In 2005, Japan’s unmanned Hayabusa spacecraft (illustration) successfully landed on an asteroid.
Getting to Mars is going to involve building a huge spacecraft and loading it up with tons of fuel and radiation shielding. Unless, that is, we could just tag along with a spacecraft that’s already headed in that direction, like an asteroid.
For an asteroid ferry to work, you’d have to be able to find a rock that’s going to pass within a million miles or so of both Earth and Mars, which seems like a long shot. But, if you can wait until 2068, there will be two of them, both of which would drop you off close to Mars in under a year. The plan is pretty simple: just wait around in Earth orbit for the right asteroid to come by, jump on with all your gear, and then jump off again when it swings past Mars.
In addition to not having to, you know, build your own spaceship and stuff, asteroids also offer lots of radiation protection. This is important, since a round-trip voyage to Mars has the potential to increase cancer risk by up to 20% thanks to those nasty cosmic rays. In addition to being big and made of rock, space rocks would potentially have craters and caves that astronauts could hide out in, Millennium Falcon style. Just so long as they make sure it’s actually a cave first.
Instead of just waiting around for a Mars-bound asteroid to randomly swing by Earth, another option is to just go wrangle an asteroid ourselves and push it where we want it to go. If we can get to one early enough, even a relatively small solar sail could provide enough thrust over time to nudge an asteroid into a useful Earth to Mars (or Mars to Earth) orbit. The eventual plan would be to stick one or more asteroids in permanent round-trip orbits, so you’d have a continual free ferry service from Earth to Mars and back.