Question: On the mars mission in a few years time the astronauts won't be able to return to Earth, I assumed this was to do with the difference in Gravity between Earth and Mars but I'm not sure, do you know why?

Alexander Henderson answered on 6 Mar 2015:

I think to get to Mars, with all the equipment and supplies that would be required, does not leave much space for fuel for the return journey. There are many issues why it’s a one-way trip; technology for one but you’re correct in thinking gravity is one too. For a spacecraft to escape Earth’s gravitational pull, a huge amount of fuel is needed – just think of the Space Shuttle on take off – it’s more fuel than anything else. Although Mars’ gravitational pull is smaller than Earth’s, trying to take off would require lots of fuel that they may not have. Good luck to the astronauts!