Radiation
I read an article about the misconceptions that we have about the effects of radiation on space habitats. A space habitat in which people will be living for a long term requires radiation shielding. Unfortunately, radiation shielding can actually make the problem of radiation in space worse instead of better, because cosmic rays (very fast moving large atomic nuclei) will produce secondary radiation when they strike thin shielding.
If the shielding is made sufficiently thick, of course, then all but a minute fraction of the cosmic rays and the secondary radiation they produce will be blocked by the shielding. After all, the Earth is located in space; if its atmosphere and magnetic field provide adequate shielding from the radiation in space for people on the ground, then a shield of equivalent thickness will provide enough protection for people in space.
Previous designs for space habitats, however, have limited the amount of radiation shielding that could be included. In some, the shielding material partakes in the rotation which produces artificial gravity for the space colonists, which means that its thickness is limited by the structural strength of the space habitat. In others, sunlight reaches the habitat through an expanse of clear glass.
There is a habitat design that makes completely practical to use leftovers from mining to produce shielding several feet thick, or, in fact, arbitrarily thick, which surrounds the people in the habitat from absolutely all directions. Thus, if the shielding provided by thirty feet of solid rock is desired, it can be obtained. If it is desired to put a team of scientists within the heart of Jupiter's radiation belts to remotely operate a rover on one of Jupiter's moons, a design using this method would be a lot safer than the common conceptions of space habitat design