The effect of prolonged radiation exposure on the human body is one of the potential show stoppers of long term human spaceflight. Yesterday we heard Zubrin present his traditional "don't worry about it" speech and another speaker refer to his reasoning as "the numbers game". By this, he meant that no-one in the audience is going to check those numbers so Zubrin can say whatever he wants. I think this speaker was missing the point. Let's look at what Bob was actually saying.
- Six month transit times to Mars are available every 2 years
- Radiation exposure on the Mars surface is negligible due to the availability of dirt for shielding
- Solar radiation in deep space can be shielded against, so galactic cosmic radiation is all that matters
- The GCR dose in LEO is about half that of deep space. It's about 55%
- Mars crews would consist of 5 people
- The ISS will continue to be crewed permanently by 7 people
Under these assumptions, a Mars program for 10 years results in a total crew dose of 5 * 10 / 2 * Deep_Space_Dose. Whereas the total crew dose for the ISS will be 7 * 10 * 0.55 * Deep_Space_Dose. If we normalize we get 25 < 38.5, so clearly Mars is a safer mission than ISS in terms of radiation dose, right?
Well no, we can argue with almost all of his assumptions. Transit times are more like 8 months. Radiation exposure on the Mars surface isn't negligible because astronauts will be doing lots of EVAs. Solar radiation shielding makes the galactic comic radiation dose worse, etc, etc. But I think this is the wrong way to go about it. Arguing about the assumptions draws us into the trap of thinking about overall program doses which are irrelevant.
What matters is the individual astronaut's lifetime dose. In the ISS program today, astronauts are limited to 9 months non-consecutive stays on the station (either three 3 month stays or a 6 month and a 3 month stay). Under the assumptions above, an individual crew member's dose in the Mars program is 1 * 1 * 1 = 1 normalized year. In the ISS program the crew member's dose is 1 * 0.75 * 0.55 = 0.4125 normalized years.
As such, an individual astronaut is more than twice as likely to die of cancer when they return from the Mars mission than an individual astronaut in the ISS program, and that's under the bad assumptions. In reality it's much worse.
Now that doesn't mean they shouldn't go. Even if it was absolutely certain that 10 years of your life would be cut off by cancer from going to Mars there would be a line of volunteers snaking out the door. What it means is that NASA is legally not allowed to send astronauts under the existing radiation exposure laws (which ISS strictly follows). It means those laws need to be changed or we need technology to make that radiation exposure less.