During the final minutes of a space shuttle launch countdown, you can almost hear the proverbial pin drop across Launch Complex 39 at NASA's Kennedy Space Center in Florida. Inside Firing Room 4, launch controllers monitor their systems and listen intently as the ground launch sequencer takes over the countdown and the final prelaunch milestones are checked off. Three miles to the east, strapped into their seats inside the shuttle's crew module, the astronauts hear the orbiter test conductor's voice in their headsets as the countdown clock ticks downward.

T-2 minutes and counting.

"Close and lock your visors and initiate O2 flow," Orbiter Test Conductor Roberta Wyrick instructs the flight crew. It's a scripted line, instructing the astronauts to pull down their helmet visors and turn on the air inside their pressurized suits. It's also the last time during the countdown that anyone from the firing room will talk directly to the flight crew.

The United Space Alliance Test Operations Group includes 24 orbiter test conductors (OTCs), with six scheduled to support each launch. And although the position gets the most visibility during the final hours of the launch countdown, the OTC is an integral part of the team that shepherds each shuttle orbiter through its complex schedule of prelaunch preparations known as the "processing flow."

"From the time the shuttle is handed over to Kennedy after landing -- whether it's here or at Edwards Air Force Base in California -- until they launch again, anything that has to do with the orbiter involves us," explains Wyrick, an OTC since 1981. "We also support the external tank and solid rocket booster work during normal processing."

The space shuttle comprises three primary components: the white, winged orbiter, the twin solid rocket boosters and the bulky, orange external fuel tank. Prepping the shuttle for an upcoming flight begins almost immediately following its last landing, when the vehicle is towed to the nearby processing facility where it undergoes several months of refurbishment, clean-up, testing and checkout. At the same time, the boosters are stacked and any remaining tank work is completed.

Orbiter test conductors are involved every step of the way, working closely with NASA's shuttle managers to coordinate the intricate processing schedule and integrating it with other center operations. Team members follow the shuttle's progress on a daily basis, monitoring activities either in person or from the OTC console in the firing room's second row.

At this same console, the test team members take turns through the three-day launch countdown, ensuring two test conductors are present on each shift, with the "prime" scheduled for the liftoff shift. A tank/booster test conductor -- known by the call sign TBC -- joins the OTC during the final three shifts of the countdown to look after those components, freeing up the OTC to focus entirely on orbiter systems and activities.

The console provides up to eight video screens the OTC can use to display system status or live operational video from the launch pad. Additionally, up to eight audio channels allow launch controllers to listen and interact on several communication loops simultaneously. During the countdown, the OTC answers to the NASA test director and speaks with the support astronauts who prepare the shuttle's crew cabin for the astronauts about to board. Once the flight crew is strapped in, the OTC performs a voice communication check with each astronaut and offers periodic reminders to complete specific preflight tasks.

Wyrick quickly points out that the astronauts don't need reminders. These short exchanges simply help the launch team keep track of which tasks are complete.

"They know the script, and they know there are certain things they have to do," she explains. "We're not telling them anything they don't know. We just want to hear their positive feedback that something's been done."

After the final "go for launch" polls and the launch director's official farewell to the crew, the countdown resumes at T-9 minutes and the ground launch sequencer (GLS) assumes control of all the shuttle's critical functions.

"We can watch the GLS screen on one of our monitors," Wyrick says. "It's there, sending all the commands. We're telling the flight crew to start the auxiliary power units, clear caution and warning memory, close and lock their visors . . . but everything else is by the computer.

"We're watching, just like everybody else."

As the orbiter access arm is retracted with seven-and-a-half minutes to go, the OTC wishes the crew a successful flight. From that point on, all spoken lines are scripted, including the cue to close their visors with only two minutes left before liftoff.

Wyrick has been with the team through all but the first shuttle mission, but insists it's never routine.

"I don't care how many times you've done it or how many times you've seen it," she says. "Your heart beats harder, and you just want to see everything go smoothly."