[SciShow intro plays]

Caitlin: So you want to build a space station? Maybe you got a cardboard box, scissors and some glue to get started and hopefully billions of dollars. But where do you actually start?

Well let's look at some of the space stations that have orbited the earth and had astronauts or cosmonauts inside. The first was in 1971, when the Soviet Union had designed the Salyut-1 which was basically a two room satellite with a docking department so cosmonauts could enter. Over the next decade the Soviet Union launched more versions of  the Salyut all the way up to the Salyut-7.

Most of them had crews for just a couple months at most, before the empty space stations were sent back into Earth’s atmosphere to safely burn up over the oceans. But a big breakthrough came in the 1980s, when the Soviet Union built a brand new space station called the Mir. The Mir was occupied by cosmonauts for over 12 of the 15 years it spent in space, and it was unique because it was a modular space station.

Basically, every component of the space station was made separately on Earth, and everything was put together in space. This means you can add in new parts, depending on the mission. Kind of like playing with Legos.

But the tricky part is connecting these pieces together. There are different ways to do a space rendezvous, but one of the key techniques is called a Hohmann transfer. To start out, you want to launch your module into orbit, but closer to Earth than the rest of the space station.

Then, you ignite the module’s engines with two carefully-timed pulses -- the Hohmann transfer -- to push the module into a higher circular orbit, closer to the station. When the moment is just right, the module performs another Hohmann transfer to get right in front of the space station. Then, the module does a 180 degree U-turn and is steered into a docking platform, where robotic arms or astronauts can help make the final connections if needed.

Pull it off, and you’ve built a modular space station like the Mir -- the biggest man-made object ever to be in space, before its mission ended and it had a controlled re-entry into Earth’s atmosphere in 2001. But in 1998, the International Space Station program was launched and quickly broke that record. The ISS is currently the size of a US football field, weighs over 300,000 kilograms, and has been occupied by astronauts for over 15 years now! It’s also a modular space station, made up of 16 pressurized modules.

And 16 countries helped with the work, including Canada, the US, Russia, Japan, Brazil, and some in the European Space Agency. All of these modules have their own role, but we’ll start with the basics: Zarya, Unity, and Zvezda -- the first three modules of the ISS. Zarya was the very first module and responsible for the main propulsion, some communication with Earth, and had two solar arrays to provide electrical power.

But now that we have new modules, Zarya is mainly just used for storage. Unity is a big connector piece that has over 50,000 cables, pipes, and mechanical parts -- which connect electronic systems and transport fluids to create a stable, livable environment. Zvezda is responsible for some of the main controls, and has life support systems, with enough room for two astronauts to live.

These starting modules were built in Russia and the US, and didn’t meet until they were orbiting around the Earth... at over 20,000 kilometers per hour. They all pulled off the proper maneuvers in space, including the Hohmann transfer, and formed the humble beginnings of the ISS! Then, more modules were added, such as science laboratories like Destiny, Columbus, and Kibo. Or more connectors, like Tranquility and Harmony.

The ISS also added the Integrated Truss Structure, which is designed to connect external equipment to the modules -- like big solar arrays. They’ve even got a big robotic arm that can do work outside the space station, like moving supplies, maintaining the station, and even attaching more modules. And right now, the ISS is experimenting with a new expandable module called BEAM.

It was compressed into a small capsule when it was launched in April 2016, was expanded, and is currently being tested for its living and working capabilities. As of today, there are only two space stations in orbit -- the ISS, and China’s Tiangong-1, which is much smaller and part of a CNSA program to create their own large modular space station by 2023.

Right now, only trained astronauts are living on these space stations. But some companies hope that inflatable modules like BEAM could eventually host tourists. And if that’s true, maybe someday you can go up yourself and get a real feeling for how these space stations are built.

Thanks for watching this episode of SciShow Space, and thanks especially to our patrons on Patreon like Amy Cohen and Samuel E Joseph who help make this show possible. Thanks Amy and Sam for supporting SciShow and sending us your great, thought-provoking questions. If you want to help us keep making episodes like this, just go to Patreon.com/SciShow to learn more, and don’t forget to go to YouTube.com/SciShowSpace and subscribe!