This article was authored by Chris James and originally published on The Conversation.
There are many things humanity must overcome before any return journey to Mars is launched.
The two major players are NASA and SpaceX, which work together intimately on missions to the International Space Station but have competing ideas of what a crewed Mars mission would look like.
Size matters
The biggest challenge (or constraint) is the mass of the payload (spacecraft, people, fuel, supplies etc) needed to make the journey.
We still talk about launching something into space being like launching its weight in gold.
