International Space Station

International Space Station
The International Space Station (ISS) is a habitable artificial satellite in low Earth orbit. It follows the Salyut, Almaz, Skylab and Mir stations as the ninth space station to be inhabited. The ISS is a modular structure whose first component was launched in 1998. Now the largest artificial body in orbit, it can often be seen at the appropriate time with the naked eye from Earth. The ISS consists of pressurised modules, external trusses, solar arrays and other components. ISS components have been launched by American Space Shuttles as well as Russian Proton and Soyuz rockets. Budget constraints led to the merger of three space station projects with the Japanese Kibō module and Canadian robotics. In 1993 the partially built components for a Soviet/Russian space station Mir-2, the proposed American Freedom, and the proposed European Columbus merged into a single multinational programme.
The ISS serves as a microgravity and space environment research laboratory in which crew members conduct experiments in biology, human biology, physics, astronomy, meteorology and other fields. The station is suited for the testing of spacecraft systems and equipment required for missions to the Moon and Mars.
The station has been continuously occupied for over 12 years, having exceeded the previous record of almost 10 years (or 3,634 days) held by Mir, in 2010. The station is serviced by Soyuz spacecraft, Progress spacecraft, the Automated Transfer Vehicle, the H-II Transfer Vehicle, and the Dragon spacecraft. It has been visited by astronauts and cosmonauts from 15 different nations.
The ISS programme is a joint project between five participating space agencies: NASA, the Russian Federal Space Agency, JAXA, ESA, and CSA. The ownership and use of the space station is established by intergovernmental treaties and agreements. The station is divided into two sections, the Russian orbital segment (ROS) and the United States orbital segment (USOS), which is shared by many nations. The ISS is maintained at an orbital altitude of between 330 km (205 mi) and 410 km (255 mi). It completes 15.7 orbits per day. The ISS is funded until 2020, and may operate until 2028. The Russian Federal Space Agency (RSA/RKA) has proposed using ISS to commission modules for a new space station, called OPSEK, before the remainder of the ISS is de-orbited.

Contrary to popular belief, the earth's gravity is only slightly less at the altitude of the ISS as at the surface. According to the equivalence principle, gravity only seems absent because, like any orbiting object, it is in continuous freefall. This state of perceived weightlessness is not perfect however, being disturbed by five separate effects:
Drag from the residual atmosphere; when the ISS enters the Earth's shadow, the main solar panels are rotated to minimise this aerodynamic drag, helping reduce orbital decay:
  • Vibration from movements of mechanical systems and the crew.
  • Actuation of the on-board attitude control moment gyroscopes.
  • Thruster firings for altitude or orbital changes.
  • Gravity-gradient effects, also known as tidal effects. 
Items not at the exact ISS center of mass would, if not attached to the station, follow slightly different orbits than that of the ISS as a whole. Those closer to the earth would tend to follow faster, shorter orbits and move forward along the velocity vector. Those farther away would have slower, longer orbits and move rearward against the velocity vector. Those to the left or right of the ISS center of mass would be in different orbital planes. Being attached to the rigid ISS, however, these items experience small forces that keep them moving along with the ISS center of mass.
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.
The investigation of the physics of fluids in microgravity will allow researchers to model the behaviour of fluids better. Because fluids can be almost completely combined in microgravity, physicists investigate fluids that do not mix well on Earth. In addition, an examination of reactions that are slowed by low gravity and temperatures will give scientists a deeper understanding of superconductivity.
The study of materials science is an important ISS research activity, with the objective of reaping 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, through the study of the efficiency of burning and control of emissions and pollutants. These findings may improve current knowledge about energy production, and lead to economic and environmental benefits. Future plans are for the researchers aboard the ISS to examine aerosols, ozone, water vapour, and oxides in Earth's atmosphere, as well as cosmic rays, cosmic dust, antimatter, and dark matter in the universe.

Station structure
The ISS follows Salyut and Almaz series, Cosmos 557, Skylab, and Mir as the 11th space station launched, as the Genesis prototypes were never intended to be manned. The ISS is a third generation modular space station.
Other examples of modular station projects include the Soviet/Russian Mir, Russian OPSEK, and Chinese space station. The first space station, Salyut 1, and other one-piece or 'monolithic' first generation space stations, such as Salyut 2,3,4,5, DOS 2, Kosmos 557, Almaz and NASA's Skylab stations were not designed for re-supply. Generally, each crew had to depart the station to free the only docking port for the next crew to arrive, Skylab had more than one docking port but was not designed for resupply. Salyut 6 and 7 had more than one docking port and were designed to be resupplied routinely during crewed operation. Modular stations can allow the mission to be changed over time and new modules can be added or removed from the existing structure, allowing greater flexibility.
Below is a diagram of major station components. The blue areas are pressurised sections accessible by the crew without using spacesuits. The station's unpressurised superstructure is indicated in red. Other unpressurised components are yellow.