Operations

Expeditionsedit

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 three-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 six around 2010. With the planned arrival of crew on US commercial vehicles in the early 2020s, expedition size may be increased to seven crew members, the number ISS is designed for.

Gennady Padalka, member of Expeditions 9, 19/20, 31/32, and 43/44, and Commander of Expedition 11, has spent more time in space than anyone else, a total of 878 days, 11 hours, and 29 minutes. Peggy Whitson has spent the most time in space of any American, totalling 665 days, 22 hours, and 22 minutes during her time on Expeditions 5, 16, and 50/51/52.

Private flightsedit

Travellers who pay for their own passage into space are termed spaceflight participants by Roscosmos and NASA, and are sometimes referred to as "space tourists", a term they generally dislike.b 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 six, space tourism was halted, as the partners relied on Russian transport seats for access to the station. Soyuz flight schedules increase after 2013, allowing five Soyuz flights (15 seats) with only two expeditions (12 seats) required. The remaining seats are sold for around US$40 million to members of the public who can pass a medical exam. ESA and NASA criticised private spaceflight at the beginning of the ISS, and NASA initially resisted training Dennis Tito, the first person to pay for his own passage to the ISS.c

Anousheh Ansari 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." 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 "an age-old dream of man: to leave our planet as a "normal person" and travel into outer space."

In 2008, spaceflight participant Richard Garriott placed a geocache aboard the ISS during his flight. This is currently the only non-terrestrial geocache in existence. At the same time, the Immortality Drive, an electronic record of eight digitised human DNA sequences, was placed aboard the ISS.

Fleet operationsedit

A wide variety of crewed and uncrewed spacecraft have supported the station's activities. Flights to the ISS include 37 Space Shuttle mission, 75 Progress resupply spacecraft (including the modified M-MIM2 and M-SO1 module transports), 59 crewed Soyuz spacecraft, 5 ATVs, 9 Japanese HTVs, 20 SpaceX Dragon and 13 Cygnus missions.citation needed

There are currently 8 docking ports, 4 on the U.S. side and 4 on the Russian side.

• The 4 U.S. docking ports are Harmony forward, Harmony zenith, Unity nadir, and Tranquility aft.
  • Tranquility aft is currently occupied by the Bigelow Expandable Activity Module.
• The Russian Docking ports are Pirs nadir, Zvezda nadir, Rassvet nadir and Rassvet aft.citation needed

Crewededit

Uncrewededit

Currently docked/berthededit

Key

Spacecraft and mission			Location	Arrival (UTC)	Departure (planned)
	Progress MS No. 448	Progress MS-14	Zvezda aft	25 April 2020	1 December 2020
	Progress MS No. 444	Progress MS-15	Pirs nadir	23 July 2020	23 April 2021
	S.S. Kalpana Chawla	NG-14	Unity nadir	5 October 2020	6 December 2020
	Soyuz MS Favor	Soyuz MS-17	Rassvet nadir	14 October 2020	17 April 2021
	Crew Dragon Resilience	Crew-1	PMA 2 / IDA 2 forward	17 November 2020	TBD

Scheduled missionsedit

• 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.

Key

Dockingedit

All Russian spacecraft and self-propelled modules are able to rendezvous and dock to the space station without human intervention using the Kurs radar docking system from over 200 kilometres away. The European ATV uses star sensors and GPS to determine its intercept course. When it catches up it uses laser equipment to optically recognise Zvezda, along with the Kurs system for redundancy. Crew supervise these craft, but do not intervene except to send abort commands in emergencies. Progress and ATV supply craft can remain at the ISS for six months, allowing great flexibility in crew time for loading and unloading of supplies and trash.

From the initial station programs, the Russians pursued an automated docking methodology that used the crew in override or monitoring roles. Although the initial development costs were high, the system has become very reliable with standardisations that provide significant cost benefits in repetitive operations.

Soyuz spacecraft used for crew rotation also serve as lifeboats for emergency evacuation; they are replaced every six months and were used after the Columbia disaster to return stranded crew from the ISS. Expeditions require, on average, 2,722 kg of supplies, and as of 9 March 2011update, crews had consumed a total of around 22,000 meals. Soyuz crew rotation flights and Progress resupply flights visit the station on average two and three times respectively each year.

Other vehicles berth instead of docking. 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 a robotic arm to grapple and berth the vehicle to the USOS. Berthed craft can transfer International Standard Payload Racks. Japanese spacecraft berth for one to two months. The berthing Cygnus and SpaceX Dragon were contracted to fly cargo to the station under the phase 1 of the Commercial Resupply Services program.

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. On 25 May 2012, SpaceX delivered the first commercial cargo with a Dragon spacecraft.

Launch and docking windowsedit

Prior to a ship's docking to the ISS, navigation and attitude control (GNC) is handed over to the ground control of the ship's country of origin. GNC is set to allow the station to drift in space, rather than fire its thrusters or turn using gyroscopes. The solar panels of the station are turned edge-on to the incoming ships, so residue from its thrusters does not damage the cells. Before its retirement, Shuttle launches were often given priority over Soyuz, with occasional priority given to Soyuz arrivals carrying crew and time-critical cargoes, such as biological experiment materials.

Repairsedit

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. Most 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 provide electricity for many parts that could be damaged by the cold of space and require heating. The larger logistics carriers also have local area network (LAN) connections for 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. Serious problems include an air leak from the USOS in 2004, the venting of fumes from an Elektron oxygen generator in 2006, and the failure of the computers in the ROS in 2007 during STS-117 that left the station without thruster, Elektron, Vozdukh and other environmental control system operations. In the latter case, the root cause was found to be condensation inside electrical connectors leading to a short circuit.

During STS-120 in 2007 and following the relocation of the P6 truss and solar arrays, it was noted during the solar array had torn and was not deploying properly. An EVA was carried out by Scott Parazynski, assisted by Douglas Wheelock. Extra precautions were taken to reduce the risk of electric shock, as the repairs were carried out with the solar array exposed to sunlight. 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 bearing surfaces, so the joint was locked to prevent further damage. Repairs to the joints were carried out during STS-126 with lubrication and the replacement of 11 out of 12 trundle bearings on the joint.

In September 2008, damage to the S1 radiator was first noticed in Soyuz imagery. The problem was initially not thought to be serious. The imagery showed that the surface of one sub-panel has peeled back from the underlying central structure, possibly because of micro-meteoroid or debris impact. 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 then used to vent the ammonia from the damaged panel, eliminating the possibility of an ammonia leak. It is also known that a Service Module thruster cover struck the S1 radiator after being jettisoned during an EVA in 2008, but its effect, if any, has not been determined.

In the early hours of 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. 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 because of an ammonia leak in one of four quick-disconnects. A second EVA on 11 August successfully removed the failed pump module. A third EVA was required to restore Loop A to normal functionality.

The USOS's cooling system is largely built by the US company Boeing, which is also the manufacturer of the failed pump.

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. Each MBSU has two power channels that feed 160V DC from the arrays to two DC-to-DC power converters (DDCUs) that supply the 124V power used in the station. In late 2011 MBSU-1 ceased responding to commands or sending data confirming its health. While still routing power correctly, it was scheduled to be swapped out at the next available EVA. A spare MBSU was already on board, but a 30 August 2012 EVA failed to be completed when a bolt being tightened to finish installation of the spare unit jammed before the electrical connection was secured. The loss of MBSU-1 limited the station to 75% of its normal power capacity, requiring minor limitations in normal operations until the problem could be addressed.

On 5 September 2012, in a second six-hour EVA, astronauts Sunita Williams and Akihiko Hoshide successfully replaced MBSU-1 and restored the ISS to 100% power.

On 24 December 2013, astronauts installed a new ammonia pump for the station's cooling system. The faulty cooling system had failed earlier in the month, halting many of the station's science experiments. Astronauts had to brave a "mini blizzard" of ammonia while installing the new pump. It was only the second Christmas Eve spacewalk in NASA history.

Mission control centresedit

The components of the ISS are operated and monitored by their respective space agencies at mission control centres across the globe, including RKA Mission Control Center, ATV Control Centre, JEM Control Center and HTV Control Center at Tsukuba Space Center, Christopher C. Kraft Jr. Mission Control Center, Payload Operations and Integration Center, Columbus Control Center and Mobile Servicing System Control.