Dain bramage and brittle bones....
NASA Likely to Break Radiation Rules to Go to Mars
12 Apr 2017
https://www.pbs.org/wgbh/nova/next/space/nasa-mars-radiation-rule/
A somewhat heated discussion of one paper on the radiation subject in the comments below this article:
Debunking the invalid claims of a space radiation paper
May 11, 2015
https://www.thespacereview.com/article/2749/1
Radiation and Bone Loss: Deep Space Mission Concerns
July 18, 2006
https://www.space.com/2642-radiation-bone-loss-deep-space-mission-concerns.html
The new study shows that on longer flights, such as a 6-month trip to the Moon or 30-month trip to Mars, the bone lost as a result of microgravity will be compounded by more extensive bone loss as a result of radiation exposure. Up to now, NASA has focused on radiation's cancer-causing properties and effect on the central nervous and immune systems. The effect on bone health has been unexamined.
"Now we're concerned that radiation and reduced gravity are both going to contribute to bone loss," Bateman said.
It is estimated that severe bone density loss is possible on a Mars mission. ISS occupants lose 2% per month.
The only real fixes are artificial gravity and shielding en-route, both very heavy and driving up the cost of a mission even beyond the levels no one apparently wants to spend even at current guesstimated mission costs.
Spend a fraction of the money of manned missions on robotics and AI research, both technologies hugely useful here on Earth, and send robotic missions everywhere. If something promising is found like evidence of past or current life and robotics/AI still still aren't up to snuff, send humans.
Rather than worrying right now about Mars colonization for a guarantee of continued human survival in the solar system, spend more than lip service on planetary impact defense. That's much more doable right now and much cheaper than a Mars mission.
But, in the end, lobbyists will determine the policy based upon what Congress is willing to spend.