Highways to the Solar System

This chart shows the amount of velocity change a rocket needs to impart, to get from one place to another within the Earth System (in kilometres per second). Roughly speaking, this shows how much fuel is needed for the trip. The biggest effort is to get off the surface of Earth, to reach the speed needed to stay in orbit, at LEO (Low Earth Orbit). Once you have done that, anywhere else is easy. All these transitions can also be completed in a maximum of three days, so even a crewed ship can be relatively simple.

Roads to Mars

Here is the equivalent chart for the transition to Mars and its moons. Not shown here is the biggest of the barriers to travel to Mars, the time it takes. These "economy orbits" require relatively little fuel, but take at least nine months for the transition to Mars. This requires a ship with more complex life support, a spinning section for artificial gravity, and extra energy for the crew to survive. Faster transitions are possible, but require more delta V. The other factor to consider is that these moves can only be made at specific times in the 2.1 year cycle of relative Earth-Mars positions. If you miss the "window" for this year's trip, it is 25 months before you can try again.