As the space shuttle Columbia and STS-107 tragedy unfolds, the overall health of the human spaceflight fleet -- particularly the age of the space planes -- is likely to be a consideration in finding the true cause of the catastrophe.

Columbia is the oldest orbiter in NASA's stable of space planes. Its maiden liftoff took place on April 12, 1981, and was returning to Earth after completing its 28th space voyage.

Columbia was one of four ships that comprised NASA's original fleet, followed by Challenger, Discovery, and Atlantis. With the loss of Challenger in January 1986 -- just over 17 years ago -- that destroyed craft was replaced by orbiter Endeavour.

While far too early to identify what took place at high altitude as Columbia sliced through Earth's atmosphere, the wear and tear on NASA's space planes from repeated liftoffs and landings has come under scrutiny.

Last April, Richard Blomberg, former chair of the Aerospace Safety Advisory Panel (ASAP) -- an independent review group to NASA -- testified before the House Subcommittee on Space and Aeronautics, highlighting his worries about an aging shuttle fleet.

"In all of the years of my involvement, I have never been as concerned for Space Shuttle safety as I am right now," Blomberg said.

"That concern is not for the present flight or the next or perhaps the one after that. In fact, one of the roots of my concern is that nobody will know for sure when the safety margin has been eroded too far. All of my instincts, however, suggest that the current approach is planting the seeds for future danger," Blomberg testified.

Blomberg said that, due to budget shortfalls, many already planned and engineered improvements to the Space Shuttle system have had to be deferred or eliminated. Some of these would directly reduce flight risk. Others would improve operability or the launch reliability of the system and are therefore related to safety, he said.

"Moreover, the current plans and budgets are not adequate even to retain the present Space Shuttle risk levels over the entire likely service life of the system," Blomberg stated.

SPACE.com could not reach Blomberg to further comment on his testimony.

In a recent interview with Ira Flatow, host of the National Public Radio program, "Talk of the Nation: Science Friday" NASA's chief, Sean O'Keefe discussed the aging shuttle fleet.

Responding to the fact that NASA's space shuttle program is more than 20 years old, O'Keefe said shuttle orbiters are not old in the sense of lifetime left to serve.

"All of them have fewer than one-quarter of their operational life expended at this stage," O'Keefe said. "So we've got a lot of miles left," he said December 27 on the NPR program.

O'Keefe noted that the overhaul program per shuttle orbiter has been significant over the years. This type of servicing is dedicated to keeping the fleet in top-notch health. Furthermore, money is being sought by NASA to better coordinate a comprehensive and on-going shuttle upgrade program.

Columbia itself was last refurbished in 1999, undergoing a long line of upgrades and improvements - from better brakes, as well as an enhancement of its TPS, or thermal protection system. Each orbiter is outfitted with a myriad of temperature-thwarting tiles and blankets that protect Columbia's structure during heated reentry through Earth's atmosphere.

Last month, the White House gave NASA a go-ahead to start work on an Orbital Space Plane -- an alternative way to launch crews to the space station on a more rapid and routine basis.

In this regard, the shuttle is like "an 18-wheeler truck type asset," O'Keefe said last month. The Orbital Space Plane "is more like a roadster."

The hope is for an Orbital Space Plane that is capable of "on-demand" takeoffs.

It would be built for speed and greater maneuverability in space, far more than shuttle orbiters. Using the vehicle, NASA could rotate crews in and out of the station more rapidly. Also, select research experts can be flown to the orbiting research base as needed, the space agency chief said.

The target date for the Orbital Space Plane to streak skyward is 2010. NASA has said in recent statements that it sees Space Shuttle operations continuing into 2015, and perhaps beyond.

In his testimony to Congress, Blomberg, said that as systems continue to age, they tend to change.

"Some of these changes are predictable. Others, however, are subtle and often unpredictable. As components and subsystems age beyond their design lives, they may fail more often and with new and unanticipated failure modes," Blomberg said.

Thus, the well-established characterization of the system is no longer fully valid, Blomberg said. "The Aerospace Safety Advisory Panel believes that the Space Shuttle is heading in this direction."

Given the expected long-lived nature of Shuttle orbiters, that fact was flagged by last year's Aerospace Safety Advisory Panel (ASAP) report to NASA.

"A satisfactory crew escape system could be a major source of risk reduction if the Space Shuttle is to be flown for an extended number of years," the independent advisory group on NASA safety reported.

NASA has been exploring crew escape options. Crew ejection seats, crew extraction systems, and a crew compartment/capsule escape system have been under study. A hybrid system -- drawing upon several concepts -- has also been evaluated.

In his Congressional testimony last April, former ASAP chairman, Blomberg, said that the safety group believes that the postponements of shuttle safety upgrades, and delay in fixing aging infrastructure, was troubling. Also, the failure to look far enough ahead to anticipate and correct shortfalls in critical skills and logistics availability, he said, "will inevitably increase the risk of operating the Space Shuttle."

"The problem is that the boundary between safe and unsafe operations can seldom be quantitatively defined or accurately predicted," Blomberg advised. "Even the most well meaning managers may not know when they cross it. This is particularly true for an aging system," he said.