When space shuttle astronauts are living and working in Earth orbit, their mission - and, ultimately, their lives - depend on all the shuttle orbiter's systems and controls working exactly as expected.

From the time a shuttle lands after a mission until it launches on its next spaceflight, a small group of specially certified United Space Alliance aerospace technicians called spacecraft operators act as the eyes, ears and hands of the test team at NASA's Kennedy Space Center in Florida.

Their workspace is the orbiter crew module, the part of the shuttle where astronauts live and work in space. They focus heavily on the cockpit - the nerve center of the space shuttle orbiter.

"The astronauts have a good job. They make it look easy," says Bill Powers, a spacecraft operator (SCO) since 1985. "Our job is just to make sure when they get the vehicle, there aren't any surprises."

Spacecraft operators are an integral part of the processing and test teams that ensure the shuttle is ready to fly.

Any time the orbiter is powered up - meaning that its systems are on - spacecraft operators are on duty.

"What SCOs do is take information from Engineering and the test conductor, and they do what the astronauts do on orbit," explains Chris Meinert. He's worked on the space shuttle since 1981, and became certified as a spacecraft operator in 1984.

Orbiter systems can be controlled in three ways. In "command," a ground controller can send a command through the shuttle's S-band antenna and, for example, turn on a fan or pump while the astronauts are asleep. "Copper path" involves a crew member physically throwing a switch in the cockpit to accomplish a task. Finally, the Launch Processing System, or LPS, is the ground command in which a signal from the Launch Control Center (LCC) travels down a wire to the launch pad.

Spacecraft operators take part in testing all three methods.

"In the LCC, you've got 15 or 20 different consoles, with many feet or yards separating them. But in the cockpit, you can see several different systems, all within arm's reach," says Meinert. "If there's something going on, we can give a report pretty quick as to what's the initial system that's either good or bad, and how the other systems are reacting to it."

More than 2,000 switches, displays and controls fill almost every surface of the flight deck, beginning with the center console in front of the commander's and pilot's seats and spreading out across the ceiling and walls. All this instrumentation is labeled and grouped into panels according to system. In order to operate the spacecraft, it's critically important to know its systems and how they interact.

"The better your knowledge of the systems, the better an SCO you are," says Meinert. "If you look at it as a number, who can remember 2,400 different things? But the panels are laid out for the crew, kind of logically, either as current flows, or as water flows, or as air flows."

The spacecraft operator certification requires one to two years of additional training. Not only must candidate technicians learn the shuttle's systems and the inner workings of the crew module, they also must learn "communication etiquette" and spend some time working in a Launch Control Center firing room.

"When you're on the headset and you're working on a job, all you hear is a voice," says spacecraft operator Jay Beason. "The only thing you see is your end of the job. So we're required, as part of our training, to sit in the firing room on the test project engineer's console and do what they do. I found that experience invaluable."

Some spacecraft operators concentrate on horizontal processing, which primarily takes place inside the orbiter processing facility. Others specialize in vertical operations inside the Vehicle Assembly Building and at the launch pad. But all are able to fill in wherever they're needed.

The shuttle arrives in the orbiter processing facility (OPF) after landing, kicking off several months of work to "safe" the spacecraft and prepare it for its next flight - an effort known to the shuttle team as the "processing flow." During horizontal processing - while the orbiter still is oriented like an airplane in a hangar - two spacecraft operators normally take turns working inside the crew module during the day, taking over for each other every two hours.

"On a typical SCO day, I might come in here and power up the ship, get it configured for whatever engineering needs or any testing that's going on - whether it be aft, midbody or forward operations," says Greg Rose, who earned his SCO certification about a year ago.

"Days when I'm not in the 'SCO loop,' I could be working on anything outside the ship: forward reaction control system, doing thruster desiccant changeouts, hooking up quick disconnects here and there, hooking up water servicing, working on the nose landing gear," he adds.

Once the orbiter is brought to the Vehicle Assembly Building, lifted to vertical and attached to its external fuel tank and solid rocket boosters, the context of the work changes.

"In the OPF, you stand on the floor and you throw switches, and it's all logically laid out where you can read them," Meinert says. "But at the pad, you're standing on a wall and all of the overhead switches are upside down. You have to read upside down and backwards in order to get the right one."

But he explains that's not the only change.

"When the vehicle goes out to the launch pad, we arm it with pyrotechnics and we put nasty hypergolics inside it. We put ammonia in it. There are systems that are armed and ready, and they need monitoring. Testing at the pad has a bit more danger to it."

About three days before liftoff, the countdown officially begins. The orbiter is powered up the entire time, and four SCOs are there at any given time: two in the cockpit and two to provide relief. Every system has to be ready before the Astronaut Support Person (ASP) - a NASA astronaut not part of the flight crew - climbs in to configure the cockpit switches prior to boarding.

Some launch pad SCOs, such as Meinert, also are members of the Closeout Crew, helping the astronauts climb aboard, making sure the hatch closes correctly and ensuring the climate-controlled White Room is set up correctly for launch.

But a successful liftoff doesn't signal the end of the spacecraft operators' role in a mission. They also have a critical role on landing day and are some of the first people to approach an orbiter that's just returned from space.

"In the wintertime, when it's cool outside, the tiles make a crinkling sound. And sometimes the panels on the hatch are still pretty warm. It's just so alive," he says. "You know it's been somewhere."

After the returning astronauts, medical staff and ASP leave the crew module, control of the ship is transferred from NASA's Johnson Space Center in Houston back to Kennedy. At that point, two SCOs will settle into the cockpit and begin monitoring systems, flipping switches and installing guards on switches that need to stay in their current position. They'll stay on board until the shuttle is towed back to the processing facility. The procedure is the same at Edwards Air Force Base, Calif.

The space shuttle era is set to conclude with the STS-135 mission, when Atlantis' wheels roll to a stop for the last time. The spacecraft operators who remain will prepare the orbiters for their next assignment: as museum pieces.

They also have a tremendous record of success to enjoy.

"When you reach that moment at T-0, knowing that approximately eight hours earlier you were up there getting everything ready for the Closeout Crew and the flight crew to get on board, it gives you a real sense of pride," Beason says.

"It's the culmination of a lot of time, a lot of effort and a lot of love."

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