Astronauts who have served extended stays on the ISS are actually showing a variety of chronic health problems at rates exceeding the general population (not including cancers because the population is too small for medical statistics on particular carcinomas) including macular degeneration, cardiopulmonary disorders, and a variety of metabolic issues. How much of this is due to radiation exposure versus the effects of long duration freefall or other non-space related lifestyle impacts is unknown, but there are clearly health risks and medical impacts from exposure to the space environment that likely also translate to the low gravity and lack of a radiation filtering atmosphere on the surface of Mars.
We have yet to develop and demonstrate the capability to move large aggregate bodies around interplanetary space, and the chemical rocket propulsion systems we currently use for most interplanetary missions are at best marginal in terms of being able to reach—much less redirect—water-containing asteroids and comets, but there is nothing fundamentally impossible about doing so, especially once we have developed an extensive capability for automation in space operations and a high specific impulse propulsion system with adequate thrust (i.e. something other than tiny electric ion thrusters suitable only for orbital stationkeeping and tiny spacecraft).
But then, that begs the question of why go to Mars at all? Mars is the most difficult solid body in the Solar System to controllably land upon, and it is unsuitable for habitation in a multitude of ways. It would make far more sense to use this technology to move and process asteroid materials in situ to build solar orbiting habitats where near-terrestrial conditions and protection from radiation can be provided to occupants without having to cope with the limitations and issues of a planet which has never been able to support plant and animal life and will is not capable of being made to do so outside of the dreams of science fiction authors.
Stranger