system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why can't we send fuel to Mars for a return journey.

Excuse my basic knowledge of space travel. From what I understand one of the biggest issues of heading to Mars is the return journey. So why couldn't we send fuel to Mars ahead of time so we can refuel the ship for a return journey?
gpt
This is known as the Rocket Problem. You have to carry all the fuel you will need and it means that your craft will get very heavy very very fast.

We simply can't get the return fuel there, if we could the problem wouldn't exist in the first place. If we could send them on one mission then send the fuel on another we could just send it with them on the first.
If you want a good idea about this you should check out the game Kerbal Space Program, it's a rocket simulator and you will learn very quickly just how much of a pain in the ass the Rocket Problem is.