Tech entrepreneur Elon Musk has spoken often of colonizing Mars with his company SpaceX.
Many experts believe that setting up a crewed Martian outpost is possible if not inevitable.
However, the technologies required to establish a permanent, thriving colony on the red planet don't yet exist.
Biosphere 2 — a large-scale experiment from the early 1990s — remains one of the best illustrations of the challenges that lie ahead.
