But in recent years, the security badges around the necks of most of the people in Mission Control have changed. They no longer work for NASA.

Instead, all but four of the people in Mission Control during recent shuttle flights were drawing their paychecks from a partnership between two of the world's biggest aerospace corporations.

The privatization of much of NASA's most visible work in 1996 drew little attention outside the insular world of aerospace management and engineering. But the United Space Alliance, a 50-50 partnership between Boeing Co. and Lockheed Martin Corp., is certain to come under increased scrutiny after the loss of Columbia while under the management of alliance employees Saturday morning.

"Most of the people I know at NASA have a feeling that contractors have gained too much influence there," said Robert Park, a University of Maryland physicist and space program expert. "The difference is, they (contractors) have to show a profit."

From the start, the contract generated controversy. Bryan O'Connor, then-director of the space shuttle program, resigned in what associates called a protest against the privatization plan. He later rejoined the agency as a safety administrator.

The move to assign some NASA work to the United Space Alliance came amid a major profit-driven consolidation of space and defense companies. During the past two decades, more than 70 private companies morphed into just five: Lockheed Martin, Boeing, General Dynamics, Raytheon and Northrup Grumman. The contract between NASA and the alliance was intended to save government money by reducing the number of separate contractor arrangements the space agency has to manage. Instead of working with more than 25 contractors, NASA could let the alliance handle it.

The initial deal was for $7-billion for six years. But incentives built into the contract brought its value to about $9-billion. Already, the contract has been extended for an additional two years for nearly $3-billion, and that number could grow.

Eighteen percent of the contract is contingent on meeting efficiency goals through cost-cutting. An additional 40 percent is an incentive "earned by safely achieving mission and schedule objectives, with positive financial incentives and penalties ascribed to each space shuttle mission."

"We don't make our money by saving money for NASA," United Space Alliance spokesman Mike Curie said. "We make money through safely operating the program."

Curie said the alliance's legal staff was researching the contract to determine what, if any, financial penalties it might face for the loss of Columbia and the death of its seven astronauts.

Although there are incentives in place for operating safely, the alliance -- and NASA's oversight of it -- has been criticized twice by the space agency's chief watchdog, the inspector general.

The first report, in March 2001, found that NASA staff at the Johnson Space Center in Houston had failed to monitor the work of alliance employees. As a result, United Space Alliance employees failed to make proper reports of unusual incidents during space shuttle planning and flights.

The second, in June, found that NASA employees at the Kennedy Space Center also were not keeping an eye on alliance employees. Instead of constantly monitoring the ground operations around the shuttles, NASA conducted audits and occasional surveillance to make sure alliance workers were doing their jobs right.

Additionally, the report said, United Space Alliance workers had used ground equipment without making sure it was safe, potentially damaging sensitive shuttle hardware.

All but four people directly managing or supporting a shuttle mission are typically employees of the alliance. The flight director, the flight surgeon, the public affairs officer and the person primarily responsible for talking to the crew are NASA employees. But thousands of others, from the people training the astronauts to those making the sensitive thermal tiles, are among the more than 9,200 employees of the alliance.

When news reports refer to "NASA engineers," the people they describe are frequently employees for United Space Alliance. NASA still has its own engineers, but alliance experts work side-by-side with them.

Curie said the mingling of government employees with corporate engineers should not be a concern for the public.

"Many (of the alliance employees) have been here since the shuttle program began, and they've worn a lot of different security badges during that time," Curie said. "But they consider themselves working for the shuttle program."

As the flight director and other NASA employees sought opinions on whether a piece of foam from the external fuel tank damaged Columbia during its launch Jan. 16, many people who advised them were working for the United Space Alliance, not NASA.

And now, as an independent panel and hundreds of others try to determine the cause of Columbia's disintegration, the NASA inspector general's office will review all the decisions made before and during the flight to see whether NASA rules were violated.

"We also plan to closely track the official agency investigation as it unfolds, to assure the administrator, the Congress and the taxpayers that all issues associated with the Columbia tragedy are satisfactorily dispositioned," the inspector general said.

Although "safety first" was the watchword of shuttle launches, aerospace engineers have repeatedly complained that belt-tightening and shifting priorities were denying Columbia and the three other shuttles the necessary upgrades and improvements.

As recently as last April, the chairman of the Aerospace Safety Advisory Panel warned Congress that NASA's management of the shuttle program had drawn "the strongest safety concern the panel has voiced" in 15 years. "I have never been as worried for space shuttle safety as I am right now," said Richard Blomberg, who was then chairman of the panel.

None of this was supposed to happen. The last lethal shuttle disaster, 17 years ago, provoked calls for revolutionary changes in the program's management. The agency promised that safety would henceforth be put far ahead of all other considerations, including budget constraints, the demands of its users and political pressures.

"We will never launch when it is unsafe," Fred Gregory, then NASA's director of space flight, promised the House science and space subcommittee nine months ago.

While none of those who issued warnings pointed specifically to a defect immediately known to be implicated in yesterday's disaster, they warned repeatedly that safety was losing the battle for scarce NASA funds. The program's 40 percent budget decline over the past decade had undermined its ability to guarantee flawless performances, they said.

NASA's response was mostly to say it disagreed: The problems were not that bad; safety was still the top priority; and the number of shuttle "anomalies" or defects was dropping fast. "NASA will continue to ensure that an adequate staff and shuttle workforce" is available to maintain a perfect record, Gregory promised.

But safety experts have long said NASA's claim that safety was improving stemmed from an illusion. The shuttle, they said, was an aging, balky and delicate space truck that exceeded NASA's own risk limits for manned flight. Time was not its friend.

The ungainly glider was created in the 1970s through a marriage of adventurous design and well-known technology, and it was considered underfunded from the outset. By all accounts, the program has never really embraced the past decade's stunning advances in aerospace engineering and safety testing.

After the shuttle Challenger exploded on launch in 1986, for example, numerous safety advisers urged that a crew ejection capsule be added to save lives even in the midst of calamity. "There is a clear need . . . to develop a plan to address the absence of an escape system by either upgrading the space shuttle or initiating a program with a realistic timetable to replace it," the Aerospace Safety Advisory Panel concluded last year.

NASA has studied the problem for years, but the costs of retrofitting such a device kept it from acting. As a result, Columbia's crew had no choice but to follow the craft's fate as it broke up around the point of reentry into Earth's atmosphere.

No new shuttle has been in development, and, in fact, many of the most recent safety alarms stemmed from the agency's recent plan to try to extend the life of the current shuttles by an additional 25 years. Blomberg warned, in particular, that budget-tightening compelled the shuttle program to spend most of its resources on current operations while planned improvements, including some that would "directly reduce flight risk," were deferred or eliminated.

"The concern is not for the present flight or the next or perhaps the one after that," Blomberg said last April. "One of the roots of my concern is that nobody will know for sure when the safety margin has been eroded too far."

"Repeated government and contractor hiring freezes" during the shuttle's operating life "have led to a lack of depth of critical skills" that become more troubling as the system ages, Blomberg said.

In an implied criticism of Congress and the White House, the panel said in its most recent published report that NASA's budgets were "not sufficient to improve or even maintain the safety risk level of operating the space shuttle. Needed restorations and improvements cannot be accomplished under current budgets and spending priorities."

In mid-2001, five of the nine members of the aerospace advisory panel and two consultants were asked to step down after NASA changed its charter and required rotating memberships. They were replaced, but the year "was one of significant upheaval on the panel," the panel said in a report last March.

The most detailed independent assessment of shuttle safety in recent years, completed in March 2000, pointed to specific problems that analysts yesterday said may have played a role in Columbia's breakup around the time of its reentry into Earth's atmosphere.

It called for scrutiny of wiring problems in "difficult-to- inspect regions" of the Columbia shuttle, in particular, a problem that NASA said it had fixed. It also said that NASA was not using the latest scientific techniques to find and fix structural cracks and other consequences of routine aging. The panel said further that NASA was not working hard enough to find and fix corrosion beneath the tiles that protect the shuttle from intense heat during reentry, and that the agency was not working hard enough to find a way to probe or study portions of each shuttle's structure -- one-tenth, on average -- that are entirely inaccessible.

"The large reduction in NASA quality assurance inspectors for each shuttle is very disturbing," said the panel, which was chaired by Henry McDonald, director of NASA's own Ames Research Center.

Some of the safety alarms stemmed from what experts have described as inadequate NASA oversight of those parts of the program that have been privatized. Just three days ago, for example, the General Accounting Office (GAO) described NASA's management of its major contractors as "weak" and "debilitating," and accused the space agency of placing "little emphasis on end results [or] product performance."

The GAO report was the latest in a series by the congressional auditing agency faulting NASA's management of major programs, including the shuttle. Weak contract management had been ranked as a "high-risk" problem at NASA since at least 1990, the report said.

The increased fiscal pressure on NASA is partly the result of steep budget cuts over the past decade. Funding for NASA and other civilian agencies involved in the space program was slashed by $1 billion in fiscal 2002, while Defense Department spending on space programs rose by $600 million, according to a recent study by the Aerospace Industries Association, an industry trade group.

"The civil space program that NASA runs has been neglected for a generation, and as a consequence we find ourselves flying increasingly aged technology," said Loren Thompson, a defense industry analyst for the Lexington Institute, a think tank.

In 1996, NASA turned over space shuttle flight operations to the United Space Alliance, a private firm owned by Boeing Co. and Lockheed Martin Corp. Under pressure from the Clinton administration and Congress to cut costs, NASA had gradually shifted many responsibilities to the private sector.

United Space is now considered the prime contractor for the space shuttle program and manages about a third of the program's budget. In addition to its role as part of United Space, Bethesda-based Lockheed also provides many crucial functions, including construction of the external tank that feeds liquid propellant to the shuttle's three main engines. It also develops the electronic systems that perform navigation, guidance and flight control for the space program and manages data collection, said spokesman Tom Jurkowsky.

While NASA managers have described their contractor oversight as adequate, NASA's Office of Inspector General disagreed. "The lack of systematic and well-documented contract surveillance is a particular area of concern," the inspector general said in a report last June.

In response to such reports, NASA has sometimes sought increased funding, added backup systems and new safety routines, and has taken other steps designed to bolster its already complex procedures for preventing accidents. But NASA has always acknowledged that the program would never be 100 percent reliable.

After the post-Challenger safety upgrades, NASA estimated there was a 1-in-250 chance of catastrophic failure and fatalities. "The shuttle is a wonderful machine, but it is at that left-hand bar of risk to humans," Sam Venneri, head of NASA's aerospace technology office, said in congressional testimony last year.

John Pike, a director of GlobalSecurity.org and a longtime critic of the agency's shuttle management, said even this estimate is speculative. "NASA has been wildly unrealistic about shuttle reliability. If you go back and look at their reliability estimates, I think that they basically just made them up," Pike said.

"There's a statistical chance every time you launch that there could be an accident. This is just a very hard thing to do," John Logsdon, director of the Space Policy Institute at George Washington University, said yesterday. "It's only in retrospect that you can say that it wasn't enough. Second-guessing is easy. Hindsight is wonderful."

"What people have done to keep an old system flying is just amazing. But it's an old system. At some point, they had to expect something to go wrong," said Donna Shirley, a former Mars exploration manager at NASA and now an instructor in aerospace engineering at the University of Oklahoma in Norman. "It's remarkable that they've kept it going this long."

Brian Chase, executive director of the National Space Society, noted that NASA had also struggled with aging support facilities crucial for maintaining the fleet. "There's definitely been concern about the facilities on the ground," Chase said. "Everything from rusting pipes to crumbling concrete."

Staff writer Eric Pianin contributed to this report.