> It is outside Jupiter's massive radiation belt, which for reference does envelop the larger moon Ganymede.

On the surface of Callisto, you'd experience about 10x-12x the background radiation that you'd get here on Earth. That's manageable with strong radiation attenuating glass (according to this). However, many of these cosmology & space blogs ignore galactic cosmic rays/radiation (GCRs). Glass won't protect you from those.

According this this study, the amount of GCR hitting Europa's surface is reduced thanks to being inside the Jovian magnetosphere - this prevents a significant fraction of the GCR spectrum from reaching Europa's surface. But this is a double-edged sword, Callisto is less protected from GCRs since it's further away from Jupiter's protection. In fact, the study goes on to say:

> "In addition, the cut-off rigidity threshold is expected to decrease in energy as one moves outward in the Jovian magnetosphere. Therefore, the GCR contribution to surface radiation processing may be more significant at these bodies, particularly at Callisto, which is bombarded by magnetospheric particles at a much lower rate than Europa, and which does not possess Ganymede's intrinsic dynamo magnetic field."

GCRs aren't a huge issue if you're taking short jaunts to the Moon or a space station, but that (combined with radiation, solar flares/ejecta/particles, etc) isn't something you'd want to experience long term. And it's not just about raising your risk for cancer by orders of magnitude, it can make you sick/unwell which puts the entire mission at risk. See here and also here.

In this study about dealing with radiation on a potential manned-trip to Mars, they find the "overwhelming health risk" comes from GCRs. This is because they cascade and create secondary radioactive particles and so on. In fact, he thicker the material they pass through, the more secondary particles there will be in the cascade. In condensed matter (liquid or solid) the shower of a 1 GeV proton continues to grow until it reaches about 200 g/cm2.