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The orbiter is both the brains and heart of the STS, and it contains the latest advances in flight control, thermal protection, and liquid-rocket propulsion. About the same size and weight as a DC-9 aircraft (a fairly small two-engine jet airplane), the orbiter is composed of the pressurized crew compartment (which can carry up to eight crew members under normal conditions and as many as ten in an emergency), the huge cargo bay, and the three main engines mounted on its aft, or rear, end.

The crew cabin has three levels: the flight deck, the mid-deck, and the utility area. Uppermost is the flight deck, where the commander and pilot control the craft, surrounded by an array of switches and controls. During launch of a seven-member crew, two additional astronauts are positioned on the flight deck behind the commander and pilot. The three other crew members are in launch positions in the mid-deck, which is below the flight deck.

The galley, toilet, sleep stations, and storage and experiment lockers are found in the mid-deck. Also located in the mid-deck are the side hatch for passage to and from the vehicle before and after landing, and the airlock hatch into the cargo bay and space beyond. Astronauts pass through this hatch to don their space suits and maneuvering units (called Simplified Aid for EVA Rescue, or SAFER, these units strap on an astronaut's back over the space suit and allow an astronaut to move about in space without being tethered to the shuttle). This equipment prepares astronauts for extravehicular activities (EVAs), more popularly known as spacewalks. Below the mid-deck's floor is a utility area for air and water tanks.

The space shuttle's cargo bay is adaptable to hundreds of tasks. Large enough to accommodate a tour bus at 18 by 4.6 m (60 by 15 ft), the cargo bay carries satellites, spacecraft, and scientific laboratories for the modular Spacelab system to and from orbit around Earth. It also is a workstation for astronauts to repair satellites, a foundation from which to erect space structures, and a storage area for satellites retrieved from space to be returned to Earth.

Mounted on the port (left, as seen while facing the nose of the shuttle) side of the cargo bay behind the crew quarters is the remote manipulator system (RMS), developed and funded by the Canadian government. The RMS (about 15 m/50 ft in length) is a robot arm and hand with three joints analogous to those of the human shoulder, elbow, and wrist. Two television cameras mounted near its elbow and wrist provide visual cues to the crew member who operates it from the rear station of the orbiter's flight deck. The RMS can move anything from satellites to astronauts to and from the cargo bay or to different points in nearby space. It has been used on many missions, deploying and retrieving various scientific and communications satellites.

Two of the orbiters, Atlantis and Discovery, have carried special adapters in their cargo bays for attaching to the Russian Mir space station. A tunnel connected the airlock of the shuttle to a circular mechanism that latched onto the docking module on Mir. Astronauts and cosmonauts could move between the two spacecraft without having to don spacesuits. Atlantis’s docking mechanism was installed in 1995, and Discovery got its own docking mechanism in 1997. Shuttle/Mir missions ended in 1998.

Thermal tile insulation and larger flexible sheets of insulating material (also known as the thermal protection system or TPS) cover the underbelly, bottom of the wings, and other heat-bearing surfaces of the orbiter and protect it during its fiery reentry into Earth's atmosphere.

In contrast to earlier piloted spacecraft such as the Apollo command module, which used material that burned and melted off in layers during reentry and could never be used again, the shuttle's silicate fiber tiles were designed to be used for 100 missions before requiring replacement.

Some 24,000 individual tiles must be installed by hand on the orbiter's surfaces. These tiles are incredibly lightweight, about the density of balsa wood, and dissipate heat so quickly that a white-hot tile with a temperature of 1260°C (2300°F) can be taken from an oven and held in bare hands without injury.


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