Just last week President Obama again announced his ambitious goal to send humans to Mars by the 2030s. His plan calls for a safe return of astronauts after visiting the Red Planet. After that first trip, he hopes that humans will establish activities that will lead to colonies on the planet. A key point in his announcement is that there should be continued cooperation between the government and the private sector, consisting of more than 1,000 companies. This is a very ambitious undertaking.

It all sounds very exciting, but the challenges are great. There are probably hundreds of science fiction stories about Mars and colonization of the Red Planet. In recent years many have seriously proposed sending humans to Mars, either one way or round trip. Others have suggested colonization with existing technologies, but no actual Mars-transport systems have yet been built.

Sending humans to Mars, even a "quick" round trip, presents tremendous challenges and risks. To start with, the minimum-energy transfer time from Earth to Mars will take roughly eight months and the departure window for such flights opens only once every two years.

During that eight-month "coasting" period, the crew will be exposed to severe levels of solar radiation that are otherwise deflected away from humans of Earth. Once you get to Mars, there is a mandatory minimum stay time, until the return window to Earth opens again.

For minimum-energy transfers this visit could last a year or more. Then, there is the return transit time of about eight months, again exposed to severe radiation, at the end of which you will be approaching Earth at a speed well over 10 km/sec. Just slowing down in order to safely land could be very risky.

To summarize, for each person traveling to Mars, you will spend roughly 30 months away from home, during which you will be in either a weightless or a low-gravity environment and exposed to severe radiation during most of that time. We have not even touched upon the risks associated with hardware failures nor the psychological issues associated with being in a capsule for eight months in transit each way and in cramped quarters for the stay on Mars.

Is any ready to sign up?