NASA details the three phases in reaching Mars with human astronauts, including Earth Reliant, Proving Ground, and Earth Independent. The first of these, Earth Reliant, requires that we make certain we're capable of existing outside of our own atmosphere for long periods of time. This includes astronaut missions aboard the International Space Station. Part of NASA's plan requires that we extend ISS operations until at least the year 2024. NASA also suggests that they'll need to pursue an evolvable SLS via Exploration Upper Stage before advanced solid rocket boosters if they're ever to make a craft that'll bring humans to Mars AND be able to bring them back to Earth.