|Fig. 1: Artist's rendering of an Interplanetary Transport System rocket launching from the pad. Elon Musk has stated that such vehicles are designed to carry 100 passengers, at low enough cost to allow the possibility of Mars colonization.  (Source: Wikimedia Commons|
Elon Musk recently made public his long-hinted plans for a rocket capable of transporting sufficient mass at low enough cost to allow the possibility of Mars colonization.  Dubbed the BFR, the rocket would have a staggering 42 engines in the first stage, and be fully reusable; an artist's rendering is shown in Fig. 1. In the rocket science sense, this is not a revolutionary advance; put enough engines and fuel on something, and you can get anything to Mars. The reusability and retropropulsion aspects have been developed and improved on the Falcon 9 launch vehicle, and can be expected to work on Mars. However, a large outstanding question remains: even if you can get people to Mars, what does it take to get them there safely?
The key biological challenge is mitigating the effect of the interplanetary radiation environment, particularly of ionizing radiation that has the ability to harm tissues. These exposures are measured in Gray, which is the standard unit measuring absorbed radiation. Hoever, for health concerns, not all radiation is created equal; in particular, radiation from protons, neutrons, or nuclei is more damaging. The Sievert is the unit of effective dose, obtained by multiplying the exposure in Gray by a weighting factor to account for the different effects of different types of radiation.
On Earth, the combined effects of atmosphere and magnetic field reduce the magnitude of ionizing radiation present at the surface; the typical person gets about 3 mSv (millsieverts) dose annually.  Astronauts on the ISS remain within the Earth's magnetosphere, which still stops most radiation that would be present in interplanetary space. The ISS also has quite thick shielding in comparison to other spacecraft, having shielding mass typically 50 g/cm2, ranging up to 100g/cm2.  Combined, these mean the annual dose on board the ISS is about 50 mSv to 100mSv.  NASA has set a lifetime radiation dose for astronauts at 1 Sv, corresponding to a 3% risk of exposure-induced death from cancer. 
How does the radiation expected on a trip to Mars compare? Data from Curiosity taken during its 253-day trip to Mars showed it absorbing nearly 0.5 Sievert, one-way.  Most mission plans for human trips to Mars use a shorter transfer time, with round-trips of about a year; for such a mission, the expected dose would be around 0.66 Sv.  This is within the lifetime exposure limit, but does not account for any time actually spent at Mars. Unlike Earth, Mars has thinner atmosphere and no magnetic field, and so without special attention to shielding on the surface any stay of length sufficient to justify the trip would likely put an astronaut over the career limit. [4,5]
It seems that a trip to Mars is feasible; radiation dose is likely to remain below the current career limit for a single trip. However, there are indications that highly ionizing radiation such as galactic cosmic rays and proton flux from the sun have different effects, and may not be appropriately measured by the current calculation of effective dose in sievert.  Computations suggest that a Mars trip will incur a 4% fatal risk, but may be as high as 14%.  There is also the effect of radiation on Apollo astronauts. On average, the Apollo astronauts had a radiation dose of 3.8 mGy; assuming all of the dose is from protons due to solar wind the weighting factor is 2, and this corresponds to an effective dose of about 8 mSv. [7,8] Even with this small dose compared to the career limit, Apollo astronauts showed a statistically significant higher degree of cardiovascular disease, as compared to both non-flight and astronauts who only flew in low Earth orbit. 
Given this effect on the Apollo astronauts, who only had missions lasting weeks, and the large uncertainty in the actual risk of radiation on a Mars voyage, it seems clear that there is much work to be done in improved shielding and understanding the effect of highly ionizing radiation on the body before even accurate risk assessments can be made. There is no reason to think these problems are insurmountable, but the current state of knowledge is not quite sufficient to start designing the solutions.
© Arul Suresh. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 K. Chang, "Elon Musks's Plan: Get Humans to Mars, and Beyond," New York Times, 27 Sep 05.
 "Annex B: Exposures of the Public and Workers from Various Sources of Radiation," in Sources and Effects of Ionizaing Radiation - UNSCEAR 2008 (Untied Nations, 2010).
 S.L. Koontz, et al., "The Ionizing Radiation Environment on the International Space Station: Performance vs. Expectations for Avionics and Materials," IEEE 1532675, 11 Jul 05.
 R. A. Kerr, "Radiation Will Make Astronauts' Trip to Mars Even Riskier," Science 340, 1031 (2013).
 C. Zeitlin, et al., "Measurements of Energetic Particle Radiation in Transit To Mars on the Mars Science Laboratory," Science 340, 1080 (2013).
 F. A. Cucinotta and M. Durante, "Cancer Risk From Exposure to Galactic Cosmic Rays: Implications For Space Exploration by Human Beings," Lancet 7, 431 (2006).
 R. A. English, et al., "Apollo Experience Report: Protection Against Radiation." NASA TN D-7080, March 1973.
 "The 2007 Recommendations of the International Commission on Radiological Protection," International Commission on Radiological Protection, ICRP Publication 103, Ann. IRCP 37, Nos. 2-4, 1 (2007).
 M.D. Delp,