Space Stations, International Space Station

The International Space Station (ISS) program is an international collaboration of the U.S., Europe, Japan, Canada, and Russia, each of which is expected to provide modules and equipment designed to support a crew of up to six for at least 15 years.

The ISS will be the largest structure ever built in space: at completion its overall dimensions will be approximately 110 by 60 meters, about the size of a soccer pitch. Its mass will be some 420,000 kilograms and its total pressurized volume about 900 cubic meters, equivalent to the passenger cabins of two Boeing 747 aircraft.

U.S. President Reagan directed National Aeronautics Space Administration (NASA) to build a space station in 1984, but redesigns and cost overruns delayed the beginning of orbital assembly until November 1998. Over that period, its original name Freedom was changed first to Alpha and then to ISS, as the team of international partners was amassed.

The first module to reach orbit, on 20 November 1998, was Zarya, formally known as the functional cargo block (or, from its Russian name, FGB). It is a self-contained space-craft—a power, propulsion, and orbital-control module that is pressurized and thus provides habitable accommodation. A U.S.-built connection node called Unity was docked to Zarya on 7 December 1998. Then, following significant delays in its construction, the Russian-built service module Zvezda, a habitable command and control center, was added on 26 July 2000.

Modules expected to join the ISS in later years include the Columbus Orbital Facility (COF), a general-purpose science and technology laboratory supplied by the European Space Agency (ESA), and the Japanese Experiment Module (JEM) supplied by the Japanese space agency NASDA (see Figure 24).

Figure 24. A view of the station showing the Japanese experiment module (JEM), complete with external payloads and remote manipulator arm. Note the windowed cupola module at center, which will be supplied by Europe

Although the station is being assembled at an orbital altitude of about 380 kilometers, its operational orbit, attained late in the assembly sequence, is expected to have an altitude of 426 kilometers and an inclination of 51.6 degrees. The lower initial orbit allows the Space Shuttle to deliver some 18 kilograms of additional payload on each mission, while the inclination is designed to allow the station to over-fly some 85 percent of the earth’s landmass and 95 percent of its population.

The six main modules and a number of connecting nodes will be mounted on an ‘‘integrated truss structure,’’ the station’s structural backbone, along with eight two-panel solar arrays designed to rotate to maximize their exposure to the sun and provide a total of some 110 kilowatts of power. The station’s location in low-earth orbit means that it will be eclipsed by the planet on each orbit, obliging its systems to draw power from its nickel-hydrogen (NiH₂) batteries which will be recharged from the arrays during the sunlit portion of the orbit.

The station’s orbital track and orientation, or attitude, was determined initially by gyroscopes alone but later used an inertial navigation system incorporating global positioning system (GPS) receivers. Attitude control is engineered using electrically powered momentum wheels, which exchange angular momentum with the station’s structure to rotate it in any axis. In common with all unmanned spacecraft, such as communications satellites, adjustments can also be made by bipropellant thrusters, and backup systems are provided for all subsystems in case the primary equipment should fail. The station’s altitude is boosted periodically, to overcome atmospheric drag, by firing rocket engines on visiting spacecraft.

In addition to the standard subsystems, there are several novel technological aspects to the station. One is the inclusion of a Canadian-built manipulator arm, which can move along the station’s truss on a mobile transporter enabling the crew to perform assembly and maintenance work without leaving the safety of the modules.

Although the arm is based on the Space Shuttle’s remote manipulator system, it has a ‘‘hand’’ at either end and is not permanently fixed to the station’s structure so that it can crawl along the truss. Also new for the ISS is a pair of cupola modules, supplied by ESA, each of which has seven relatively large windows to allow observations of the earth and stars and monitoring of work on and around the station.

There are many technical challenges inherent in the design and construction of such a large structure in space, not least its structural integrity, which despite numerous computer simulations, remained to be proven in flight. Also, because of its size, the probability of impact from space debris will be statistically greater than for previous spacecraft. As a result, the module shells are designed to withstand impacts from objects less than 1 centimeter in diameter without significant damage, while their life support systems should allow at least three minutes for evacuation following a 10 centimeter puncture.

One of the most important questions posed throughout the station’s design and development concerned what it would be used for. The answers usually included microgravity research into growing more perfect semiconductor crystals than available on earth; various aspects of biomedical research that could lead to new drugs and procedures for use on earth; and space science applications such as astronomy.

Over and above such scientific endeavors, the ISS is likely also to be used as an in-orbit model shop for future spacecraft technologies; for example, in the testing of large deployable structures such as solar arrays and communications antennas, or for space-qualifying new materials, thermal control hardware and ion propulsion systems, to name but a few. Interestingly, the ISS has already proved itself as a destination for space tourists, the first of which, Dennis Tito, paid a reported US$20 million for a week on the station in 2001. Moreover, the potential for the ISS to act as a departure point for further exploration of the Moon and Mars should not be ignored.

In addition to the engineering challenge of establishing a small community in low-earth orbit, the ISS program has proved to be a political challenge. Agreement on financing and respective responsibilities among international partners has proved difficult and has resulted in modifications and delays. For example, the inclusion of Russia relatively late in the program—following an end to Cold War hostilities—led to a major redesign to incorporate a significant amount of Russian hardware.

Although this led to a station in orbit sooner than the U.S. could have produced alone, it did so at the expense of the Russian station, Mir, which was deorbited in March 2001 because Russia could not afford to operate both. Despite the problems, the ISS seems likely to be a model for future international space programs, such as a manned Mars mission, simply because they are unlikely to prove affordable for a single nation.