The process of assembling the International Space Station (ISS) has been under way since the 1990s. Zarya, the first ISS module, was launched by a Proton rocket on 20 November 1998. The STS-88 shuttle mission followed two weeks after Zarya was launched, bringing Unity, the first of three node modules, and connecting it to Zarya. This bare 2-module core of the ISS remained unmanned for the next one and a half years, until in July 2000 the Russian module Zvezda was added, allowing a maximum crew of two astronauts or cosmonauts to be on the ISS permanently.
The ISS has a planned pressurized volume of approximately 1,000 cubic meters, a mass of approximately 400,000 kilograms, approximately 100 kilowatts of power output, a truss 108.4 meters long, modules 74 meters long, and a crew of six. Building the complete station will require more than 40 assembly flights. As of March 2011, 26 Space Shuttle flights have docked with ISS to add elements, and 9 other Shuttle flights have flown logistics-servicing missions to ISS without adding major external elements. These 35 Shuttle missions include 9 SpaceHab and 10 MPLM logistics-servicing missions in various combinations. The last two planned Shuttle flights are due to add one of the two final elements of ISS, followed by one last Proton launch with the planned delivery of the ERA. Other assembly flights have consisted of modules lifted by the Russian Proton rocket or in the case of Pirs and Poisk by a Soyuz-U rocket.
Some of the larger modules include:Zarya (launched 20 November 1998)
Unity Module (launched 4 December 1998, also known as Node 1)
Zvezda (launched 12 July 2000)
Destiny Laboratory Module (launched 7 February 2001)
Columbus orbital facility (launched 7 February 2008)
Japanese Experiment Module, also known as Kibo (launched in multiple flights between 2008-2009)
The truss and solar panels are also a large part of the station. (launched in multiple flights between 2000-2009)
The space station is located in orbit around the Earth at an altitude of approximately 410 km (250 mi), a type of orbit usually termed low Earth orbit (the actual height varies over time by several kilometers due to atmospheric drag and reboosts). It orbits Earth in a period of about 90 minutes; by August 2007 it had completed more than 50,000 orbits since launch of Zarya on 20 November 1998.
A total of 14 main pressurized modules are scheduled to be part of the ISS by its completion date in 2010. A number of smaller pressurized sections will be adjunct to them (Soyuz spacecraft (permanently 2 as lifeboats - 6 months rotations), Progress transporters (2 or more), the Quest and Pirs airlocks, as well as periodically the Multi-Purpose Logistics Module, the Automated Transfer Vehicle and the H-II Transfer Vehicle).
The ISS, when completed, will consist of a set of communicating pressurized modules connected to a truss, on which four large pairs of photovoltaic modules (solar panels) are attached. The pressurized modules and the truss will be perpendicular: the truss spanning from starboard to port and the habitable zone extending on the aft-forward axis. Although during the construction the station attitude may vary, when all four photovoltaic modules are in their definitive position the aft-forward axis will be parallel to the velocity vector.
In addition to the assembly and utilization flights, approximately 30 Progress spacecraft flights are required to provide logistics until 2010. Experimental equipment, fuel and consumables are and will be delivered by all vehicles visiting the ISS: the Shuttle, the SpaceX Dragon, the Russian Progress, the European ATV and the Japanese HTV, and space station downmass will be carried back to Earth facilities on both the Shuttle and the Dragon.
At one point, there was some uncertainty over the future of the ISS. The Space Shuttle Columbia disaster on 1 February 2003, the subsequent two and a half-year suspension of the U.S. Space Shuttle program, followed by problems with resuming flight operations in 2005, were major obstacles.
The Space Shuttle program resumed flight on 26 July 2005, with the STS-114 mission of Discovery. This mission to the ISS was intended both to test new safety measures implemented since the Columbia disaster, and to deliver supplies to the station. Although the mission succeeded safely, it was not without risk; foam was shed by the external tank, leading NASA to announce future missions would be grounded until this issue was resolved.
Between the Columbia disaster and the resumption of Shuttle launches, crew exchanges were carried out solely using the Russian Soyuz spacecraft. Starting with Expedition 7, two-astronaut caretaker crews were launched in contrast to the previously launched crews of three. Because the ISS had not been visited by a shuttle for an extended period, a larger than planned amount of waste accumulated, temporarily hindering station operations in 2004. However Progress transports and the STS-114 shuttle flight took care of this problem.
ISS construction experienced delays due to the halting of all NASA Shuttle flights following the Columbia disaster in early 2003, although there had been prior delays due to Shuttle problems, and the Russian space agency's budget constraints. During the shuttle stand-down, construction of the ISS was halted and the science conducted aboard was limited due to the crew size of two. Many changes were made to the originally planned ISS, even before the Columbia disaster. Modules and other structures were cancelled or replaced and the number of Shuttle flights to the ISS was reduced from previously planned numbers. However more than 80% of the hardware intended to be part of the ISS in the late 1990s was orbited and is now part of the ISS's configuration.
In March 2006, a meeting of the heads of the five participating space agencies accepted the new ISS construction schedule that plans to complete the ISS by 2010. A crew of six has been established as of May 2009, following 12 Shuttle construction flights after the second "Return to Flight" mission STS-121. Requirements for stepping up the crew size included enhanced environmental support on the ISS, a second Soyuz permanently docked on the station to function as a second 'lifeboat', more frequent Progress flights to provide double the amount of consumables, more fuel for orbit raising maneuvers, and a sufficient supply line of experimental equipment.
Later additions included the Bigelow Expandable Activity Module (BEAM) in 2016, and numerous Russian components are planned as part of the in-orbit construction of OPSEK.
The ISS is made up of 15 pressurized modules: five Russian modules (Zarya, Pirs, Zvezda, Poisk and Rassvet), seven US modules (Leonardo, Harmony, Quest, Tranquility, Unity, Cupola, and Destiny), two Japanese modules (the JEM-ELM-PS and JEM-PM) and one European module (Columbus). One more Russian pressurized module (Nauka) is scheduled to be added to the station.
Although not permanently docked with the ISS, Multi-Purpose Logistics Modules (MPLMs) form part of the ISS during some Shuttle missions. An MPLM is attached to Harmony (initially to Unity) and is used for resupply and logistics flights.
Spacecraft attached to the ISS also extend the pressurized volume. At least one Soyuz spacecraft is always docked as a 'lifeboat' and is replaced every six months by a new Soyuz as part of crew rotation.
The table below shows the sequence in which these components were or will be added to the ISS. The numbers provided are indications and represent launch weight and dimensions.Interim Control Module - not needed once Zvezda was launched
ISS Propulsion Module - not needed once Zvezda was launched
Habitation Module (HAB) - With the cancellation of the Habitation Module, sleeping places are now spread throughout the station. There are two in the Russian segment and four in the US segment. It is not necessary to have a separate 'bunk' in space — many visitors just strap their sleeping bag to the wall of a module, get into it and sleep.
Crew Return Vehicle (CRV) - replaced by two Soyuz spacecraft
Centrifuge Accommodations Module (CAM) - would have been attached to Harmony (Node 2)
Science Power Platform (SPP) - power will be provided to the Russian segments partly by the US solar cell platforms
Russian Research Modules (RM1 and RM2) - to be replaced by single Multipurpose Laboratory Module (Nauka)
Universal Docking Module (UDM) - canceled along with the Research Modules which were to connect to it
The following modules are proposed, but not yet confirmed in the ISS launch manifests.American Node 4 - Also known as the Docking Hub System (DHS), would allow the station to have more docking ports for visiting vehicles and would allow inflatable habitats and technology demonstrations to be tested as part of the station.
Nautilus-X Centrifuge Demonstration - If produced, this centrifuge will be the first in-space demonstration of sufficient scale centrifuge for artificial partial-g effects. It will be designed to become a sleep module for the ISS crew.