JOHNSON SPACE CENTER, HOUSTON, July 18, 2011  -- For 30 years, every time a space shuttle has flown, so has OV-095, the ghost orbiter. You've probably never heard of it, but in its way it has been as important as Atlantis or Endeavour.

It sits inside a nondescript building at the Johnson Space Center here in Houston. The group that runs it has an acronym -- SAIL, short for Shuttle Avionics Integration Laboratory.

OV-095 has never left the ground and never will, but it has the same cockpit, the same computers and the same wiring as the ones that have flown millions of miles. The circuitry is exposed for easy access in an emergency. Walking under the shuttle's massive cargo bay helps one appreciate the complexity of the orbiter.

Sitting inside the cockpit of OV-095 is eerie – it's tracking the mission in progress, so you listen to the air-to-ground communications between Mission Control and the crew of Atlantis.

"On my mark go to operate – start all systems - start clock." It can be a prosaic conversation. You hear Shannon Lucid, the astronaut in Mission Control who talks to the crew, reminding Commander Chris Ferguson to empty his Outlook inbox.

It is when there is a problem that the SAIL team springs into action. Since OV-095's systems are virtually identical to those of Atlantis, they can be used to replicate a problem in orbit and thus fix it.

Even its name, bland as it sounds, links OV-095 to Atlantis in orbit. "OV" stands for Orbiter Vehicle. When the shuttle program was being put together in the 1970s, the ships that flew were designated OV-102 (Columbia), OV-104 (Atlantis) and so forth. Test ships, not meant to fly, were given numbers below 100. OV-095 was meant for troubleshooting. OV-099 was a complete ship, later reconfigured for spaceflight as the shuttle Challenger. It was destroyed during liftoff in 1986; seven astronauts died.

The space shuttle was the world's first re-usable spacecraft, capable of lifting 30 tons of cargo, maneuvering in earth orbit like a spacecraft, and landing like an airplane. It is, says engineer Frank Svrcek, a very complicated machine, one that requires incredible attention. Svrcek and his team test the avionics, flight software and flight procedures for each mission

"It takes something like this to really emphasize the complexity, how did we ever do that, to get there – how did we ever fly the shuttle given its complexity," he said. "We have to simulate the world, the environment. We have to simulate the main engines, it has to all work together, time and time again."

A flight-rated shuttle contains more than 2.5 million parts, 230 miles of wire, 1060 valves, 1,440 circuit breakers. It accelerates to an orbital velocity of 17,500 miles per hour, 25 times faster than the speed of sound, in just over eight minutes. On liftoff it weighs 4,525,808 pounds.

Atlantis has now flown 33 flights since it was delivered April 1985. It is named after the two-masted ship that was the primary research vessel for the Woods Hole Oceanographic Institute from 1930 to 1966. Atlantis has flown 120,650,907 miles to date and will add at least 4 million miles when it lands after this mission. The five orbiters in the shuttle fleet have flown 537,114,016 miles.

The SAIL team has flown all those miles with the orbiters, all from this building. "Words can't tell you how proud we are of what the shuttle has done," said Svrcek, "what our country has done, that this worked, that it worked repeatedly, to bring lots of people up and bring them back, to bring heavy payloads back. It is just an amazing accomplishment that I can't put totally into words how proud I should be."

OV-095's mission will be complete when Atlantis rolls to a stop at the Kennedy Space Center. Workers will come in to dismantle the orbiter, and by the end of December, it will truly be a ghost orbiter, just a memory for the hundreds of workers who made the space shuttle program work.