Radioactively hot, that is.
While the Curiosity Mars Rover was cruising for eight months between Earth and Mars, one of the instruments aboard was measuring the radiation levels encountered along the way. The Radiation Assessment Detector—RAD—was inside the spacecraft, shielded in much the same way as astronauts would be on an interplanetary mission. Now the same instrument is continuing to monitor those radiation levels on the Martian surface. The results from the interplanetary cruise phase of the mission were recently released, and while they don’t rule out interplanetary voyages by humans, they do highlight a significant risk.
Let’s do a Q & A to address some background material and some of the issues involved in order to better understand this.