By: Zach Rosenberg
In constant low Earth orbit (LEO) at 51.6˚ orbital inclination, the International Space Station is the closest to colonising space that humanity has ever been. Founded in the wake of the Cold War and the resulting end of competing Russian Mir-2 and US Freedom station concepts, among others, the ISS is maintained by a 15-nation group, and provides an international laboratory for technology in a microgravity environment.
The ISS is modular, and for the most part any of the nodes or modules can be detached and reattached in different configurations - the space station has been rearranged several times as it has grown. The first piece of the ISS in orbit was the Russian-built Functional Cargo Block (FGB, using Cyrillic initials), launched in 1998, followed quickly by Node 1 and the service module. The first permanent crew entered the ISS in 2000, and people have resided aboard ever since in teams of three to six (more when the Space Shuttle visited).
The space station is divided into a Russian half and a US/international half, connected by the Node 1 module (also called Unity). In addition to being a crucial connector, there are four sleeping stations aboard Node 1. Connected to Unity are the FGB (via Pressurised Mating Adaptor 1), the US laboratory (called Destiny), the Permanent Multipurpose Module (mainly used for storage) and Node 3 (or Tranquility). A similar node, Node 2 (Harmony) extends from the other side, anchoring the European Columbus lab and pressurised Japanese sections.
Node 3 contains much of the non-Russian half's life-support equipment, including the oxygen and water generation systems. Water is initially sent up by visiting vehicles, but as much is recycled as possible, including waste water from urine, via a complex series of filters and treatment. Much of it is stored in bags throughout the ISS, but some is transformed into oxygen and hydrogen.
"There's an oxygen generator, and it electrolyses water into oxygen and hydrogen," says Greg Gentry, Boeing's chief engineer for environmental control and life-support systems. "In the past we would vent hydrogen overboard as we did CO2, but now we have a device called a Sabatier reactor that will recombine the carbon dioxide and the oxygen to make methane and water, so we do get some of the water back, but not all of it." The methane is then vented overboard.
Also supplying oxygen (and a small amount of nitrogen) is a system of pressurised tanks located on the outside of the Quest airlock. In addition to supplying the ISS with gas, the tanks are used to charge space suits in advance of the occasional space walks required for maintenance or experimentation.
Tying the ISS together is the immense integrated truss structure, upon which are mounted the solar arrays and heat dispensers, connected to the pressurised areas via an attachment to the Destiny laboratory. The truss is delineated along "port" and "starboard" lines and further broken down into segments. Starboard section zero (S0) connects port and starboard sides in the centre, mounted to the Destiny laboratory from which branch the other pressurised parts of the space station. The life support and electrical equipment is essentially identical for both port and starboard sides.
On both sides are four solar arrays, capable of producing between 20 and 30 kilowatts each, depending on factors such as the panels' angle towards the sun - well over the continuous 75kW required minimum total. Although solar cells degrade over time, the degradation measures only around 1% of generating capacity each year, enough to keep the ISS running until 2028 and beyond. An alpha gimbal orients the panels along the truss, while beta gimbals change the panels' angle towards the sun.
The base loads to the space station consume roughly two-thirds of the generated power at any given time, with approximately one-third going to experiments. The biggest source of power consumption is "probably the fans and the pumps - it's the motors that are driving either fluid or air around the station", says Jeff Donoughue, Boeing's subsystem manager for electrical power systems. "There's hundreds and hundreds of loads, but the pumps - the 3kW pumps - are probably some of our biggest loads."
Depending on the time of year, the ISS will see 16 sunsets and 16 sunrises in a 24h period, and of course the solar arrays produce no power in the dark. To overcome this, mounted in S6, P6, S4 and P4 truss segments are 48 nickel-hydrogen batteries.
The external structure of the entire space station is mostly built of very high-grade aluminium, and is subject to the same fatigue pressures as an aircraft. The sections exposed to sunlight can get very hot, while the shaded sections get very cold. The temperature difference between the light and shaded areas, and the rapid change between light and dark areas as it crosses through Earth's shadow, are the biggest structural stresses the ISSregularly undergoes.
"Thermal's [one of] the biggest loads since [the Space] Shuttle's stopped flying," says Mark Mulqueen, Boeing's deputy programme manager for ISS. "[ISS is] under tension and compression as you go around, and that's how you get those peak loads."
The Space Shuttle, now retired, was once the biggest structural stressor by virtue of its considerable mass. Smaller visiting vehicles - Soyuz capsules carrying astronauts, and Progress, ATV, HTV and Dragon capsules with supplies - have a much lower mass, and are therefore not as stressful. Although Soyuz is the lightest of the visiting capsules, its faster approach means more stress.
Maintaining a good environment for the onboard computers is a necessity, and computers run hot. To cool them down, heat is filtered out via water cooling, which meets a liquid ammonia reservoir that in turn sends heat out via large radiators mounted along the truss.
Protecting the crew are integrated micrometeorite shields, made of aluminium and fixed in place. The exposed cupola on Node 3, permanently pointed towards Earth's spectacular scenery, also has deployable shields. To guard against the possibility of sparks and fire, two plasma generators are mounted outside to envelop the ISS in a sort of protective bubble.
Located outside the pressurised areas of the ISS, mainly along the truss, are external storage and logistics carriers, which contain the all-important orbital replacement units (ORUs) for replacing entire systems, and provide necessary electrical and data transmission lines. The Express Logistics Carriers (ELCs) can be used to mount any of the several mechanical arms that are used for everything from docking visiting spacecraft to performing repairs and inspections of the space station.
The ISS receives regular resupply flights from four vehicles - the Russian Progress and Soyuz, the Japanese HTV, European ATV and newly-qualified US Dragon. If all goes according to plan, they will be joined shortly by the US-built Cygnus. As of now only Soyuz can carry crew, but the US Commercial Crew Integrated Capability (CCiCap) initiative is funding three more: a crewed version of Dragon, the Boeing CST-100 capsule and Sierra Nevada's Dream Chaser winged lifting body.