When Discovery launches within the July 13-31 window from Cape Canaveral, Fla., its enhanced safety measures will include a new external fuel tank designed to keep foam pieces of any significant size from breaking off and possibly seriously damaging the shuttle.

Foam debris was determined to have caused of the Columbia accident. During liftoff on Jan. 16, 2003, a briefcase-sized chunk of foam pealed off the external fuel tank and punched a hole in the protective heat shield on Columbia's left wing.

As the orbiter maneuvered toward Earth on Feb. 1 after its two-week scientific mission, hot gases generated during re-entry seeped through the hole in the wing, melted the aluminum structure underneath and caused the shuttle to destabilize and disintegrate in the skies above Texas, killing all seven astronauts aboard.

The investigation into Columbia's demise was triggered that day -- a mechanism put in place after the Challenger explosion in 1986.

And in the summer of 2003, the Columbia Accident Investigation Board declared the cause of the accident the foam strike. The CAIB gave the National Aeronautics and Space Administration a series of benchmarks it had to meet before launching another shuttle.

The board's 15 return-to-flight recommendations included mechanical safety upgrades and procedures, but also changes in the NASA environment to foster communication among the agency's ranks.

(Click here for the full investigation report.)

Another independent board, called the Return-to-Flight Task Group, led by retired Air Force Lt. Gen. Thomas Stafford and former astronaut Richard Covey, monitored NASA's compliance with the CAIB recommendations.

The panel, made up of members from industry, academia and the government, visited NASA sites and contractor facilities, conducted interviews, collected information from outside experts, attended NASA meetings and observed NASA tests to conduct its reviews, according to the panel's spokesman Dave Drachlis.

The eventual retirement of the space shuttle, set for 2010 under President George W. Bush's directive, had little impact on the task group's assessment activities, which focused on NASA's compliance to the CAIB report, Drachlis said.

The task group determined at its last meeting on June 27, 2005 that NASA had met all but three of the recommendations: eliminating the chance that foam and ice could break off the external tank and damage the orbiter, making the shuttle more resilient to debris hits, and coming up with a practical way astronauts could fix the heat shield in orbit.

Although NASA hadn't reached full compliance in its efforts to address those critical safety needs, the task force still deemed the shuttle ready to return to use.

"We feel that it is a safe vehicle to fly," panel member Joseph Cuzzupoli said.

Both the CAIB and oversight panel emphasized that risk can never be eliminated. "We have often heard the safest shuttle is one that never leaves the ground," the Return-to-Flight Task Group wrote in its Jan. 28, 2005 report.

NASA culture
The Columbia Accident Investigation Board found NASA's culture to be as much responsible for the shuttle disaster as technical problems.

CAIB Chairman retired Navy Adm. Hal Gehman said in a NewsHour interview when the board's final report came out in August 2003 that NASA had emphasized cost constraints and sticking to schedules over not just safety, but research and development as well.

"As that process kind of morphed itself over the years, questions like why is this happening and this is a violation of our rules but should we continue to live with it, those questions aren't even asked anymore," Gehman said. "Instead questions like are you going to make the schedule and can you get the cost controls -- and those are the kinds of questions that will get asked."

(Click here for the full interview with Gehman.)

In response, NASA promised to create a culture that fostered communication, encouraged employee ownership over work, ensured procedures are fully understood and followed, and integrating checks and balances of technical and safety standards.

The long-term cultural changes that the space agency needed to make did not have to be completed, just plotted out, before the next shuttle launch.

According to Return-to-Flight (RTF) spokesman Drachlis, "The CAIB realized that cultural changes occur over time and did not specifically address cultural changes in its RTF recommendations.

"They did, however, recommend organizational changes that might begin to drive behavioral changes that could eventually affect culture over the long term," he said. "The CAIB expectation was that a plan for those organizational changes would be in place prior to the next space shuttle flight. In fact, most of the organizational changes have been or are in the process of being implemented."

Redesigned fuel tank
One of the main modifications necessary before the space shuttle could fly again was the creation of an external fuel tank that would not shed its foam insulation on lift-off. In January 2005, NASA rolled out its new tank.

However, subsequent engineering tests showed the shuttle could still be vulnerable to ice breaking off the external fuel tank, so program managers at NASA delayed the shuttle's target launch twice.

Although the oversight panel said NASA fell short of eliminating the risk of falling foam, NASA Administrator Michael Griffin announced in late June 2005 that Discovery would launch as planned in July and proceed with its mission to restock the International Space Station and monitor and test the new safety devices.

Tests on the new 154-foot, bullet-shaped tank proved foam chunks no larger than .03 pounds -- about the size of two marshmallows -- would fall from the tank during shuttle launches.

"This is the safest, most reliable tank NASA has ever produced," said Sandy Coleman, NASA's external tank program manager, in a telephone news conference from Marshall Space Flight Center in Huntsville, Ala., according to the Associated Press.

The external fuel tank contains engine propellants liquid hydrogen and oxygen, which -- in their supercooled state -- can cause ice to form on the tank's metal skin. The foam was intended to prevent ice from forming and breaking off, possibly injuring the orbiter.

The fuel tank, which is jettisoned 8.5 minutes into the shuttle's flight, also no longer has two ramp-shaped wedges of foam once placed around metal struts connecting the tank to the orbiter. The investigation revealed it was one of these wedges of foam that slid off the left side of the tank and crashed into Columbia's wing.

Instead, the tank now has electric heaters to prevent ice formations and a video camera to monitor the tank during launch. NASA now has one person spray the foam on the tank while another person watches, along with a videotape and high-fidelity mockup to check the procedure, said Neil Otte, NASA's chief engineer for the external tank, reported the Washington Post.

Additional safety measures
Another requirement in the resumption of shuttle flights is the ability of astronauts to repair damage to the orbiter in space.

Discovery's astronauts plan to test two methods of repairing the reinforced carbon-carbon heat shield -- with a caulk gun to seal small cracks and with a "wash" that acts like a heat reflector on damaged tiles -- on test panels in the open shuttle bay.

The crew also will test plugs for reinforced carbon-carbon by screwing flexible patches over holes in the heat shield.

If the shuttle is damaged beyond repair, NASA must be prepared to send a rescue mission, the recommendations say. NASA said it would not launch a shuttle unless a second shuttle can be prepared and launched within the timeframe that the International Space Station can provide accommodations, including food, water and oxygen, for the stranded crew.

In addition, NASA is using new sensors, images and cameras to more thoroughly monitor the shuttle's heat shield, and getting at least three useful views during liftoff to help engineers identify any damage that would need to be repaired while the shuttle is in orbit.

Additional Outside Links:
Independent Return-to-Flight board

NASA's return to flight Web site

Columbia Accident Investigation Board

-- By Larisa Epatko, Online NewsHour