CAPE CANAVERAL ? NASA is moving to protect ground crews, astronauts and VIPs from potential disaster, closing shuttle Atlantis' launch pad to all but essential personnel even before a three-day countdown starts Tuesday.

No up-close-and-personal tours will take place at Kennedy Space Center's launch pad 39A next week after NASA finishes pressurizing helium and nitrogen tanks aboard the 22-year-old spaceship, which is scheduled to launch at 7:38 p.m. Friday.

The precaution is being taken to reduce the risk created by aging pressure vessels in the orbiter that could burst, triggering a rocket fuel fire or explosion that might injure or kill workers and seriously damage the launch pad.

The tanks "can explode without warning at normal operating pressures," a safety bulletin sent to workers Friday said. Shrapnel from an exploding tank "would be dangerous to personnel and could potentially puncture adjacent hypergolic fuel systems resulting in fire, explosion and toxic cloud release," the bulletin added.

NASA records obtained by FLORIDA TODAY under the Freedom of Information Act show that the problem is one of the top risks facing the shuttle program. It is also an example of obsolescence issues bound to crop up before NASA retires the three orbiters in 2010.

"It's a serious problem, and it is one of a number of problems that I would call 'aging aircraft' issues that we face with these vehicles," shuttle program manager Wayne Hale said. "So we're trying to understand exactly what the hazard is, and how to accommodate it."

Here's the situation:

Shuttle orbiters are equipped with 24 helium and nitrogen gas tanks that pressurize the shuttle's main propulsion system, orbital maneuvering engines and nose-and-tail steering thrusters.

The spherical tanks provide pressure needed to push rocket propellants into shuttle engines and thrusters at very specific rates required to keep the spaceship on its proper course. Some of the propellants are highly volatile and ignite on contact.

Ranging in diameter from 19 to 40 inches, the tanks have lightweight titanium or steel shells wrapped with the same type of fabric used to make bulletproof vests -- Kevlar -- or carbon graphite. They hold helium and nitrogen gas at extremely high pressures (up to 4,600 pounds per square inch) and are extraordinarily dangerous.

"You certainly wouldn't want a 4-foot-diameter helium bottle that's pressurized to about 4,000 psia to burst on you," Hale said. "That would be a bad thing."

A tank rupture on the ground could lead to a fire or explosion that could injure or kill workers in the launch pad area. A failure in flight could lead to the loss of a shuttle and the astronauts inside.

NASA nonetheless is poised to proceed with the launch of Atlantis and seven astronauts on an International Space Station assembly mission.

"I would characterize it as serious but not a showstopper kind of problem," Hale said. "It's certainly not something somebody has trumped up to get attention. I mean, this is a nontrivial engineering problem. It's quite complex."

Built for NASA in the late 1970s and early 1980s, the copper-colored spheres were designed, developed, manufactured and tested for 10 years of shuttle fleet operations.

The NASA records show that proper engineering analyses were done in 1988 to certify the tanks for an additional decade of use. But no subsequent recertification was done in 1998 when the agency's extended warranty expired.

NASA engineers raised questions about the tanks, which are named Composite Overwrap Pressure Vessels, as the agency was struggling to return the shuttle fleet to service after the 2003 Columbia accident.

The aerospace industry already had expressed concern about the structural integrity of similar tanks on satellites and aircraft, and the agency's newly anointed NASA Engineering and Safety Center took up the cause in 2004.

The safety center's engineers concluded the orbiter tanks are much more likely to fail than NASA previously thought.

Past NASA analyses assumed the tanks would leak before they burst. New studies and tests show that they would explode before they leaked, increasing the hazard considerably.

The new tests were done at NASA's White Sands Testing Facility in New Mexico, and another series aimed at more accurately pinpointing the risk is getting under way.

New tanks are not an option. The original vendor is out of the business and the agency would not be able to qualify a new producer before the shuttles' retirement. So NASA is changing the way it operates to reduce the risk.

Pressure within the tanks aboard Atlantis will be brought up to 80 percent -- rather than 100 percent -- this weekend, a move meant to lessen the amount of time full pressure is maintained before launch. The launch pad will be cleared of all but essential personnel when pressure is increased to full flight levels Monday.

"We go through our standard two-stage pressurization trick now and do that as close to launch countdown as possible. The principal risk is to the ground crews, and so we clear the pad when we go through the pressurizations," launch director Mike Leinbach said.

"This is an ongoing activity," Hale said. "We believe that we have an adequate level of safety for the upcoming flight, and we have a longer program of engineering tests to try to more specifically indicate what we might do to mitigate those problems."

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