During his hour-long announcement of the SpaceX Mars colonization plan, CEO Elon Musk didn't say where exactly Martian colonists will live once they arrive on the planet — and how exactly they'll survive given the harsh environment.

Musk seemed particularly unconcerned about solar radiation. "The radiation thing is often brought up, but it’s not too big of a deal," he says. There is a "slightly increased risk" of cancer, he says, and there will probably be some sort of shielding. He talked of creating an artificial magnetic field on Mars to deflect high-energy particles, especially to protect colonists from solar flares. But Musk didn't provide any information of how this magnetic field would be created.

Watch: SpaceX Mars safety and habitat Q&A

Musk says SpaceX's goal is to build the transport system, like building the Union Pacific Railroad. "Once that transport system is built," Musk says, "there's a tremendous opportunity for anyone who wants to go to Mars and create something new or build the foundations of a new planet." People will be able to go to the planet and build "anything from iron refineries to the first pizza joint."

In a video SpaceX released to detail its "Interplanetary Transport System," the colonists are seen arriving on Mars and finding no habitat on its surface.

Here are some questions about the Mars habitats that we still think should be addressed: what will the habitats look like? Will they be underground? Is there assembly required? What about life support? Water? Air? Food? Will resources come from Earth or will the colonists live off the land? How will the colonists deal with the effects of reduced gravity?

Colonists will need a life-support system to create breathable air, a way to recycle water, and a way to feed themselves. No one has ever tried doing any of these things on Mars and the challenges are of course enormous.