Space stations enable us to experience and take advantage of spaceflight over long periods of time. Astronaut Chris Hadfield has describes them as “purpose-built laboratories orbiting the world.” Their unique environment allows us to conduct experiments that aren’t possible on Earth—directly observing the universe and our planet with multiple sensor types, and using the unending, near-perfect vacuum and microgravity environment for research and manufacture. They are also a testbed for spaceship hardware, to gain proven experience and confidence close to home before we head further into the solar system.

The ISS is an internationally operated, habitable artificial satellite in low-earth orbit. It is jointly operated the United States (NASA), Russia (Roscomos), Japan (JAXA), Canada (CSA ASC), and the European Space Agency (ESA), and is governed by treaties between these nations.

It is the largest structure ever in space. It began assembly in orbit in 1998 and has had crews aboard continuously since 2000. With 15 countries cooperating on a daily basis to operate it, the lessons learned have been political as well as operational, technical, and scientific. ISS was designed for 30 years of life, through 2028, when the cost of maintenance is expected to start becoming prohibitive. At that time, to avoid an unplanned orbital decay and random impact with Earth somewhere, it will be deliberately driven into the empty expanse of the South Pacific.

Astronauts aboard the ISS undertake experiments in:

• Biology and human biology
• Meteorology
• Physics
• Astronomy

The ISS orbits Earth in what is called low-earth orbit outside of planet’s atmosphere. The ISS average altitude makes it subject to orbital debris, space debris, and atmospheric drag. However, moving the station’s orbit to a higher altitude would not be practical as the current altitude of the ISS is optimal.

In the early 1980s, NASA began plans to launch a modular space station as an answer to the Soviet space station Mir. Over the next several years, Japanese and European (particularly German and Italian) space agencies became closely involved with the U.S.-led project.

Then, in 1993, as plans were still underway, U.S. Vice President Al Gore and Russian Prime Minister Viktor Chernomyrdin reached an agreement to begin constructing a joint space station. This effort, when fused with the existing American-led international effort, became the International Space Station, as the partner agencies from Japan, Europe, and Canada joined the U.S.-Russian project.

In 1998, the first components of the ISS were launched by the Russian space agency, and over the next decade, the station was built to host long term human occupation, a milestone achieved for the first time in 2000. Since then, the ISS has been continuously occupied by a rotating cast of international crew members. People and cargo arrive via space shuttle on craft like the Space Shuttle Endeavor and Russian Soyuz spacecraft.

Though it has been long term habitable since 2000, the ISS is never complete; the station has undergone many changes as, by design, it can receive new docking sections and discard those no longer in use.

As initially envisioned, the purpose of the ISS was multifold:

• A research laboratory to conduct experiments at various gravitational levels
• An observatory from which to monitor conditions on earth and in space
• A factory to convert raw materials into useable objects for further space exploration
• A waystation and launch point for space missions to the moon, Mars, asteroids, and other points in the solar system.

Over time, the purposes as outlined between the U.S. and Russia in the Memorandum of Understanding have shifted.

• The ISS has yet to become a launch point for missions beyond low earth orbit, though this may come in time.
• Education, diplomacy, and commerce have been added as additional purposes of the ISS
• There are hundreds of experiments running on the ISS simultaneously, including those sponsored by governments, space agencies, universities, and private companies. The eventual intent is that the cost of launching and operating a space station will become low enough that they can be purely commercial platforms.

As the station continues to evolve, so too does its mandate. Its future purpose may include space missions from private rocket launches for commercial purposes, like Space X.

Since it is an international effort, the ISS was made through international cooperation, with individual nations constructing parts that would fit with those made in other nations. The space station was made by fusing together components built to specification in particular countries.

Separate components, including labs, modules, robotic arms, and solar arrays, have been manufactured in each of the five-member nations—they are then launch processed and sent into orbit via cargo spacecraft to dock with the existing space station structure. The station is always a work in progress with new modules being connected as necessary.

The ISS was constructed and assembled module by module. The size of each module was limited by the place and way in which they were launched to space—for example, by the dimensions of the payload bay of the Space Shuttle, or the height of the railway overpasses between Moscow and the Russian launch site in Baikonur, Kazakhstan.

• The ISS central modules are pressurized habitats where the crew lives and works, while the main transverse truss supports power, cooling, communications, external stowage, and scientific test equipment.
• The ISS is electrically powered using solar arrays that rotate to track the sun throughout each orbit. Waste heat is gathered by cooling loops inside the ship and expelled out into space through large, flat radiators.
• The station is primarily flown in an orientation known as LVLH, which stands for “local vertical, local horizontal,” such that the “bottom” of the ISS faces Earth and the “nose” is pointed into the velocity vector (the direction it’s moving).
• There are multiple docking ports on the ISS to allow for different types of spacecraft to dock, including the Space Shuttle, the Russian Soyuz spacecraft, planned human-rated capsules, and various resupply craft. The specific docking mechanisms vary from port to port.
• The Canadian-built robotic manipulators Canadarm (Shuttle-based) and Canadarm2 (ISS-based) helped assemble, grow, and repair the ISS. The ISS also has a two-armed dexterous robot called Dextre for delicate work outside, and a Japanese-built robotic arm for deploying experiments externally.

The ISS is visible every day to amateur astronomers from the earth’s surface with the naked human eye in the night sky; viewing ISS passes simply requires you to know when and where to look up, but does not require additional equipment such as a telescope. The space station appears like a bright star in motion, one of the brightest objects in the sky, roughly the luminosity of Venus and more visible than Jupiter.

From the earth’s surface, the ISS orbit appears to be a floating white dot that moves across the sky after sunset and just before sunrise—when the sun’s light is still on the station but not the earth below it. Weather permitting, it can be spotted from 95% of the earth’s surface—it is not visible from the northern or southern poles but can be seen from every other location.

Several websites and smartphone applications will provide users all the information they need should they choose to view the space station. Based on the user’s longitude and latitude, such tools can give users information regarding:

• When the ISS will be visible in local time
• Where it will appear to rise
• The altitude above the horizon the station will reach
• The duration of visibility before the station enters the Earth’s shadow

Additionally, starting in 2012, NASA began offering text and email alerts to subscribers who wanted to “spot the station” when it appeared over their hometown.