After years on the drawing board, construction finally began on the International Space Station (ISS) a little over two and a half years ago. And for the last nine months, astronauts have been living and working there. When completed in 2006, the ISS will house seven scientists roughly 220 miles above the Earth.
Inside, pressurized living quarters and six laboratories will be spread over a volume about the size of a 747 jetliner's passenger cabin (43,000 cu ft). And by the time the station is completed, more than a million pounds of equipment will have been launched into orbit and assembled into a structure that stretches more than 360 ft from end to end. The ISS will be an in-orbit lab for research into living and working in space, material science, and biology, as well as providing a platform for studying the Earth.
Truss spans station
One of the major features of the station will be its 328-ft-long aluminum truss. The truss will hold four solar wings, two at each end, as well as electrical and cooling lines, and other equipment, such as the four control moment gyros. The 300-kg, two-degree-of-freedom gyros, each capable of generating 256.9 Nm of torque, will help maintain ISS's attitude.
To make it easier to build the truss and then maintain it and its associated equipment, NASA's engineers have added a cart that rides on rails mounted to the truss. Astronauts place their feet in straps on the cart, tether themselves to it, then pull themselves hand-over-hand across the truss, the cart trailing behind. Having the cart carry gear lets the crew avoid having to tether and untether while traversing the truss.
Other maintenance tools include the Autonomous Extravehicular Activity Robotic Camera Sprint (AERCam Sprint) designed by NASA to use two color cameras and small floodlight to give crews a view of the ISS exterior. It is a 35-lb, 14-in. sphere powered by 12 nitrogen gas thrusters. The jets put out 0.08 lb of thrust, giving the orb a speed of 0.25 ft/sec (15 ft/min). Remote control is through two-way UHF communications. Lithium batteries limit the Sprint to 7-hr missions, which is also the length of most space walks. A 0.6-in.-thick covering of Nomex should cushion it against inadvertent bumps and collisions, while a yellow LED and surface markings make it easy for operators to see and position.
Securing the basics
Power for the ISS currently comes from the Zarya, a Russian module with solar arrays. But plans call for attaching four solar wings (or 16 arrays) to the truss that will span the station. The 16 arrays, covering 27,000 sq ft, will be the largest deployable space structures ever built, according to NASA. Rotating joints on the truss will let station operators keep the arrays optimally pointed to receive sunlight. When new, each array generates 31 kW of dc voltage, which slowly degrades to 24 kW over their operational life.
Although the rotary joints let data and power pass through, they can't handle fluids. Therefore, each pair of arrays is equipped with a thermal-control system that draws conditioned electricity from the arrays and keeps temperatures within operating limits. (Temperatures range from –184 to 149°C at the ends of the truss.) The thermal systems use pumps, coldplates, ammonia as a liquid coolant, and radiators jutting into space at right angles from the arrays to shed heat. A small heater keeps fluid lines from freezing while the ISS traverses the Earth's shadow.
Total power generation at peak conditions will be about 240 kW, a third of which powers the thermal controls for the arrays. Another third or so recharges the nickel-hydrogen batteries (six batteries per array). These batteries provide power while the ISS is out of the sunlight, or 30 min out of every 90-min revolution around the Earth. By design, the batteries shouldn't fall below 35% of a full charge. This lets them fully charge in the 60 min the ISS is in sunlight, and lets them power the station at a reduced level for one entire orbit following an orbital eclipse.
The last third of the power is distributed throughout the station at