Before we discuss the scientific research and technology development on-orbit, let’s give the ISS itself a proper introduction. The Space Station is a joint project of NASA, the Russian Federal Space Agency, the Japan Aerospace Exploration Agency, the Canadian Space Agency, and the European Space Agency.
Now the largest spacecraft ever built, the ISS grew out of the plans of these separate agencies to build their own space stations. After the end of the Cold War, the United States, Russia, Japan, Canada, and Europe agreed in the early 1990s to pool their resources and expertise to build one space station that everyone could share.
Begun in November 1998, with the launch of the Zarya module on a Russian Proton rocket, the modular construction of the ISS is largely complete. The only significant delays resulted from the 18-month gap between the Node 1 launch and delivery of the Zvezda module and the hiatus imposed after the loss of Columbia. Future expansions could include Russian multi-purpose modules and an inflatable test section.
Today, we have thirteen pressurized modules on-orbit – including laboratories, airlocks, and crew living spaces – and a complete Integrated Truss Structure with all four US-built solar arrays providing power. Node 3 and the Cupola were brought to the Station on STS-130 by the crew of Space Shuttle Endeavour. Discovery and the STS-133 crew delivered a pressurized cargo module to the Station that serves as a permanent storage area – the PMM. The final mission of the Space Shuttle Program, STS-135, was a cargo and support flight to the ISS.
The Space Station is serviced by the self-traversing Canadian-built robot arm and its Special Purpose Dexterous Manipulator attachment, a smaller robot arm attached to the Japanese Experiment Module, and two cargo cranes on the Russian segment. In 2013, a third robot arm produced by the European Space Agency will be launched with the Russian “Nauka” Multipurpose Laboratory Module.
Five different vehicles have visited the Space Station. Soyuz crew capsules and Progress unmanned cargo vehicles launch from Russia’s Baikonur Cosmodrome in Kazakhstan. Soyuz has always been the primary crew transfer vehicle for the ISS and serves as a lifeboat for the crew on-board. The European Space Agency’s Automated Transfer Vehicle acts as an unmanned resupply and reboosting craft and docks on the Russian side. Japan’s H-II Transfer Vehicle is unique among the unmanned visiting vehicles in its ability to carry both unpressurized and pressurized cargo. Because the HTV berths at the Station like the cargo modules carried by the Shuttle, it can also carry large internal payload racks. The now-retired Space Shuttle fleet delivered most of the large ISS components to orbit.
ATV HTV Progress
Starting this year, the SpaceX Dragon capsule and Orbital Sciences’ Cygnus spacecraft will begin delivering cargo to the ISS under the fixed-cost Cargo Resupply Services contract. NASA is also looking to companies like SpaceX, Boeing, Sierra Nevada, and Blue Origin for commercially-developed crew vehicles that will maintain independent US crew access for the remainder of the ISS service life.
Our work on the Station has taught us how to build complicated structures in space, develop sustained international operations on-orbit, and manage a diverse space traffic system consisting of a variety of visiting vehicles. More than that, though, the ISS gives us a unique platform for long-term studies in “weightlessness”, access to little-understood regions of the upper atmosphere, and an adaptable vantage point for observing our planet.
Gravity is the fundamental force here on the ground. The immense mass of the Earth pulls us to it. We feel gravity in the ground pushing back against our feet or in the tension in our arms from gravity pulling them against our fixed shoulders. Physics, chemistry, and biology are all dominated by the presence of gravity, so much so that other important phenomena are often obscured or suppressed.
Newton theorized that, if you throw a ball progressively higher and faster, you will eventually throw it high enough and fast enough that it will fall “around” the Earth. Newton’s thought experiment becomes reality when we fly in space. The centripetal force from the circular path around the planet is equal and opposite to the force from gravity while in orbit.
If you built a tower up to the altitude of the Space Station, you would still feel nearly 90 percent of the Earth’s gravity. If you stepped off the edge of that tower, you would not hover in orbit. Instead, you would immediately fall right back down because you wouldn’t have enough rotational speed to counteract gravity’s pull. The ISS and its menagerie of visiting vehicles would just whiz right past you.
On those orbiting vehicles, we experience a state called microgravity. Because the centripetal and gravitational forces balance each other out, the only gravity-like forces are felt when we fire the thrusters or from other disturbances inside the Station, like an astronaut running on the treadmill.
After the Russian Mir station was retired and until the International Space Station was built, microgravity studies could only be done for brief moments on drop towers, sounding rockets, and parabolic aircraft or for days at a time on vehicles like the Space Shuttle. The Station has the permanency and accommodations to support microgravity and space environment studies of weeks, months, or years in duration. Some of those accommodations include isolation racks to minimize the vibrations on sensitive instruments, freezers and incubators for temperature control, fluid and combustion facilities for controlled experiments with liquids or fire, and growth chambers for both biological and non-biological science.
One particularly interesting phenomenon in microgravity is the lack of buoyancy. On the ground, materials separate by density. We can see this every day – steam rises from boiling water, ice cubes come up to the surface when you fill your glass, and a column of various colored oils will stratify into a beautiful rainbow. The heavier fluids sink to the bottom, while the lighter fluids rise.
In microgravity, we lose this density-driven separation. Underlying processes on Earth, such as diffusion, conduction, capillary action, and surface tension, emerge as dominant. In a pipe on Earth, you will see a fluid flow along the bottom with any air on top. In space, you might see the fluid stick to the surface of the pipe and the air flow through the middle! Surface tension effects on a water film
On Earth, a candle has the characteristic teardrop shaped flame that results from the convection of the heated gas and burning fuels. In space, the flame takes a spherical shape and can burn itself out without a flow of fresh oxygen.
Thus, the processes that we rely on here on the ground for industry and sustaining life may not work the same, or at all, in space. We must understand the fundamental physics, chemistry, and biology in microgravity to build a sustainable space infrastructure that truly brings benefits back to our civilization. The Space Station provides an ideal laboratory environment for that research.
Thermosphere and Observational Studies
The ISS orbits the Earth in a part of the atmosphere called the thermosphere. This is a region of rarified gasses with high exposure to solar ultraviolet radiation and cosmic rays. Most of the thin atmosphere is atomic oxygen. The high UV absorption and low density allows for a relatively high level of ionization to occur, so the ISS also resides in the upper part of the ionosphere.
Due to both this hostile environment and the presence of orbital debris, the Station serves as an excellent platform for studying the long-term effects of the space environment on new materials and coatings. In fact, the ISS itself is an on-going study of those effects on structures and materials. Partners from various industries, government agencies, and academia have also shared space on external platforms attached to the Station to conduct materials research.
The Station can also be used for fundamental physics research. Because samples are more uniform in microgravity, the critical points where a material transitions between one state of matter and another are easier to observe. Direct testing of gravitational theories are also possible on the Station because it orbits with enough velocity that we can observe the predicted differences in time measurement between an atomic clock on-orbit and a synchronized companion on the ground.
Perhaps the most appreciated function of the Space Station, though, is Earth observation. The arrival of the Cupola has given us unprecedented views of our planet from orbit. These aren’t just pretty pictures, though.
The angle of the Station’s orbit allows it to view up to 95% of the world’s inhabited land area. With professional cameras and motion compensation, a spatial resolution of less than six meters has been achieved by ISS crews. Astronauts on the Station have recorded volcanoes erupting, patterns of land use and urban development, and the extent of visible pollution – be it from light, airborne emissions, or in the water.
Because we have human ingenuity and intuition guiding those cameras, we have been able to record events that unmanned satellites might have missed and provide near-real time feedback with the people who use the imagery for their work. These pictures and the trends they capture have been used to help with city planning, geological surveys, agricultural planning, and the monitoring of natural resources impacted by human use.
Think of this as the 200-mile-high view of what we do on the Station.