The International Space Station (ISS) is a modular space station (habitable artificial satellite) in low Earth orbit. It is a multinational collaborative project involving five participating space agencies: NASA (United States), Roscosmos (Russia), JAXA (Japan), ESA (Europe), and CSA (Canada). The ownership and use of the space station are established by intergovernmental treaties and agreements.
The station serves as a microgravity and space environment research laboratory in which scientific research is conducted in astrobiology, astronomy, meteorology, physics, and other fields. The International Space Station is suited for testing the spacecraft systems and equipment required for possible future long-duration missions to the Moon and Mars.
The station is divided into two sections: the Russian Orbital Segment (ROS) is operated by Russia, while the United States Orbital Segment (USOS) is run by the United States as well as many other nations. Roscosmos has endorsed the continued operation of ROS through 2024, having previously proposed using elements of the segment to construct a new Russian space station called OPSEK.
The first International Space Station component was launched in 1998, and the first long-term residents arrived on 2 November 2000 after being launched from the Baikonur Cosmodrome on 31 October 2000. The station has since been continuously occupied for 20 years and 223 days, the longest continuous human presence in low Earth orbit, having surpassed the previous record of 9 years and 357 days held by the Mir space station.
Purpose of International Space Station
The International Space Station was originally intended to be a laboratory, observatory, and factory while providing transportation, maintenance, and a low Earth orbit staging base for possible future missions to the Moon, Mars, and asteroids. However, not all of the uses envisioned in the initial memorandum of understanding between NASA and Roscosmos have been realized. In the 2010 United States National Space Policy, the ISS was given additional roles of serving commercial, diplomatic, and educational purposes.
The International Space Station provides a platform to conduct scientific research, with power, data, cooling, and crew available to support experiments. Small uncrewed spacecraft can also provide platforms for experiments, especially those involving zero gravity and exposure to space, but space stations offer a long-term environment where studies can be performed potentially for decades, combined with ready access by human researchers.
The ISS simplifies individual experiments by allowing groups of experiments to share the same launches and crew time.
Research is conducted in a wide variety of fields, including astrobiology, astronomy, physical sciences, materials science, space weather, meteorology, and human research including space medicine and the life sciences. Scientists on Earth have timely access to the data and can suggest experimental modifications to the crew. If follow-on experiments are necessary, the routinely scheduled launches of resupply craft allow new hardware to be launched with relative ease. Crews fly expeditions of several months’ duration, providing approximately 160 person-hours per week of labor with a crew of six. However, a considerable amount of crew time is taken up by station maintenance.
Gravity at the altitude of the ISS is approximately 90% as strong as at Earth’s surface, but objects in orbit are in a continuous state of freefall, resulting in an apparent state of weightlessness. This perceived weightlessness is disturbed by five separate effects:
- Drag from the residual atmosphere.
- Vibration from the movements of mechanical systems and the crew.
- Actuation of the on-board attitude control moment gyroscopes.
- Thruster firings for attitude or orbital changes.
- Gravity-gradient effects, also known as tidal effects.
If not attached to the station, items at different locations within the ISS would follow slightly different orbits. Being mechanically interconnected these items experience small forces that keep the station moving as a rigid body.
Researchers are investigating the effect of the station’s near-weightless environment on the evolution, development, growth, and internal processes of plants and animals. In response to some of this data, NASA wants to investigate microgravity’s effects on the growth of three-dimensional, human-like tissues, and the unusual protein crystals that can be formed in space.
Investigating the physics of fluids in microgravity will provide better models of the behavior of fluids. Because fluids can be almost completely combined in microgravity, physicists investigate fluids that do not mix well on Earth. In addition, examining reactions that are slowed by low gravity and low temperatures will improve our understanding of superconductivity.
The study of materials science is an important ISS research activity, intending to reap economic benefits through the improvement of techniques used on the ground. Other areas of interest include the effect of the low gravity environment on combustion by studying the efficiency of burning and controlling emissions and pollutants. These findings may improve current knowledge about energy production, and lead to economic and environmental benefits.
The ISS provides a location in the relative safety of low Earth orbit to test spacecraft systems that will be required for long-duration missions to the Moon and Mars. This provides experience in operations, maintenance as well as repair and replacement activities on-orbit, which will be essential skills in operating spacecraft farther from Earth, mission risks can be reduced and the capabilities of interplanetary spacecraft advanced.
Referring to the MARS-500 experiment, ESA states that “Whereas the ISS is essential for answering questions concerning the possible impact of weightlessness, radiation and other space-specific factors, aspects such as the effect of long-term isolation and confinement can be more appropriately addressed via ground-based simulations”. Sergey Krasnov, the head of human space flight programs for Russia’s space agency, Roscosmos, in 2011 suggested a “shorter version” of MARS-500 may be carried out on the ISS.
The critical systems are the atmosphere control system, the water supply system, the food supply facilities, the sanitation and hygiene equipment, and fire detection and suppression equipment. The Russian Orbital Segment’s life support systems are contained in the Zvezda service module. Some of these systems are supplemented by equipment in the USOS. The Nauka laboratory has a complete set of life support systems.
The atmosphere onboard the ISS is similar to that of Earth. Normal air pressure on the ISS is 101.3 kPa, the same as at sea level on Earth. An Earth-like atmosphere offers benefits for crew comfort, and is much safer than a pure oxygen atmosphere, because of the increased risk of a fire such as that responsible for the deaths of the Apollo 1 crew. Earth-like atmospheric conditions have been maintained on all Russian and Soviet spacecraft.
The Elektron system aboard Zvezda and a similar system in Destiny generate oxygen aboard the station. The crew has a backup option in the form of bottled oxygen and Solid Fuel Oxygen Generation (SFOG) canisters, a chemical oxygen generator system. Carbon dioxide is removed from the air by the Vozdukh system in Zvezda. Other by-products of human metabolisms, such as methane from the intestines and ammonia from sweat, are removed by activated charcoal filters.
Part of the ROS atmosphere control system is the oxygen supply. Triple-redundancy is provided by the Elektron unit, solid fuel generators, and stored oxygen. The primary supply of oxygen is the Elektron unit which produces O2 and H2 by electrolysis of water and vents H2 overboard. The 1 kW system uses approximately one liter of water per crew member per day. This water is either brought from Earth or recycled from other systems. Mir was the first spacecraft to use recycled water for oxygen production. The secondary oxygen supply is provided by burning O2 producing Vika cartridges.
Power and thermal control
Double-sided solar arrays provide electrical power to the International Space Station. These bifacial cells collect direct sunlight on one side and light reflected off from the Earth on the other, and are more efficient and operate at a lower temperature than single-sided cells commonly used on Earth.
The Russian segment of the station, like most spacecraft, uses 28 V low voltage DC from two rotating solar arrays mounted on Zvezda. The USOS uses 130–180 V DC from the USOS PV array, power is stabilized and distributed at 160 V DC and converted to the user-required 124 V DC. The higher distribution voltage allows smaller, lighter conductors, at the expense of crew safety. The two station segments share power with converters.
The USOS solar arrays are arranged as four wing pairs, for a total production of 75 to 90 kWs. These arrays normally track the Sun to maximize power generation. Each array is about 375 m^2 in the area and 58 m long. In the complete configuration, the solar arrays track the sun by rotating the alpha gimbal once per orbit, the beta gimbal follows slower changes in the angle of the Sun to the orbital plane. The Night Glider mode aligns the solar arrays parallel to the ground at night to reduce the significant aerodynamic drag at the station’s relatively low orbital altitude.
The station’s large solar panels generate a high potential voltage difference between the station and the ionosphere. This could cause arcing by insulating surfaces and sputtering conductive surfaces as ions are accelerated by the spacecraft plasma sheath. To mitigate this, plasma contactor units (PCU)s creates current paths between the station and the ambient plasma field.
Communications and computers
Radio communications provide telemetry and scientific data links between the station and mission control centers. Radio links are also used during rendezvous and docking procedures and for audio and video communication between crew members, flight controllers, and family members. As a result, the ISS is equipped with internal and external communication systems used for different purposes.
The Russian Orbital Segment communicates directly with the ground via the Lira antenna mounted to Zvezda. The Lira antenna also has the capability to use the Luch data relay satellite system. This system fell into disrepair during the 1990s, and so was not used during the early years of the ISS, although two new Luch satellites—Luch-5A and Luch-5B—were launched in 2011 and 2012 respectively to restore the operational capability of the system. Another Russian communications system is the Voskhod-M, which enables internal telephone communications between Zvezda, Zarya, Pirs, Poisk, and the USOS and provides a VHF radio link to ground control centers via antennas on Zvezda’s exterior.
The US Orbital Segment (USOS) makes use of two separate radio links mounted in the Z1 truss structure: the S-band (audio) and Ku band (audio, video, and data) systems. These transmissions are routed via the United States Tracking and Data Relay Satellite System (TDRSS) in geostationary orbit, allowing for almost continuous real-time communications with Christopher C. Kraft Jr. Mission Control Center (MCC-H) in Houston. Data channels for the Canadarm2, European Columbus laboratory, and Japanese Kibō modules were originally also routed via the S-band and Ku band systems, with the European Data Relay System and a similar Japanese system intended to eventually complement the TDRSS in this role. Communications between modules are carried on an internal wireless network.
Cost for International Space Station
The ISS has been described as the most expensive single item ever constructed. As of 2010, the total cost was US$150 billion. This includes NASA’s budget of $58.7 billion (inflation-unadjusted) for the station from 1985 to 2015 ($72.4 billion in 2010 dollars), Russia’s $12 billion, Europe’s $5 billion, Japan’s $5 billion, Canada’s $2 billion, and the cost of 36 shuttle flights to build the station, estimated at $1.4 billion each, or $50.4 billion in total. Assuming 20,000 person-days of use from 2000 to 2015 by two- to six-person crews, each person-day would cost $7.5 million, less than half the inflation-adjusted $19.6 million ($5.5 million before inflation) per person-day of Skylab.