International Space Station Background
The International Space Station is the most complex engineering project ever attempted in space and, at some $150 billion to date, one of the most expensive.
The first element of what would become the International Space Station was the NASA-owned, Russian-built Zarya propulsion and storage module, also known as the Functional Cargo Block, or FGB. It was launched Nov. 20, 1998, by a Proton rocket. Two weeks later, a space shuttle carried the first NASA component into orbit, the Unity connecting node, and the two were "mated" to form the core of the station.
NASA modified the assembly sequence in the wake of the 2003 Columbia disaster and a subsequent decision by the Bush administration to retire the shuttle by the end of the decade. The U.S. segment of the outpost was declared complete after the final shuttle flight in July 2011.
To understand the engineering challenge facing space station operators, it helps to visualize the 924,000-pound structure as it orbits the Earth 260 miles up, streaking through space at 5 miles - 86 football fields - per second and enduring temperature swings of 500 degrees Fahrenheit as it moves from sunlight to shadow and back again.
The long axis of the lab complex, normally oriented in the direction of travel, generally stretches out like a train, with pressurized modules connected fore and aft like passenger cars. At the front end of the complex -- the locomotive in the train analogy -- the U.S. Harmony module leads the way, with the European Space Agency's Columbus laboratory attached to a right-side port and Japan's Kibo lab extending to the left.
An Earth-facing port in the Harmony module serves at the attachment point for visiting Japanese HTV cargo ships, SpaceX Dragon capsules and Orbital Sciences Cygnus supply ships. All three are berthed using the station's robot arm. Harmony also features four U.S.-supplies sleep stations, or berths, for NASA sponsored crew members.
Harmony's aft port is connected to the U.S. Destiny laboratory module, which in turn is bolted to the central Unity connecting node. The U.S. Quest airlock extends to the right and the Tranquility module extends to the left. A cargo storage compartment is attached to Tranquility's forward port and a set of four gyroscopes, used to re-orient the station and maintain its commanded position, or "attitude," is housed in a short truss that extends from Unity's top port.
The U.S. segment of the station, which includes ESA, Japan and the Canadian Space Agency, extends from Unity forward. The Russian segment begins just beyond Unity's aft port where the Zarya module is attached. The Russian Rassvet module extends down from Zarya and serves as a docking port for unmanned cargo ships and manned Soyuz spacecraft. Bringing up the rear of the space station "train" is the Russian Zvezda command module.
The Poisk docking compartment extends upward from Zvezda while the Pirs module, which serves as a docking port and an airlock, extends straight down. An aft port is available for manned and unmanned vehicles. The Russians plan to replace Pirs next year with a large laboratory module. Later, they plan to attach a multi-port docking compartment to the new lab and then a solar array assembly that will extend from that module to the right.
Mounted at right angles to the long axis of the station is its primary solar power truss, a huge assembly spanning the length of a football field that houses critical electrical components, ammonia coolant loops and steerable radiator panels. The truss is anchored to the long axis of the station by 10 massive struts that connect the central S0 truss segment to the top of the Destiny laboratory.
On the front side of the truss, a mobile platform mounted on rails can carry the Canadian robot arm to various work sites. On each end of the truss, four huge sets of solar arrays rotate like giant paddle wheels to track the sun.
The arrays generate, on average, some 80 kilowatts of power, enough to supply 55 average homes. Electricity from the solar arrays, known as "primary power," is routed to components in the S0 truss called main bus switching units, or MBSUs.
The four MBSUs take that 160-volt primary power and route it to transformers known as DC-to-DC Converter Units, or DDCUs, which lower the voltage to a precisely controlled 124 volts DC. This so-called "secondary power" is then directed to the station's myriad electrical systems using numerous electro-mechanical switches known as remote power controllers.
The space station cooling system features two independent ammonia loops - loop A and B - that include large ammonia reservoirs, pumps, cold plates and the plumbing required to route the coolant through the big radiators to dissipate heat.
The entire lab complex can be maneuvered, or re-oriented, by firing Russian rocket thrusters or by changing the speed of NASA's gyroscopes inside the Z1 truss atop the Unity module. Rocket thrusters are typically used for major maneuvers while the gyros are primarily used for more minor attitude changes.