Those six guys are about as close as we've gotten to sending people to Mars. The six-person crew of the 520-day Mars500 mission underwent the grueling simulation of a flight to the red planet.

Photo courtesy ESA/IBMP - Oleg Voloshin

Why Send Rovers?

So if we're so advanced and fancy that we can build extremely complicated robots to Mars, why can't we just send Terry the Astronaut? The most important reason is also probably the most obvious: Terry probably just wouldn't make it there.

That is, only about a third of the missions launched thus far have been "successful," meaning that they've made a trip to Mars intact. While it's easy to be optimistic about the nearly one-third of rovers that have provided us with valuable information, it's not as easy to cheerlead a track record like that when Terry the Astronaut is in the picture. Few of us enjoy the odds of dying every three days at work.

Cost, of course, is another factor. While Curiosity, the most recent rover that's part of NASA's Mars Science Laboratory mission, cost a whopping $2.47 billion to build, NASA still didn't have to account for pesky things like allowing someone to breathe oxygen [source: Space.com]. Or return from Mars, for that matter. Keep in mind that the rovers get to stay on Mars forever when we're done with them, but Terry the Astronaut's trip is more a vacation than a move. And that means food, fuel, waste disposal and a plethora of other costs -- twice.

Beyond logistics and cost are all the vast unknowns about how the human system could react to an atmosphere like Mars. Because Mars has no magnetic field, humans would receive whopping doses of cosmic radiation -- not a problem on Earth, where the planet's magnetic field works to block it out. A 1,000-day trip to Mars has the potential to result in a 40 percent chance of the astronaut developing cancer after returning to Earth -- not necessarily something a lot of people are looking for when interviewing for a job [source: NASA Science]. Keep in mind, too, that if Terry the Astronaut is also Terry the Woman, she's at even more risk: Having breasts and female reproductive organs present nearly double the risk for cancer [source: NASA Science].

So without Terry the Astronaut signing up for massive doses of cancer-causing rays, we're left with robotic explorers. Jet over to the next page to learn about some of the missions to Mars.