The only physical factor preventing a human journey to Mars has been weight  Carrying enough fuel to propel a conventional spacecraft to Mars and back would make even the lightest craft too heavy to be launched from Earth  A device has recently been invented, however, that allows an otherwise conventional spacecraft to refill the craft' s fuel tanks with fuel manufactured from the Martian atmosphere for the return trip
Ttherefore, it is possible for people to go to Mars in a spacecraft that carries this device and then return
The device for manufacturing fuel for the return to Earth weighs less than the tanks of fuel that a conventional spacecraft would otherwise need to carry from Earth for the return trip.