too bad some of the posters around here can't be bothered quoting their actual links (especially when starting a new thread)

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.