Chicago, ILL-- As the nation marks the sixth anniversary of the Space Shuttle Columbia disaster, questions remain about the future of the space program now that there's a new president. There is particular concern about the aging fleet of orbiters and proposals to extend the life of the shuttle program beyond its planned retirement next year.

Columbia broke apart while re-entering the Earth's atmosphere early on the morning of Feb. 1, 2003, bringing a tragic end to what had until then been a successful 16-day science mission. The orbiter suffered a catastrophic failure and broke apart just 15 minutes short of the Kennedy Space Center landing strip. The shuttle's destruction claimed the lives of mission commander Rick Husband, pilot Willie McCool and mission specialists Michael Anderson, Kalpana Chawla, David Brown, Laurel Clark and Ilan Ramon, Israel's first astronaut.

Investigators would later trace the physical cause of the accident to a suitcase-sized chunk of foam that popped free from Columbia's external fuel tank during launch. The foam punched a hole in the orbiter's heat shield along its left wing leading edge leaving it vulnerable to the superheated atmospheric gases during re-entry.

Now, just a year from the shuttle program's scheduled retirement and after six years of safe and successful missions, NASA finds itself defending further use of the aging shuttles.

"It's not necessarily a question of are they're old or decrepit or reached the end of their service life," said Wayne Hale, NASA Deputy Associate Administrator of Strategic Partnerships. "I think there's an inherent risk level in flying the space shuttle." Hale is the former Space Shuttle Program Manager and had a key role in the Columbia disaster investigation.

"We know now that the design has some unforgiving characteristics and every time we fly the shuttle we do it at significant risk despite our best efforts to control that," said Hale during an interview with Newsradio 780 WBBM. "I think every day about Columbia and how that came about and how we can prevent similar events."

Lessons learned from Columbia have been applied to every shuttle mission since the tragedy.

"Columbia really hammered home the notion that the spacecraft is pretty fragile and even the slightest damage could result in the deaths of the astronauts," said journalist Pat Duggins who is also the author of "Final Countdown."

During his 20 year career, Duggins has covered more than 85 shuttle missions. His book chronicles the highs and lows of the shuttle program and details some of the safety measures put in place after Columbia.

"When the astronauts launch, the first two days of the mission are spoken for," he said. "The astronauts go through this really laborious process of scanning all the tiles, all the heat shields up and down the shuttle very meticulously to make sure there are no problems or cracks. After that NASA decides whether to make repairs in orbit. All of that has become part of the culture of NASA," Duggins told Newsradio 780.

NASA's three shuttle fleet - including Discovery, Atlantis and Endeavor - has nine flights remaining before the 2010 retirement deadline. According to Duggins, the orbiters - initially designed for just a 10 year lifespan - will soon require service.

"Just like you take your car in for a 3,000 or 6,000 mile check, the shuttles are coming due for their 3,000 or 6,000 mile check and that's going to be money NASA doesn't have."

NASA must also address the five-year gap between the shuttle retirement and the first manned flight of Orion. Proposals being considered call for either two year or five year extensions of the shuttle program to include at least two missions per year. The space agency estimates it would cost three-billion dollars a year to keep the orbiters flying past 2010 and raises the risk of losing a vehicle or crew to one in four. The risk of losing an orbiter or crew on any given mission is 1 in 77.

"If we were to lose another shuttle it would definitely disturb the program drastically," said space expert and author Robert Zimmerman. "You would only have two shuttles and you would have to find out why the shuttle was lost and fix it."

But another shuttle accident would probably not end NASA's manned space program, according to Zimmerman who cited he Columbia disaster as an example.

"After Columbia was lost in '03, not only did NASA get a boost in funding but it got a whole new clear vision for how it should move forward and a commitment to go to the moon. That was something it had not had since President Kennedy in the 60's. It was a complete boost," said Zimmerman.

According to experts, each shuttle was designed for 100 trips into space. Discovery, the dean of the fleet, has launched only 30 times. The entire fleet has gone up only 113 times.

Many of NASA's top scientists and engineers oppose plans to continue shuttle flights, Hale among them.

"It takes a lot of money to fly the shuttle and we believe it would be better for the country and the agency to put our resources into the next generation space vehicle which is the capability to go to the moon and on to Mars."

Former NASA Administrator Michael Griffin also urged the Obama Administration to retire the shuttle as planned. "The recommendation I would have for the new administration would be to stay the course," he said.

On Thursday, NASA's Kennedy Space Center paid tribute to the crews of Apollo 1 and space shuttles Challenger and Columbia as well as other NASA colleagues during the agency's Day of Remembrance observance. Kennedy Space Center Director and former astronaut Bob Cabana took part in a wreath-laying ceremony at the Space Mirror Memorial at the Kennedy Space Center Visitor Complex.

President Barack Obama issued this statement:

"The arrival of a new year reminds us that life is a journey, one that takes us on many unexpected paths. NASA's role is to pioneer journeys into the unknown for the benefit of humanity. Along the way, we sometimes experience tragedy instead of triumph. Today, we pause to reflect on those moments in exploration when things did not go as expected and we lost brave pioneers. But what sets us apart as Americans is our willingness to get up again and push the frontiers even further with an even stronger commitment and sense of purpose."

"On this Day of Remembrance, we remember the sacrifices of those who dared to dream and gave everything for the cause of exploration. We honor them with our ongoing commitment to excellence and an unwavering determination to continue the journey on the path to the future."

The Astronauts Memorial Foundation, a private, not-for-profit organization, built and maintains the Space Mirror Memorial. It was dedicated in 1991 to honor all astronauts who lost their lives on missions or during training and since has been designated a National Memorial by Congress.