On that, if you orient the ship so that the rear is pointed directly at the sun (trivial to do once you're coasting) you'll have a hundred feet of liquid fuel and other solid material which will block the primary source of radiation (the sun).

I too, read Project Hail Mary (great book btw).

I was always led to believe that the primary source of radiation we need to worry about for space travel was Cosmic Radiation [1]. The shielding requirements for CR relative to solar radiation requires much more material, to protect from rays from every angle. [1] https://www.nasa.gov/missions/analog-field-testing/why-space...

It's not that different between the two; Mars lacks a magnetosphere, so the planet itself is left to block about half of what'd hit you in interplanetary space.

The atmosphere, though thin, does a lot of work too.