The Moon is a harsh place, as decades of direct exploration have demonstrated. It carries all of the risks of conventional spaceflight, then adds to them. The tyranny of distance from Earth makes emergency returns difficult. The dust on the surface could be dangerous to inhale. Meteorites pepper the surface.
These are serious problems, but the greatest threat to future human explorers will probably be radiation.
Astronauts in low Earth orbit receive protection from the Earth’s magnetic field, which shields out some of the heavier subatomic particles that stream in from space. It’s just as well. These cosmic rays are energetic and dangerous to life. Go beyond this region of space, and this natural protection disappears.
The Moon itself has essentially no magnetic field, and no atmosphere. There’s little to stop the barrage of particles and rays that stream in from the Sun and beyond.
Some of this material actually changes the lunar surface. It can cause erosion of some rocks, and alter their chemistry. Particles from the solar wind probably contribute to the thin layer of water on some lunar rocks, by stimulating chemical reactions. The Moon’s deposits of Helium-3, which could be a future energy source, have been collected from eons of solar emissions.
A short mission to the Moon will be survivable for astronauts, mainly because exposure times will be low. Astronauts staying for longer periods will need shielding, to guard against the long-term effects of exposure. The thin walls of spacecraft will not be enough. Bases on the Moon will probably need to be buried, or at least covered with a layer of soil.
Studies of the levels of radiation around the Moon were performed by the Indian Chandrayaan-1 orbiter. A radiation monitoring instrument on board the NASA Lunar Reconnaissance Orbiter is still active. This research is certainly useful, but it will not answer all our questions.
Radiation sensors need to be placed on the surface, and at different regions, to assess the full nature of the Moon’s radiation environment.
The most critical challenge will come when astronauts face the fury of a large solar flare while on the Moon. Radiation levels can increase enormously, and fatal doses can be absorbed by unprotected astronauts within minutes.
At least one part of a lunar base will need to be equipped as a radiation shelter, to protect against the most extreme radiation events. Shielding will be thicker, and provisions will be made for stays of several hours or days.
Being ready for a radiation storm is just half of the solution. Astronauts will need an early warning system to alert them of an impending event. Will they have enough time to retreat to a shelter if they are a long way from the base? If not, can they improvise protection inside a rover?
These challenges will need to be addressed long before we place our next footprints on the Moon.