international space station (page 4)

Power and thermal control

Double-sided solar, or Photovoltaic arrays, provide electrical power for the ISS. These bifacial cells are more efficient and operate at a lower temperature than single-sided cells commonly used on Earth, by collecting sunlight on one side and light reflected off the Earth on the other.^[169]

The Russian segment of the station, like the Space Shuttle and most spacecraft, uses 28 volt DC from four rotating solar arrays mounted on Zarya and Zvezda. The USOS uses 130–180 V DC from the USOS PV array, power is stabilised 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 ROS uses low voltage. The two station segments share power with converters.^[134]

The USOS solar arrays are arranged as four wing pairs, with each wing producing nearly 32.8 kW.^[134] These arrays normally track the sun to maximise power generation. Each array is about 375 m² (450 yd²) in area and 58 metres (63 yd) long. In the complete configuration, the solar arrays track the sun by rotating the alpha gimbal once per orbit while 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.^[170]

The station uses rechargeable nickel-hydrogen batteries (NiH₂) for continuous power during the 35 minutes of every 90 minute orbit that it is eclipsed by the Earth. The batteries are recharged on the day side of the Earth. They have a 6.5 year lifetime (over 37,000 charge/discharge cycles) and will be regularly replaced over the anticipated 20-year life of the station.^[171]

The station's large solar panels generate a high potential voltage difference between the station and the ionosphere. This could cause arcing through insulating surfaces and sputtering of conductive surfaces as ions are accelerated by the spacecraft plasma sheath. To mitigate this, plasma contactor units (PCU)s create current paths between the station and the ambient plasma field.^[172]

The large amount of electrical power consumed by the station's systems and experiments is turned almost entirely into heat. The heat which can be dissipated through the walls of the stations modules is insufficient to keep the internal ambient temperature within comfortable, workable limits. Ammonia is continuously pumped through pipework throughout the station to collect heat, and then into external radiators exposed to the cold of space, and back into the station.

The International Space Station (ISS) External Active Thermal Control System (EATCS) maintains an equilibrium when the ISS environment or heat loads exceed the capabilities of the Passive Thermal Control System (PTCS). Note Elements of the PTCS are external surface materials, insulation such as MLI, or Heat Pipes. The EATCS provides heat rejection capabilities for all the US pressurised modules, including the JEM and COF as well as the main power distribution electronics of the S0, S1 and P1 Trusses. The EATCS consists of two independent Loops (Loop A & Loop B), they both use mechanically pumped Ammonia in fluid state, in closed-loop circuits. The EATCS is capable of rejecting up to 70 kW, and provides a substantial upgrade in heat rejection capacity from the 14 kW capability of the Early External Active Thermal Control System (EEATCS) via the Early Ammonia Servicer (EAS), which was launched on STS-105 and installed onto the P6 Truss.^[173]

Communications and computers

Radio communications provide telemetry and scientific data links between the station and Mission Control Centres. Radio links are also used during rendezvous and docking procedures and for audio and video communication between crewmembers, flight controllers and family members. As a result, the ISS is equipped with internal and external communication systems used for different purposes.^[174]

The Russian Orbital Segment communicates directly with the ground via the Lira antenna mounted to Zvezda.^[15][175] The Lira antenna also has the capability to use the Luch data relay satellite system.^[15] This system, used for communications with Mir, fell into disrepair during the 1990s, and as a result is no longer in use,^{[15][176][177]} 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.^[178] Another Russian communications system is the Voskhod-M, which enables internal telephone communications between Zvezda, Zarya, Pirs, Poisk and the USOS, and also provides a VHF radio link to ground control centres via antennas on Zvezda's exterior.^[179]

The US Orbital Segment (USOS) makes use of two separate radio links mounted in the Z1 truss structure: the S band (used for audio) and K_u band (used for audio, video and data) systems. These transmissions are routed via the United States Tracking and Data Relay Satellite System (TDRSS) in geostationary orbit, which allows for almost continuous real-time communications with NASA's Mission Control Center (MCC-H) in Houston.^[9][15][174] Data channels for the Canadarm2, European Columbus laboratory and Japanese Kibō modules are routed via the S band and K_u band systems, although the European Data Relay Satellite System and a similar Japanese system will eventually complement the TDRSS in this role.^[9][180] Communications between modules are carried on an internal digital wireless network.^[181]

UHF radio is used by astronauts and cosmonauts conducting EVAs. UHF is employed by other spacecraft that dock to or undock from the station, such as Soyuz, Progress, HTV, ATV and the Space Shuttle (except the shuttle also makes use of the S band and K_u band systems via TDRSS), to receive commands from Mission Control and ISS crewmembers.^[15] Automated spacecraft are fitted with their own communications equipment; the ATV uses a laser attached to the spacecraft and equipment attached to Zvezda, known as the Proximity Communications Equipment, to accurately dock to the station.^[182][183]

The ISS is equipped with approximately 100 IBM and Lenovo ThinkPad model A31 and T61P laptop computers. Each computer is a commercial off-the-shelf purchase which is then modified for safety and operation including updates to connectors, cooling and power to accommodate the station's 28V DC power system and weightless environment. Heat generated by the laptops does not rise, but stagnates surrounding the laptop, so additional forced ventilation is required. Laptops aboard the ISS are connected to the station's wireless LAN via Wi-Fi and are connected to the ground at 3 Mbit/s up and 10 Mbit/s down, comparable to home DSL connection speeds.^[184]

The operating system used for key station functions is the Debian version of Linux.^[185] The migration from Microsoft Windows was made in May 2013 for reasons of reliability, stability and flexibility.^[186]

Station operations

Expeditions and private flights

See also the list of professional crew, private travellers, or both

Each permanent crew is given an expedition number. Expeditions run up to six months, from launch until undocking, an 'increment' covers the same time period, but includes cargo ships and all activities. Expeditions 1 to 6 consisted of 3 person crews, Expeditions 7 to 12 were reduced to the safe minimum of two following the destruction of the NASA Shuttle Columbia. From Expedition 13 the crew gradually increased to 6 around 2010.^[187][188] With the arrival of the American Commercial Crew vehicles in the middle of the 2010s, expedition size may be increased to seven crew members, the number ISS is designed for.^[189][190]

Sergei Krikalev, member of Expedition 1 and Commander of Expedition 11 has spent more time in space than anyone else, a total of 803 days and 9 hours and 39 minutes. His awards include the Order of Lenin, Hero of the Soviet Union, Hero of the Russian Federation, and 4 NASA medals. On 16 August 2005 at 1:44 am EDT he passed the record of 748 days held by Sergei Avdeyev, who had 'time travelled' 1/50th of a second into the future on board MIR.^[191] He participated in psychosocial experiment SFINCSS-99 (Simulation of Flight of International Crew on Space Station), which examined inter-cultural and other stress factors affecting integration of crew in preparation for the ISS spaceflights. Commander Michael Fincke has spent a total of 382 days in space - more than any other American astronaut.

Travellers who pay for their own passage into space are termed spaceflight participants by Roskosmos and NASA, and are sometimes informally referred to as space tourists, a term they generally dislike.^{[note 1]} All seven were transported to the ISS on Russian Soyuz spacecraft. When professional crews change over in numbers not divisible by the three seats in a Soyuz, and a short-stay crewmember is not sent, the spare seat is sold by MirCorp through Space Adventures. When the space shuttle retired in 2011, and the station's crew size was reduced to 6, space tourism was halted, as the partners relied on Russian transport seats for access to the station. Soyuz flight schedules increase after 2013, allowing 5 Soyuz flights (15 seats) with only two expeditions (12 seats) required.^[197] The remaining seats are sold for around US$40 million to members of the public who can pass a medical. ESA and NASA criticised private spaceflight at the beginning of the ISS, and NASA initially resisted training Dennis Tito, the first man to pay for his own passage to the ISS.^{[note 2]} Toyohiro Akiyama was flown to Mir for a week, he was classed as a business traveller, as his employer, Tokyo Broadcasting System, paid for his ticket, and he gave a daily TV broadcast from orbit.

Anousheh Ansari (Persian: انوشه انصاری‎) became the first Iranian in space and the first self-funded woman to fly to the station. Officials reported that her education and experience make her much more than a tourist, and her performance in training had been "excellent."^[198] Ansari herself dismisses the idea that she is a tourist. She did Russian and European studies involving medicine and microbiology during her 10 day stay. The documentary Space Tourists follows her journey to the station, where she fulfilled the childhood dream 'to leave our planet as a normal person and travel into outer space.'^[199] In the film, some Kazakhs are shown waiting in the middle of the steppes for four rocket stages to literally fall from the sky. Film-maker Christian Frei states "Filming the work of the Kazakh scrap metal collectors was anything but easy. The Russian authorities finally gave us a film permit in principle, but they imposed crippling preconditions on our activities. The real daily routine of the scrap metal collectors could definitely not be shown. Secret service agents and military personnel dressed in overalls and helmets were willing to re-enact their work for the cameras – in an idealised way that officials in Moscow deemed to be presentable, but not at all how it takes place in reality."

Spaceflight participant Richard Garriott placed a geocache aboard the ISS during his flight.^[200] This is currently the only non-terrestrial geocache in existence.^[201]

Crew activities

A typical day for the crew begins with a wake-up at 06:00, followed by post-sleep activities and a morning inspection of the station. The crew then eats breakfast and takes part in a daily planning conference with Mission Control before starting work at around 08:10. The first scheduled exercise of the day follows, after which the crew continues work until 13:05. Following a one-hour lunch break, the afternoon consists of more exercise and work before the crew carries out its pre-sleep activities beginning at 19:30, including dinner and a crew conference. The scheduled sleep period begins at 21:30. In general, the crew works ten hours per day on a weekday, and five hours on Saturdays, with the rest of the time their own for relaxation or work catch-up.^[202]

The station provides crew quarters for each member of the expedition's crew, with two 'sleep stations' in the Zvezda and four more installed in Harmony.^[203][204] The American quarters are private, approximately person-sized soundproof booths. The Russian crew quarters include a small window, but do not provide the same amount of ventilation or block the same amount of noise as their American counterparts. A crewmember can sleep in a crew quarter in a tethered sleeping bag, listen to music, use a laptop, and store personal items in a large drawer or in nets attached to the module's walls. The module also provides a reading lamp, a shelf and a desktop.^{[162][163][164]} Visiting crews have no allocated sleep module, and attach a sleeping bag to an available space on a wall—it is possible to sleep floating freely through the station, but this is generally avoided because of the possibility of bumping into sensitive equipment.^[166] It is important that crew accommodations be well ventilated; otherwise, astronauts can wake up oxygen-deprived and gasping for air, because a bubble of their own exhaled carbon dioxide has formed around their heads.^[162]

Orbit and mission control

The ISS is maintained in a nearly circular orbit with a minimum mean altitude of 330 km (205 mi) and a maximum of 410 km (255 mi), in the centre of the Thermosphere, at an inclination of 51.6 degrees to Earth's equator, necessary to ensure that Russian Soyuz and Progress spacecraft launched from the Baikonur Cosmodrome may be safely launched to reach the station. Spent rocket stages must be dropped into uninhabited areas and this limits the directions rockets can be launched from the spaceport.^[205][206] The orbital inclination chosen was also low enough to allow American space shuttles launched from Florida to reach the ISS.

It travels at an average speed of 27,724 kilometres (17,227 mi) per hour, and completes 15.50 orbits per day.^[1][19] The station's altitude was allowed to fall around the time of each NASA shuttle mission. Orbital boost burns would generally be delayed until after the shuttle's departure. This allowed shuttle payloads to be lifted with the station's engines during the routine firings, rather than have the shuttle lift itself and the payload together to a higher orbit. This trade-off allowed heavier loads to be transferred to the station. After the retirement of the NASA shuttle, the nominal orbit of the space station was raised in altitude.^[207][208] Other, more frequent supply ships do not require this adjustment as they are substantially lighter vehicles.^[26][209]

Orbital boosting can be performed by the station's two main engines on the Zvezda service module, or Russian or European spacecraft docked to Zvezda's aft port. The ATV has been designed with the possibility of adding a second docking port to its other end, allowing it to remain at the ISS and still allow other craft to dock and boost the station. It takes approximately two orbits (three hours) for the boost to a higher altitude to be completed.^[209] In December 2008 NASA signed an agreement with the Ad Astra Rocket Company which may result in the testing on the ISS of a VASIMR plasma propulsion engine.^[210] This technology could allow station-keeping to be done more economically than at present.^[211][212]

The Russian Orbital Segment contains the station's engines and control bridge, which handles Guidance, Navigation and Control (ROS GNC) for the entire station.^[104] Initially, Zarya, the first module of the station, controlled the station until a short time after the Russian service module Zvezda docked and was transferred control. Zvezda contains the ESA built DMS-R Data Management System.^[213] Using two fault-tolerant computers (FTC), Zvezda computes the station's position and orbital trajectory using redundant Earth horizon sensors, Solar horizon sensors as well as Sun and star trackers. The FTCs each contain three identical processing units working in parallel and provide advanced fault-masking by majority voting. Zvezda uses gyroscopes and thrusters to turn itself around. Gyroscopes do not require propellant, rather they use electricity to 'store' momentum in flywheels by turning in the opposite direction to the station's movement. The USOS has its own computer controlled gyroscopes to handle the extra mass of that section. When gyroscopes 'saturate', reaching their maximum speed, thrusters are used to cancel out the stored momentum. During Expedition 10, an incorrect command was sent to the station's computer, using about 14 kilograms of propellant before the fault was noticed and fixed. When attitude control computers in the ROS and USOS fail to communicate properly, it can result in a rare 'force fight' where the ROS GNC computer must ignore the USOS counterpart, which has no thrusters.^{[214][215][216]} When an ATV, Nasa Shuttle, or Soyuz is docked to the station, it can also be used to maintain station attitude such as for troubleshooting. Shuttle control was used exclusively during installation of the S3/S4 truss, which provides electrical power and data interfaces for the station's electronics.^[217]

The components of the ISS are operated and monitored by their respective space agencies at mission control centres across the globe, including:

• Roskosmos's Mission Control Center at Korolyov, Moscow Oblast, controls the Russian Orbital Segment which handles Guidance, Navigation & Control for the entire Station.,^[104][213] in addition to individual Soyuz and Progress missions.^[15]
• ESA's ATV Control Centre, at the Toulouse Space Centre (CST) in Toulouse, France, controls flights of the unmanned European Automated Transfer Vehicle.^[15]
• JAXA's JEM Control Centre and HTV Control Centre at Tsukuba Space Centre (TKSC) in Tsukuba, Japan, are responsible for operating the Japanese Experiment Module complex and all flights of the 'White Stork' HTV Cargo spacecraft, respectively.^[15]
• NASA's Mission Control Center at Lyndon B. Johnson Space Center in Houston, Texas, USA, serves as the primary control facility for the United States segment of the ISS and also controlled the Space Shuttle missions that visited the station.^[15]
• NASA's Payload Operations and Integration Center at Marshall Space Flight Center in Huntsville, Alabama, coordinates payload operations in the USOS.^[15]
• ESA's Columbus Control Centre at the German Aerospace Centre (DLR) in Oberpfaffenhofen, Germany, manages the European Columbus research laboratory.^[15]
• CSA's MSS Control at Saint-Hubert, Quebec, Canada, controls and monitors the Mobile Servicing System, or Canadarm2.^[15]

Repairs

Orbital Replacement Units (ORUs) are spare parts that can be readily replaced when a unit either passes its design life or fails. Examples of ORUs are pumps, storage tanks, controller boxes, antennas, and battery units. Some units can be replaced using robotic arms. Many are stored outside the station, either on small pallets called ExPRESS Logistics Carriers (ELCs) or share larger platforms called External Stowage Platforms which also hold science experiments. Both kinds of pallets have electricity as many parts which could be damaged by the cold of space require heating. The larger logistics carriers also have computer local area network connections (LAN) and telemetry to connect experiments. A heavy emphasis on stocking the USOS with ORU's occurred around 2011, before the end of the NASA shuttle programme, as its commercial replacements, Cygnus and Dragon, carry one tenth to one quarter the payload.

Unexpected problems and failures have impacted the station's assembly time-line and work schedules leading to periods of reduced capabilities and, in some cases, could have forced abandonment of the station for safety reasons, had these problems not been resolved. During STS-120 on 2007, following the relocation of the P6 truss and solar arrays, it was noted during the redeployment of the array that it had become torn and was not deploying properly.^[218] An EVA was carried out by Scott Parazynski, assisted by Douglas Wheelock, the men took extra precautions to reduce the risk of electric shock, as the repairs were carried out with the solar array exposed to sunlight.^[219] The issues with the array were followed in the same year by problems with the starboard Solar Alpha Rotary Joint (SARJ), which rotates the arrays on the starboard side of the station. Excessive vibration and high-current spikes in the array drive motor were noted, resulting in a decision to substantially curtail motion of the starboard SARJ until the cause was understood. Inspections during EVAs on STS-120 and STS-123 showed extensive contamination from metallic shavings and debris in the large drive gear and confirmed damage to the large metallic race ring at the heart of the joint, and so the joint was locked to prevent further damage.^[220] Repairs to the joint were carried out during STS-126 with lubrication of both joints and the replacement of 11 out of 12 trundle bearings on the joint.^[221][222]

2009 saw damage to the S1 radiator, one of the components of the station's cooling system. The problem was first noticed in Soyuz imagery in September 2008, but was not thought to be serious.^[223] The imagery showed that the surface of one sub-panel has peeled back from the underlying central structure, possibly due to micro-meteoroid or debris impact. It is also known that a Service Module thruster cover, jettisoned during an EVA in 2008, had struck the S1 radiator, but its effect, if any, has not been determined. On 15 May 2009 the damaged radiator panel's ammonia tubing was mechanically shut off from the rest of the cooling system by the computer-controlled closure of a valve. The same valve was used immediately afterwards to vent the ammonia from the damaged panel, eliminating the possibility of an ammonia leak from the cooling system via the damaged panel.^[223]

Early on 1 August 2010, a failure in cooling Loop A (starboard side), one of two external cooling loops, left the station with only half of its normal cooling capacity and zero redundancy in some systems.^{[224][225][226]} The problem appeared to be in the ammonia pump module that circulates the ammonia cooling fluid. Several subsystems, including two of the four CMGs, were shut down.

Planned operations on the ISS were interrupted through a series of EVAs to address the cooling system issue. A first EVA on 7 August 2010, to replace the failed pump module, was not fully completed due to an ammonia leak in one of four quick-disconnects. A second EVA on 11 August successfully removed the failed pump module.^[227][228] A third EVA was required to restore Loop A to normal functionality.^[229][230]

The USOS's cooling system is largely built by the American company Boeing,^[231] which is also the manufacturer of the failed pump.^[232]

An air leak from the USOS in 2004,^[233] the venting of fumes from an Elektron oxygen generator in 2006,^[234] and the failure of the computers in the ROS in 2007 during STS-117 which left the station without thruster, Elektron, Vozdukh and other environmental control system operations, the root cause of which was found to be condensation inside the electrical connectors leading to a short-circuit.^{[citation needed]}

The four Main Bus Switching Units (MBSUs, located in the S0 truss), control the routing of power from the four solar array wings to the rest of the ISS. In late 2011 MBSU-1, while still routing power correctly, ceased responding to commands or sending data confirming its health, and was scheduled to be swapped out at the next available EVA. In each MBSU, two power channels feed 160V DC from the arrays to two DC-to-DC power converters (DDCUs) that supply the 124V power used in the station. A spare MBSU was already on board, but the 30 August 2012 EVA failed to be completed when a bolt being tightened to finish installation of the spare unit jammed before electrical connection was secured.^[235] The loss of MBSU-1 limits the station to 75% of its normal power capacity, requiring minor limitations in normal operations until the problem can be addressed.

As of 2 September 2012, a second EVA to tighten the balky bolt, completing the installation of the replacement MBSU-1 in an attempt to restore full power, has been scheduled for Wednesday,^[236] Yet in the meanwhile, a third solar array wing has gone off line due to some fault in that array's Direct Current Switching Unit (DCSU) or its associated system, further reducing ISS power to just five of the eight solar array wings for the first time in several years.

On 5 September 2012, in a second, 6 hr, EVA to replace MBSU-1, astronauts Sunita Williams and Akihiko Hoshide successfully restored the ISS to 100% power.^[237]

Fleet operations

Progress M-20M (ISS-52P) was the 54th progress spacecraft to arrive at the ISS, including M-MIM2 and M-SO1 which installed modules. Thirty-five flights of the retired NASA Space Shuttle were made to the station.^[2] TMA-09M is the 35th Soyuz flight, and there have been four European ATV, four Japanese Kounotori 'White Stork', and three SpaceX Dragon arrivals.

Currently docked/berthed

See also the list of professional crew, private travellers, both or just unmanned spaceflights.

Spacecraft and mission			Location	Arrived (UTC)	Departure date
	Soyuz TMA-08M	Expedition 35/36	Poisk Zenith	29 March 2013	10 September 2013
	Soyuz TMA-09M	Expedition 36/37	Rassvet nadir	29 May 2013	11 November 2013
	Albert Einstein ATV	ATV-4 Cargo	Zvezda aft	15 June 2013	28 October 2013
	Progress M-20M	Progress 52 Cargo	Pirs nadir	28 July 2013	4 February 2014

Scheduled launches and dockings/berthings

All dates are UTC. Dates are the earliest possible dates and may change. Forward ports are at the front of the station according to its normal direction of travel and orientation (attitude). Aft is at the rear of the station, used by spacecraft boosting the station's orbit. Nadir is closest the Earth, Zenith is on top.

Uncrewed cargoships are in light blue. Crewed spacecraft are in light green. Modules are white. Spacecraft operated by government agencies are indicated with 'Gov', while 'Com' denotes those operated under commercial arrangements.

Spacecraft and operator			Spaceport and mission		Launch	Docking/Berthing Port
2013 launches					2013
	Com	Cygnus COTS Demo	MARS (in USA)	Cygnus COTS Demo	17 September	Harmony nadir
	Gov	Soyuz TMA-10M	Baikonur	Expedition 37/38	25 September	Poisk Zenith
	Gov	Soyuz TMA-11M	Baikonur	Expedition 38/39	7 November	Rassvet nadir
	Gov	Progress M-21M	Baikonur	Progress 53 Cargo	20 November	Zvezda aft
	Com	Cygnus CRS Orb-1	MARS (in USA)	Cygnus 1 Cargo	11 December	Harmony nadir
2014 onwards					2014
	Com	SpaceX CRS-3	Cape Canaveral	Dragon 3 Cargo	January	Harmony nadir
	Gov	Progress M-22M	Baikonur	Progress 54 Cargo	February	Pirs nadir
	Gov	Soyuz TMA-12M	Baikonur	Expedition 39/40	March	Poisk zenith
	Com	SpaceX CRS-4	Cape Canaveral	Dragon 4 Cargo	April	Harmony nadir
	Gov	Proton	Baikonur	Module Nauka MLM	2014	Zvezda nadir
	Gov	Progress M-UM & Soyuz-2.1b	Baikonur	Module Node Module	NET 2014	Nauka nadir
	Gov	Proton-M (or Angara A5)	Baikonur	Module NEM-1	NET 2015	Node Module nadir
	Gov	Proton-M (or Angara A5)	Baikonur	Module NEM-2	NET 2015	Node Module nadir
	Com	SpaceX CRS-8	Cape Canaveral	Bigelow Expandable Activity Module	2015	Harmony nadir (BEAM to Tranquility PMA-3)

Docking

All Russian manned spacecraft, modules, and progress craft are able to rendezvous and dock to the space station without human intervention. Using Kurs radar they detect and intercept the ISS from over 200 kilometres away. The European ATV uses star sensors and GPS to determine its intercept course. When it catches up it then uses laser equipment to optically recognise Zvezda, with Russian Kurs redundancy. Crew supervise these craft, but do not intervene except to send abort commands in emergencies. The Japanese H-II Transfer Vehicle parks itself in progressively closer orbits to the station, and then awaits 'approach' commands from the crew, until it is close enough for the crew to grapple it with a robotic arm and berth it to the USOS. The American Space Shuttle was manually docked, and on missions with a cargo container, the container would be berthed to the Station with the use of manual robotic arms. Berthed craft can transfer International Standard Payload Racks. Japanese spacecraft berth for one to two months. Russian and European Supply craft can remain at the ISS for six months,^[238][239] allowing great flexibility in crew time for loading and unloading of supplies and trash. NASA Shuttles could remain docked for 11–12 days.^[240]

The American manual approach to docking allows greater initial flexibility and less complexity. The downside to this mode of operation is that each mission becomes unique and requires specialised training and planning, making the process more labour-intensive and expensive. The Russians pursued an automated methodology that used the crew in override or monitoring roles. Although the initial development costs were high, the system has become very reliable with standardizations that provide significant cost benefits in repetitive routine operations.^[241] An automated approach could allow assembly of modules orbiting other worlds prior to manned missions.

Soyuz manned spacecraft for crew rotation also serve as lifeboats for emergency evacuation, they are replaced every six months and have been used once to remove excess crew after the Columbia disaster.^[242] Expeditions require, on average, 2 722 kg of supplies, and as of 9 March 2011, crews had consumed a total of around 22 000 meals.^[2] Soyuz crew rotation flights and Progress resupply flights visit the station on average two and three times respectively each year,^[243] with the ATV and HTV planned to visit annually from 2010 onwards.^{[citation needed]} Following retirement of the NASA Shuttle Cygnus and Dragon were contracted to fly cargo to the station.^[244][245]

From 26 February 2011 to 7 March 2011 four of the governmental partners (United States, ESA, Japan and Russia) had their spacecraft (NASA Shuttle, ATV, HTV, Progress and Soyuz) docked at the ISS, the only time this has happened to date.^[246] On 25 May 2012, SpaceX became the world's first privately held company to send a cargo load, via the Dragon spacecraft, to the International Sp

Author:	Bling King
Published:	Sep 9th 2013
Modified:	Sep 9th 2013