If we can only go 150 days in space before the radiation is too high ? How will we ever get to Mars ? It takes 240 days on a good trip ?

Shielding is not the answer. A modest increase in shielding INCREASES the effective radiation. What was mildly dangerous cosmic radiation (usually zips through the body without interacting) becomes very bad ionizing radiation when it is tripped up by shielding. The ISS has no shielding because of this.

To get an overall reduction the shielding has to be MUCH thicker - water ice is very good so about two feet thickness of water ice works - only two feet of water ice in a jacket round the habitation adds a few hundred tons to the load.

The rough designs proposed (no one has a real design) have a heavily shielded "refuge" to be used in the event of a Solar storm and just take the rest of the radiation as chance. This only protects from solar events, a burster will be unexpected and do its damage.

Even using the refuge any crew would clock up several times the recomended "life time dose" on the trip out. There is a significant probability of radiation sickness on the way out - add in the time on surface and the return trip and the maths suggests they will have to decide whether corpses are frozen and brought back or just dumped.

Also bone decay in free fall plus blood vessel decay points to fragile, leaking, crew arriving and possibly dying in landing or returning.

Radiation exposure is not necessarily instantly fatal. In the olden days about 600 rems was considered a fatal dose, in that most people receiving such a dose would not survive more than a few days. That is a big exposure and wouldn't be received on a typical trip to Mars unless some badass flares happened.

However, even doses far less than this carry a high risk of genetic damage resulting in raised incidence of cancers in the following years. This is the information that prospective astronauts must consider. Is it worth being confined in a stuffy claustrophobic capsule on a pointless and probably suicidal mission for a year just to succumb to cancer a short time later?

Personally I would not touch a mission like that with a barge pole, but I'm sure there is a whole line of clueless suckers waiting in line to volunteer. People, generally, have no concept of risk versus reward.

Cheers!

Do you know why the Earth protects us from 99+% of the deadly radiations? It's a very simple natural phenomenon where the Earth produces a magnetic field that surrounds the planet and protects us. The trick will be for science to make such a device portable enough for use in space exploration. I've never seen the 150 day limit, but http://www.space.com/21353-space-radiation-mars-mi... explains the gist of it.

That is a good answer. A moderate increase in shielding means some humans are alive week 150, assuming we can provide other human needs, some of which are presently doubtful. Also for a lot more money and some moderate technical advances we can soft land one human on Mars about 100 days after lift off. You did not imply returning any survivors to Earth. Return trip, or bigger crew, or healthier, will require some bigger technical advances.

If we can only go 150 days in space before the radiation is too high - I've never heard thatr. Please stae youur source. Because I think that is false - and contridicts the data in 'The Case for Mars'. Some additional shielding will be needed (for example water) but only in the habital parts. Also a spin habitate will also be needed.

I believe the answer lies in developing a radiation-free resonance node. See some of Larry Niven's fiction for a description of such nodes used in conjunction with fusion drives. The theory behind such nodes is the that the directed radiation from the fusion drive prevents random radiation from entering the node..

Where is that figure from?

Anyway, that would be based on a certain level of exposure, so the obvious answer is "more shielding to cut the exposure".