Mass requires reaction (generally: fuel) both to accelerate and decellerate. Absent alternative delta-V mechanisms (usually: aeroraking, as lithobraking is perceived as generally too extreme), this rapidly runs into the tyranny of the rocket equation.
<https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation>
Even without earth-to-orbit costs, that mass has real costs, and reduces available payload.
If that shielding mass can be dual-use (e.g., water), reactive (e.g., electromagnets), or reduced to a very small amount (emergency shelters, wearable garments), it becomes more practicable. That still doesn't make it easy.
There's also a discontinuity in radiation exposure. Passing through radiation belts (e.g., Earth's van Allen belts, or those around Jupiter), and solar storms, are both predictable and special precautions can be taken. Cosmic ray radiation is unpredictable, high-energy, and is far harder to guard against. Some risk is inevitable.
Right, I don't think it makes sense to have our long distance transit vehicle be the same one that needs to decelerate/aerobrake - I'm assuming we'd do something like an Aldrin Cycler for transit between Earth/Mars, where we have taxis that go to/from the cyclers.
In KSR's Mars Trilogy, they had an emergency shelter, seems reasonable if the bulk of the radiation exposure is predictable. But I'd personally feel better being shielded the entire time, given cosmic radiation.
That long-distance transit vehicle has to change trajectory somehow. Orbital transfers may be cheap but they're not free, unless you're talking about a free-return trajectory in which case your specific transits are limited, and often fairly slow.
Though yes, that does make the option of providing a radiation-hardened shelter available with less concern as to total mass.
I thought i read somewhere that the soil on mars is probably pretty toxic to humans, so maybe getting there safely isn't as concerning as what you'd do once you are there...
If you mean the perchlorate concentration, it's water soluble, I think we just rinse it off as part of going inside.
> Cosmic ray radiation is unpredictable, high-energy, and is ...
IIR, the big problem is that the cosmic ray background radiation is far too predictable - there is a concerning amount of it 24x7x365. So your baseline choices are gritting your teeth and bearing it, or spending most of your time in a seriously hard shelter.
Definitely outside my realm of expertise, but I'd expect that particles have a range of energies, and that the most energetic, or most damaging (not necessarily the same) occur somewhat rarely.
But yes, generally, it's at the very least a constant background flux (with occasional peaks), and all of it high-energy enough to make lightweight shielding of limited use.