Mass is no longer a factor in rad-shielding (was Travel-time to GEO)

> Lightweight materials for shielding

Given that cosmic-radiation shielding is generally functional
based only on mass and hydrogen-richness, why would one choose
*lightweight* materials for it?
http://en.wikipedia.org/wiki/Health_threat_from_cosmic_rays#Shielding

"None of these strategies currently provides a method of protection
that would be known to be sufficient, while using known engineering
principles and conforming to likely *limitations on the mass* of the
payload. The required amount of material shielding would be *too
heavy* to be lifted into space."

> include foam and aerogel.

That is new to me. What sorts of radiation do foam and aerogel
shield the best?