A point missed...
Hi gb
Water - in quantity - could be used as reaction mass and as cosmic-ray shielding. Using a low temperature reactor to heat the comet mass, capturing the water driven off, then using it to boost tens of thousands of tons back to LEO, GEO and L1/2 for use as space-station mass-shielding and propellant on a BIG Mars vehicle... well there's a lot of positives with the idea. A reactor only has to run at about 1900 C and can use condensed water instead of hydrogen cracked from the water.
If there's water available then perhaps we should look at BIG scale ISRU.