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 160 V to prevent transmission losses, and then converted to secondary power, a tightly regulated 124 Vdc, near where it will be used. The two-level design of the power-distribution system compensates for line losses, hardware degradation, and aging solar arrays on the primary system while the secondary voltage remains constant for equipment.
This is a change from past practices in which most spacecraft, Russian and U.S., relied on 28 Vdc. The higher voltage lets NASA engineers meet the ISS's power needs and use smaller, lighter power lines. Appliances, computers, and other electrical devices will have to be designed to operate on 124 Vdc or use an adapter. Russian modules will still use 28 Vdc and converters will ensure power is at the proper voltage depending on whether crews are in the American or Russian sectors of the station.
The power system will use single-point ground architecture, making the ISS metal structure the electrical common point inside the station. But the potential difference between the ISS and the plasma environment in space could be as much as 140 Vdc during eclipse, enough to cause microarcing and damage to the arrays and thermal coatings. Two plasma contactors mounted on the truss, one operational and one backup, will generate a stream of electrons from xenon gas and emit it into space, effectively grounding the ISS to space.
The station generates oxygen primarily by electrolyzing water into oxygen and hydrogen. The hydrogen will be vented overboard. If necessary, oxygen can also be generated using chemical cartridges in an exothermic reaction.
Oxygen is also transferred from the Shuttle to the Station whenever the Shuttle repressurizes itself and the station prior to undocking and heading to Earth. The Shuttle will also leave its excess water at the ISS. Water is a byproduct of the Shuttle's fuel cells, which convert hydrogen and oxygen into electricity. NASA figures it should only take three Shuttle flights per year to keep the ISS supplied with water, which will be used for drinking and generating oxygen. The Station will also recycle water from the sinks, shower, urine, and condensation for oxygen generation, an in-orbit first.
Modules and nodes
The ISS will eventually grow to include over a dozen modules for living quarters, labs, and storage. They will be connected by airlocks and nodes, all tied to the main truss and its solar arrays.
The first module in the jigsaw-puzzle construction was the 42,600-lb Zarya, designed in the U.S. but built in Russia. It is 41-ft long and 13.5 ft at its widest, and provides power and propulsion while the ISS is being built. On the outside, it is studded with 36 steering jets for adjusting attitude, and it carries two larger main engines for reboosting and changing orbit. There are also 16 fuel tanks with a total capacity of 6 tons and two solar arrays putting out 3 kW of electrical power. Most of Zayra's functions will be carried out by the Zveda module once it is in place, leaving the Zayra as storage space and a fuel depot. Zayra's two side docking ports will accommodate Soyuz and Progress spacecraft.
There should always be enough Soyuz reentry vehicles on the station ready to act as lifeboats for the entire crew. The piloted, three-person Soyuz must always be flown by a Russian cosmonaut, according to the Russian space agency. It could eventually be replaced by spaceplanes based on the X-38, a NASA project which recently had its funding deferred.
Progress cargo rockets will deliver up to 6 tons of reboost propellant and dry cargo to resupply the station every two months. They will also serve as self-incinerating trash cans. ISS personnel will fill the Progress rockets with refuse, launch them toward Earth, and they will burn up on reentry.
The 43-ft long Russian Zveda module connects to one end of Zayra. During construction, Zveda houses the crew and main computers, distributes power, and maintains life support. All these functions will be redistributed once the ISS is completed. But Zveda will remain the heart of the Russian segment of the ISS, which will consist of a series of lab modules and a solar array and mast. The 42,000-lb module is similar in layout to the Mir with three compartments: a small spherical transfer chamber forward, a cylindrical work room in the center, and a cylindrical transfer chamber aft. It can dock with a Soyuz or Progress. Zveda also has power-producing solar arrays and an unpressurized assembly area wrapped around the aft end.
Living spaces on Zveda include personal sleeping quarters for three, each with its own window, a toilet and hygiene facilities, a galley with a refrigerator, freezer, a table for eating meals, and a treadmill and stationary bike for exercise. There is a large 16-in viewing port in Zveda's working compartment and three 9-in. ports in the transfer compartment so the crew can watch docking activities. All together, Zveda boasts 14 windows. Astronauts will live in it until the U.S. Habitation module is delivered in 2005.
At the other end of Zarya is Node One, or Unity Node, connecting it and Zveda, along with future Russian modules, to the rest of the station. It also gives engineers five additional ports for planned expansion. The 22-ft-long, 18-ft-diameter node will provide a passage to the U.S. and European labs and living quarters, and be the main structural connection to the truss. Though seemingly simple, Unity contains more than 50,000 mechanical items, 216 lines for fluids and gases, 121 internal, and external cables using six miles of wiring.
The Joint Airlock Module, which connects to the Unity node, gives astronauts a way to access space without using the Shuttle. Before it is installed, there will only be Russian-built airlocks, which are incompatible with U.S. spacesuit umbilicals. The Joint Airlock will store suits and be used for overnight "camp outs" in preparation for spacewalks. Astronauts will sleep in the lock while pressure drops from the station's normal 14.7 to 10.2 psi. This purges their bloodstreams of nitrogen, increases the oxygen in their blood, and prevents decompression sickness, or the bends, when they go to the 4.3-psi pure-oxygen atmosphere inside the suit.
The U.S. space suit has been upgraded for station use. Internal parts are now more easily replaced, metal sizing rings let the suits adjust for different-sized crew members, and new gloves are more flexible and sport fingertip heaters. A new radio lets up to five people talk at once, and the helmet carries spot and floodlights. There's also a new jet-pack lifejacket, dubbed SAFER, that will help untethered spacewalkers get back to the station in emergencies. The new suits are rated for 25 space walks before they must be returned to Earth for refurbishing.
U.S. and European lab modules are connected to the ISS by the Unity node. They include the U.S. lab Destiny and Centrifuge Accommodation Module (CAM), the European lab Columbus, and the Japanese Kibo. Each lab has access to power, cooling, communication, vacuum, exhaust, gaseous nitrogen, and microgravity measurement tools. CAM will house an 8.2-ft-diameter centrifuge, an essential tool for exploring gravitational biology.
Space has its hazards, so Kevlar debris shields will cover the labs. Aluminum shields separated from the debris shield by a 4-in. gap will then cover the Kevlar for added protection. Russian modules are protected by an outer glass cloth covering a screened aluminum honeycomb shield, a small gap, and an inner carbon-plastic-coated screen. Protection is important since there are 20,000 object in low-Earth orbit larger than 1.97 in., according to NASA. There's at least a 7% chance one such object will penetrate a U.S. module while the ISS is in orbit. Chances are somewhat less for Russian modules.
The labs are similar in size, ranging from 21 to 36-ft long and 14.6 to 16.2 ft. in diameter, and are built to house research racks. Each rack is roughly the size of a large refrigerator with a curved back to fit the modules' cylindrical outer wall. In total, the European, Japanese, and U.S. labs hold 37 racks.
One such rack, for example, will house five inserts in half of its volume. Each insert is equipped with a furnace to process materials. NASA and the European Space Agency will each supply two inserts, with the German Space Agency proving the fifth. The German furnace, unlike the others, will use a rotating magnetic field to control flow inside molten samples.
Japan's Kibo is unique in that part of the lab is "outside," to expose experiments to the environment of space. An airlock allows moving equipment in and out of the station without depressurizing the entire lab. Researchers will control a robotic arm with grippers and a camera to observe experiments exposed to space and move equipment.
A device called the Express pallet (Expedite the Processing of Experiments to Space Station) will hold up to six payloads or experiments that call for exposure to space. Built by the Brazilian Space Agency, the pallet can be located almost anywhere along the main truss, and will get its power and data handling from the ISS. Robots, such as the Canadian and European robotic arms, or spacewalks will be used to remove, replace, and service payloads.
A habitation module will connect to Node 2, which will be below, or Earthward of, Unity node. It will contain living quarters for seven crew members, eating and hygiene facilities, as well as medical and exercise equipment.
Keeping it in place
Completing the station will take more than 40 shuttle flights and a host of Progress launches. Future payloads are also scheduled for delivery on Japan's Hope spacecraft and the European Automated Transfer Vehicle. The Italian Space Agency is providing three multipurpose logistics modules named Leonardo, Rafaello, and Donatello after famed Italians, not Ninja Turtles. The modules fit inside the Shuttle bay and carry 9.1 tons of cargo packed in a pressurized environment. The modules also provide power, data, and cooling fluid to some cargo containers.
Due to the small but constant presence of atmosphere at ISS's orbiting altitude (217 to 285 miles) and the large size of the ISS, it will fall about 1 mile/wk once it is completed. To stay in orbit, a docked Progress rocket, Shuttle, or the Zarya module will fire its engines, increasing the ISS's orbital speed, and therefore, its orbital height.
Currently, Congress has approved five years of funding for ISS development and construction, about $24 billion, and a 10-yr operating budget of $13 billion. NASA expects the station to have a much longer life than the current 10-yr plan. After all, many ISS parts were originally designed for the all-American Freedom space station and a 30-yr life. NASA was instructed to use as many Freedom components and modules as they could. All station components are also designed for transport and assembly in space, so almost anything can be replaced.
ISS's extended life is not hard to accept when you consider that Pioneer 10, a mission launched 29 years ago and formally ended four years ago, is still functioning. Signals from Pioneer were detected last April when it was about 7.3 billion miles from the Earth traveling 27,830 mph. So it's likely the ISS will outlive its budget.
When the end finally comes, the ISS will be "decommissioned" by taking it apart into pieces sized to burn up on reentry and then sending them Earthward.