Congress was unwilling to provide enough money to build and operate Freedom, and demanded NASA increase international participation to defray the rising costs or they would cancel the entire project outright.

[22] The ISS 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.

Unprotected presence in space is characterised by an intense radiation field (consisting primarily of protons and other subatomic charged particles from the solar wind, in addition to cosmic rays), high vacuum, extreme temperatures, and microgravity.

Medical research improves knowledge about the effects of long-term space exposure on the human body, including muscle atrophy, bone loss, and fluid shift.

In 2006, data on bone loss and muscular atrophy suggested that there would be a significant risk of fractures and movement problems if astronauts landed on a planet after a lengthy interplanetary cruise, such as the six-month interval required to travel to Mars.

These findings supported the notion of panspermia, the hypothesis that life exists throughout the Universe, distributed in various ways, including space dust, meteoroids, asteroids, comets, planetoids or contaminated spacecraft.

Examples of Earth-viewing remote sensing experiments that have flown on the ISS are the Orbiting Carbon Observatory 3, ISS-RapidScat, ECOSTRESS, the Global Ecosystem Dynamics Investigation, and the Cloud Aerosol Transport System.

In 2010, ESA Director-General Jean-Jacques Dordain stated his agency was ready to propose to the other four partners that China, India, and South Korea be invited to join the ISS partnership.

[83][84] Much of the US Orbital Segment, including the Destiny and Unity modules, the Integrated Truss Structure, and solar arrays, were built at NASA's Marshall Space Flight Center in Huntsville, Alabama and Michoud Assembly Facility in New Orleans.

The Space Shuttles Discovery, Atlantis, and Endeavour delivered the American Destiny laboratory and Quest airlock, in addition to the station's main robot arm, the Canadarm2, and several more segments of the Integrated Truss Structure.

[112] The crew airlock, derived from the Space Shuttle, features essential equipment like lighting, handrails, and an Umbilical Interface Assembly (UIA) that provides life support and communication systems for up to two spacesuits simultaneously.

[118] Launched on 23 October 2007 aboard Space Shuttle Discovery on STS-120,[119][120] Harmony was initially attached to the Unity[121][122] before being relocated to its permanent position at the front of the Destiny laboratory on 14 November 2007.

It is the largest single module on the ISS, consisting of a pressurized lab, an exposed facility for conducting experiments in the space environment, two storage compartments, and a robotic arm.

[144][145] While these platforms allow experiments (including MISSE, the STP-H3 and the Robotic Refueling Mission) to be deployed and conducted in the vacuum of space by providing electricity and processing experimental data locally, their primary function is to store spare Orbital Replacement Units (ORUs).

It included a spare elbow joint for the European Robotic Arm (ERA) (which was launched with Nauka) and an ERA-portable workpost used during EVAs, as well as RTOd add-on heat radiator and internal hardware alongside the pressurized experiment airlock.

[149] Also installed during STS-134 was the 15 m (50 ft) Orbiter Boom Sensor System (OBSS), which had been used to inspect heat shield tiles on Space Shuttle missions and which can be used on the station to increase the reach of the MSS.

To keep the internal temperature within workable limits, a passive thermal control system (PTCS) is made of external surface materials, insulation such as MLI, and heat pipes.

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, Texas.

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.

[298][299][300][301] These soundproof, person-sized booths offer privacy, ventilation, and basic amenities such as a sleeping bag, a reading lamp, a desktop, a shelf, and storage for personal items.

[311][312][313] To simulate night conditions, the station’s windows are covered during designated sleep periods, as the ISS experiences 16 sunrises and sunsets daily due to its orbital speed.

[331] Cosmonaut Valery Ryumin wrote in his journal during a particularly difficult period on board the Salyut 6 space station: "All the conditions necessary for murder are met if you shut two men in a cabin measuring 18 feet by 20 [5.5 m × 6 m] and leave them together for two months."

The latter is still often a cause of stress on the ISS, such as when the mother of NASA astronaut Daniel Tani died in a car accident, and when Michael Fincke was forced to miss the birth of his second child.

Microbiological hazards have led to a development of the LOCAD-PTS (a portable test system) which identifies common bacteria and molds faster than standard methods of culturing, which may require a sample to be sent back to Earth.

[341] Since 2016, a series of ESA-sponsored experiments have been conducted to test the anti-bacterial properties of various materials, with the goal of developing "smart surfaces" that mitigate bacterial growth in multiple ways, using the best method for a particular circumstance.

Dubbed "Microbial Aerosol Tethering on Innovative Surfaces" (MATISS), the programme involves deployment of small plaques containing an array of glass squares covered with different test coatings.

The experiment was performed over a period of 14 months on three different flight missions, and involved taking samples from 8 predefined locations inside the station, then returning them to earth for analysis.

There have been no persistent mission-related hearing threshold shifts among US Orbital Segment crewmembers (JAXA, CSA, ESA, NASA) during what is approaching 20 years of ISS mission operations, or nearly 175,000 work hours.

[citation needed] This orbit was selected because it is the lowest inclination that can be directly reached by Russian Soyuz and Progress spacecraft launched from Baikonur Cosmodrome at 46° N latitude without overflying China or dropping spent rocket stages in inhabited areas.

[432] At the end of 2011, the Exploration Gateway Platform concept also proposed using leftover USOS hardware and Zvezda 2 as a refuelling depot and service station located at one of the Earth–Moon Lagrange points.