo 11 commander Neil Armstrong observed that the accident provided "the gift of time" — a chance to change a lot of things for the better. "We got that added benefit, but we regret the price we had to pay," Armstrong said.

Challenger's crew of seven was led by commander Dick Scobee, but the best-known flier was Christa McAuliffe, who was tapped to be the first teacher in space. The other astronauts were Michael Smith, Ellison Onizuka, Judith Resnik, Ron McNair and Greg Jarvis. Their space shuttle blew up 73 seconds after launch, due to a bad seal on one of the solid rocket boosters. The investigation led to a redesign of the boosters, which worked without fail ever since. It also pointed up the problem of "go fever," which led NASA to give the go-ahead for launch amid dangerously low temperatures. Reforms in management procedures gave astronauts, engineers and contractors more of a role in ensuring launch safety. 

Columbia's crew included Israel's first astronaut, Ilan Ramon, as well as commander Rick Husband, David Brown, Laurel Clark, Michael Anderson, Kalpana Chawla and William McCool. The shuttle broke up over Texas during its descent at the end of a 16-day science mission. Investigators concluded that flying foam insulation from the external fuel tank damaged the left wing during launch, setting the stage for the Feb. 1 tragedy. The fuel tank was redesigned, emergency rescue plans were updated, and an array of cameras was added to the shuttle to watch for damage. The investigators also pointed to lapses in NASA's "safety culture." The George W. Bush administration followed up on the investigative panel's recommendations and decided to close down the space shuttle program once construction of the International Space Station was complete. That day finally came on July 21, 2011, with the landing of the space shuttle Atlantis.

Bolden said the successful operation of the space station and the rise of a new generation of commercial space vehicles would not have been possible if it weren't for the sacrifices made by the fallen astronauts. Rather than shutting down America's space program, political leaders gave the go-ahead for more ambitious plans to go beyond Earth orbit, and ultimately to Mars.

"If we didn't have that coming along, then what would have been the point of losing them?" Bolden said. 

To recognize those sacrifices, Bolden will attend a space conference being conducted in Ramon's honor this week in Israel, and then will return to Washington in time for Friday's wreath-laying ceremony at Arlington National Cemetery. NASA's space centers are planning commemorations as well: Officials at Johnson Space Center will participate in memorial events in Texas on Thursday and Friday. Kennedy Space Center's ceremony is scheduled for 10 a.m. ET Friday at the visitor center's Space Mirror Memorial. That Florida observance is open to the public and will be broadcast on NASA TV.

Stay tuned for more about NASA's week of sad celebration in the days ahead — and feel free to add your own reminiscences and tributes as comments below.

More about NASA's space tragedies:

• Apollo 1's tale retold: 'Fire in the cockpit!'
• The chilling saga of the shuttle Challenger
• Columbia remembered, 10 years after launch
• Flash interactive: NASA's Day of Remembrance

Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log as well as NBCNews.com's other stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.