International Space Station

The International Space Station (ISS) is the largest orbital research facility ever assembled and represents one of the most extensive cooperative scientific and technological projects in history. Maintained in low Earth orbit through the partnership of five space agencies—the United States’ NASA, Russia’s Roscosmos, the European Space Agency, Japan’s JAXA, and the Canadian Space Agency—the ISS functions as a permanent microgravity laboratory, a base for long-duration missions, and a platform for advancing international cooperation in space exploration.

Development and Structure

The ISS originated from the merging of two planned space station programmes: NASA’s Space Station Freedom and the Soviet Union’s Mir-2. Construction began with the launch of the Zarya module in 1998, supported by Proton and Soyuz rocket families alongside Space Shuttle assembly flights. Long-term human occupancy commenced on 2 November 2000 with Expedition 1, marking the beginning of an uninterrupted human presence in space.
The station consists of two primary segments:

• Russian Orbital Segment (ROS), developed by Roscosmos and responsible for propulsion, attitude control, and various research facilities.
• United States Orbital Segment (USOS), developed collectively by NASA, ESA, JAXA, and CSA, hosting most scientific laboratories, habitation modules, and advanced systems.

A key engineering feature is the Integrated Truss Structure, which supports extensive solar arrays, radiators, and external payloads. The ISS includes dedicated modules for scientific experimentation, crew living quarters, spacecraft operations, storage, and airlock functions. It also offers eight docking or berthing ports for visiting spacecraft, supporting a steady cadence of crewed and uncrewed missions.
Operating at an average altitude typical for low Earth orbit, the ISS completes an orbit in approximately 93 minutes, amounting to several revolutions per day. By the mid-2020s, 279 individuals from 22 nations had visited the station, contributing to its status as a symbolic and practical centre of international scientific cooperation. Current plans foresee continued operations until 2030, after which controlled deorbiting using a dedicated US Deorbit Vehicle is expected.

Mission and Roles

The ISS was initially conceived as a multipurpose facility for scientific research, Earth observation, manufacturing, and as a staging post for future missions to the Moon, Mars, and asteroids. Although not all early ambitions were realised, the station’s purposes have evolved to include:

• Scientific investigation in microgravity
• Commercial research and private spaceflight opportunities
• Educational outreach and public diplomacy
• Testing technologies for deep-space exploration

Subsequent policy directives, such as those of the United States in the 2010s, reinforced its role in fostering commercial innovation and international collaboration.

Scientific Research

As an orbiting laboratory, the ISS provides a microgravity environment with sustained access to power, cooling, communication systems, and human operators. This unique environment enables decades-long experimentation that cannot be replicated on short-duration missions or free-flying satellites. Research covers a comprehensive range of scientific fields:

• Astrobiology and space environment studies, including the behaviour of extremophiles and the survivability of organisms in vacuum, radiation, and temperature extremes.
• Astronomy and astrophysics, such as cosmic ray investigation, dark matter studies, and solar observations.
• Physical sciences, particularly fluid dynamics, combustion, and materials science under microgravity conditions.
• Life sciences and biomedical research, examining muscle atrophy, bone density loss, fluid redistribution, and space medicine.
• Earth observation, with instruments monitoring atmospheric composition, aerosols, vegetation, and climate-related variables.

One of the station’s flagship experiments is the Alpha Magnetic Spectrometer (AMS), designed to detect cosmic antimatter and investigate the nature of dark matter. Requiring substantial power and data bandwidth, it benefits from integration with the ISS rather than operating as an independent satellite. Early AMS findings indicated an excess of high-energy positrons, a phenomenon with potential relevance to dark matter research.
Scientific operations are supported by around 160 crew labour hours per week, though a significant portion of astronaut time is dedicated to maintenance tasks. Rapid access to incoming cargo missions enables adjustments to ongoing experiments and timely deployment of new scientific instruments.

Medical and Biological Studies

Research into the biological effects of spaceflight is central to planning for long-duration missions. Studies aboard the ISS document changes in bone density, muscle mass, cardiovascular function, and immune responses. Findings from the mid-2000s indicated heightened risks of fractures and impaired mobility following long voyages, prompting intensified research into countermeasures.
A prominent medical project is the Advanced Diagnostic Ultrasound in Microgravity study, in which astronauts perform ultrasound procedures guided by remote specialists. The results inform both space-based emergency care and terrestrial medical practices, particularly in isolated or resource-limited regions.
Experiments have also examined microorganism resilience in space. Notably, highly radiation-resistant bacteria such as Deinococcus radiodurans have survived multi-year exposures on the station’s exterior, contributing to debates on panspermia and the spread of life through space.

Earth Observation and Space Science

Completion of the US Orbital Segment in 2011 expanded the ISS’s capability for Earth and space research. A series of specialised instruments observe aerosols, ozone, and atmospheric chemistry, while others investigate solar activity, cosmic dust, and deep-space phenomena. Programmes include:

• Orbiting Carbon Observatory-3
• ECOSTRESS for ecosystem temperature measurement
• Global Ecosystem Dynamics Investigation (GEDI) for forest canopy mapping
• ISS-RapidScat for ocean wind monitoring
• Neutron Star Interior Composition Explorer (NICER)
• CALorimetric Electron Telescope (CALET)
• Monitor of All-sky X-ray Image (MAXI)

These instruments collectively advance knowledge of climate processes, cosmic radiation, and fundamental astrophysical questions.

Microgravity Environment

Although gravity at ISS altitude is about 90 per cent of that at Earth’s surface, the station and its contents remain in continuous free fall, creating the sensation of weightlessness. This environment is affected by several influences, including atmospheric drag, mechanical vibrations, thruster firings, and slight differences in orbital paths across the station’s structure. Nevertheless, it offers an ideal setting for studying fluid physics, biological growth, and the formation of protein crystals, with implications for medical and industrial applications.

Broader Impact and Future Prospects

The ISS serves as a testbed for technologies, procedures, and international coordination mechanisms essential to future deep-space missions. Its contributions to human spaceflight include improved understanding of life-support systems, spacecraft operations, and the physiological challenges of long-term habitation.