Will humans ever live on other planets? The stranding of two astronauts has thrown the spotlight on the conditions that space travellers are forced to deal with.
The smelly truth about life in space
Left behind: Suni Williams and Butch Wilmore (above) said it was hard to watch their malfunctioning craft depart the International Space Station without them after being stranded in space for eight months. Glossary
International Space Station - A joint project between the US, Russia, Japan, Canada and the European Space Agency.
Free radicals - A type of unstable molecule.
Boeing - An American multinational corporation that designs, makes and sells aircraft, rockets, satellites and missiles.
Helium - A very light gas used in balloons to make them float. Old airships used hydrogen, but it burnt easily. Helium does not burn.
SpaceX - Founded in 2002 by South African entrepreneur Elon Musk, SpaceX's mission is to reduce the cost of space transportation to make possible the colonisation of Mars.
GMT - Short for Greenwich Mean Time
Gravity - An invisible force that pulls objects towards each other. Earth's gravity keeps us on the ground and makes things fall. It is less strong on Mars.