Simplest Atom Is Proving to Be Pesky Stuff to NASA : Space: Hydrogen, the most common element in the universe, is causing difficult problems as shuttle fuel.

Hydrogen, the most common element in the universe and the simplest atom of all, is the latest tyrant to bedevil the National Aeronautics and Space Administration.

Leaks of the pesky stuff grounded the nation’s shuttles, and experts with the space agency admit they are baffled about why they are having so much trouble controlling something that has been used as rocket fuel for decades. Some believe the problem has always been there and has only recently been discovered.

The latest leak, which is quite different from earlier leaks that have grounded the space shuttles for four months, resulted from the failure of a seal not much larger than a silver dollar. The Teflon-coated seal was somehow crushed when it was installed, NASA announced Wednesday.

The discovery of the failure cleared the way for the fourth attempt to launch the shuttle Columbia at 10:30 p.m. Monday as well as for an October launch of Discovery.

Nevertheless, the reasons NASA has had so much trouble finding and stopping the leaks have as much to do with the nature of hydrogen itself as they do with the extremely complex labyrinth of pipes and fittings that it must pass through to send the shuttle into orbit.

“Hydrogen is very difficult to handle,” former NASA Administrator James M. Beggs said during a recent interview. “It can pass through solid steel if the steel is even slightly porous.”

And to be compact enough to serve as rocket fuel, gaseous hydrogen must be converted to a liquid, which means it must remain below minus 420 degrees Fahrenheit. That ultra-low temperature robs normal gaskets and seals of the flexibility they need to seal joints, thus making it easier for hydrogen to escape, slipping between the atoms of other materials like sand sifting through a window screen.

If escaped gas concentrates in an enclosed area, such as the engine compartment of the space shuttle, a small spark could cause it to explode, jeopardizing the lives of the crew.

It is a strange twist for a tiny element that played such a crucial role in opening the secrets of the atom.

Hydrogen makes up about three-fourths of the universe, and with one proton and one electron, it is the simplest of all atoms.

“It’s a very light gas,” said Len Worlund, deputy director of the propulsion laboratory at NASA’s Marshall Space Flight Center in Huntsville, Ala. “The molecules are able to get through most media.”

Hydrogen is everywhere. Hydrogen and the next atom on the periodic chart of the elements, helium, make up 98% of the universe, in the form of gas or chemical compounds. All the other elements so common to humans, from the oxygen we breathe, to the materials we use to build our homes, to the rocks and mountains that make up the Earth, account for only about 2% of all matter.

Long before the word hydrogen became synonymous with bomb , the element helped unlock the secrets of the atom--partly because of its simplicity--thus launching the world toward the Atomic Age. Scientists hoped it would also make space travel common.

Hydrogen, when combined with a burning agent such as the liquid oxygen used by the space shuttle, is an extremely powerful rocket fuel.

“It gives us a very high specific impulse,” which is equivalent to “miles per gallon in gasoline,” Worlund said.

When hydrogen is burned in the shuttle’s three main engines, it is in a gaseous form, but it cannot be carried into space as a gas. The volume would be so great, and the tank would be so large, that the spacecraft would never get off the ground. It is far more compact, and thus practical for spacecraft, in liquid form.

So it is cooled to just below minus 420 degrees Fahrenheit, the temperature at which liquid hydrogen boils into gas. In its liquid form, hydrogen is “kind of like water” sitting in a kettle on a stove, waiting to boil, Worlund said.

When NASA buys hydrogen for the shuttle, it comes in liquid form in heavily insulated containers. “It’s kind of like a giant Thermos bottle,” Worlund said. “The only thing that keeps it from boiling off is our ability to insulate it.”

But since no insulation is perfect, the fluid does warm gradually, which allows some of it to boil off. As some of the warming liquid turns to gas, more liquid hydrogen is pumped in, keeping the temperature below minus 420 degrees.

When used aboard the space shuttle, liquid hydrogen is pumped into one of two giant caverns inside the huge external fuel tank attached to the shuttle’s belly. The other compartment contains liquid oxygen. When the two fuels are mixed together for liftoff, they ignite, making it extremely important to guard against leaks.

During launch, the hydrogen and oxygen power the three liquid-fueled engines in the shuttle’s tail. The spacecraft also uses two solid-fueled rockets attached to its sides for added boost during the first stage of ascent.

However, getting the liquid fuel from the tank to the engines is no simple matter. The oxygen and the hydrogen pass through two separate, 17-inch-diameter fuel lines that run from the external tank to the aft compartment of the shuttle. Each of those large lines, called manifolds, feeds into three 12-inch fuel lines running to the three engines.

It is a complex system involving various manifolds, distribution lines and valves, all of which undergo rapid thermal changes when the fuel is pumped in at minus 420 degrees.

And the fuel flows through the system for several hours before liftoff. The hydrogen is circulated through the external tank by three pumps, and then it is forced through the system and the three engines through a “special recirculation umbilical.” That process “chills down the lines between the tank and the engines so that the path is free of any gaseous hydrogen bubbles and is at the proper temperature for engine start,” according to NASA documents.

There are other pumps and valves for such things as dumping fuel during emergencies and even recycling hydrogen gas into the tank to keep the pressure at 1 atmosphere after the liquid begins to be withdrawn. All of those components depend on gaskets and seals that are designed to keep the hydrogen locked inside the system.

And that is the rub.

Normally, gaskets work by expanding or flexing to fill voids that would otherwise allow a fluid or a gas to escape. There are literally hundreds of such gaskets in a standard automobile.

However, when a liquid that has been cooled to minus 420 degrees hits almost anything, it freezes it. If the same thing happened to a car, all the seals would freeze and it would leak from every pore.

“If you put anything in there it becomes solid at liquid-hydrogen temperatures,” Worlund said. “A gasket normally remains flexible, but at the temperature of liquid hydrogen that gasket is solid. It’s like glass.”

Worlund said NASA uses seals that consist of strips of stainless steel, bent in the shape of a V and coated with Teflon. The coating makes the surface smooth, and the two legs of the V push against the sides, sealing the joint.

Even Teflon freezes at such low temperatures, Worlund said, so the strength of the seal depends largely upon the smoothness of the two surfaces.

Unfortunately, it doesn’t take much for the two sides to be pushed apart enough for hydrogen to escape. The width of the smallest atom in the universe will suffice.

NASA officials suspect that at least some of their problems were caused by microscopic glass beads that mysteriously turned up in one of the 17-inch manifolds. The beads, though far too small to be seen with the unaided eye, would be plenty large enough to create the kind of leaks that grounded the shuttle fleet.

Normally, a little hydrogen leaking into the atmosphere would not be great cause for worry, since there is plenty of hydrogen there already. And pure hydrogen is “a very volatile gas,” Worlund said, meaning it disperses very rapidly.

“If you put it out here in the air and turn it loose, it will go up at 300 feet per minute,” he added. “It’s going to get away from you in a hurry.”

But in an enclosed environment, such as the engine compartment of the space shuttle, hydrogen gas can become trapped in pockets, reaching concentrations at which it could explode.

NASA executives have stated that this is a recent problem, but many observers doubt that. Some believe that improved sensors that were installed after the Challenger accident in 1986 have simply exposed a threat that has been there all along.

“What bothers me,” Rep. George E. Brown Jr. (D-Colton) said in a recent interview, “is that after all these missions, we are just now finding these hydrogen leaks.”

Brown is a senior member of the House subcommittee that watches over NASA. There is little possibility that this problem, coming at a time of great turmoil within the space agency, will escape notice in the halls of power in Washington.

It may be that the simplest atom of all, carrying the energy potential that powers the sun, has brought NASA to its knees.