I have only 6 months of knowledge about rocket science but I was wondering why can't we do the following to go to Mars and come back to Earth: We can have enough fuel in a rocket at maximum to just go to Mars.But what if we made 100 rocket (Let's call them 1,2,3,...,100) and they were placed between Mars and Earth(1 was the closest to Mars and 100 is the closest to Earth) so when we wanted to get back to earth , we could use the fuel in 1 to get to 2 , use the fuel in 2 to get to 3 and so on.
Fuel depots might help, but only if they are at the right position, one in an orbit around Mars and one at the landing spot. Eventually another one in an Earth orbit. After a rendezvous maneuver, the spacship may dock to the fuel depot and fuel transfer could be done. But all tanks and pumps should be build for transfer at zero gravity.
Fuel depots between Earth and Mars would not stay were you put them. A useful depot should have the same position, the same speed and the same direction of flight as the spaceship to enable a rendezvous between depot and spaceship. This is only possible if the depot follows the same transfer orbit and starts at the same time. But you don't need fresh fuel while being within the transfer orbit.