Outside the Earth's relatively thick atmosphere and protective magnetic fields the universe is full of ionizing radiation. The general risks of these fast-moving particles to biology has long been apparent, but the specific dangers are still being investigated.
The major ionizing radiation hazards in our solar system come from high energy Galactic Cosmic Rays (CGRs) - that are extremely hard to shield against, and sporadic Solar Energetic Particles (SEPs) associated with coronal mass ejection events on the Sun. SEPs consist mainly of lower energy protons - but lots and lots of them.
Two recent studies add to existing evidence that brains may be particularly vulnerable to GCRs and SEPs, resulting in the insidious effects of Alzheimer-like degeneration. One investigation, by Parihar et al. looked at the detailed impact of cosmic radiation on mice. These unlucky rodents suffered degraded abilities in behavioral tasks, as well as a litany of changes in their brains - including shifts in the complexity of neural connections, morphology, and inflammation in the brain. The bottom line is that 6 months after exposure the mice still showed cognitive impairment, suggesting that humans might suffer memory problems, anxiety, and serious judgment issues.
Another study, by Kempf et al. (also using mice), that looked at the effect of low-dose ionizing radiation from terrestrial exposure (from air travel to medical scanners) also found evidence of radiation-produced molecular features. Features that are similar those seen in Alzheimer cases. That doesn't mean you're about to succumb to a degenerative neurological disorder from flying or going to the doctor's, but it does suggest that over the long term - and especially in higher radiation environments - it's the brain that can suffer.
This is not good news for any human space mission beyond low-Earth orbit. It's also not good news for long-term human occupation of places like Mars. The thin martian atmosphere, and lack of a strong dipolar magnetic field means that GCRs and SEPs penetrate more easily to the planetary surface - where they can also generate secondary particle sprays from colliding with any atomic nuclei.
Of course you can shield against radiation. But this is tricky too - it can add very significant mass to spacecraft, and extra logistical hurdles for any planet-based encampments. It's especially a problem if you want to get out and about - which our pioneering colonists on Mars would presumably want to do.
It would be fair to say that lots more research is necessary. We may find that things are not quite as bad as they seem, and we may be able to come up with workarounds (fast interplanetary travel would help). But I think these issues raise some interesting questions for much further down the line, and perhaps for any biological minds out there in the rest of the universe.
GCRs are likely to be a ubiquitous hazard. If living systems work along similar lines to ours then they probably arise and evolve in planetary environments that provide some kind of protection (atmospheres, marine systems, icy ceilings). So what happens when biological minds decide to step out into the cosmos?
Maybe they discover that biology is just not suited to exploring the universe in anything but a very parochial way. The choices are to stay close to home or to replace the organic substrate with something new - perhaps by handing the exploration task over to machines. Artificial life might also have a hard time with GCRs - it's physics after all - but perhaps it can be designed to accommodate a lot more damage.
And if machines are the true cosmic explorers there is no guarantee that they will fit any of our long cherished fantasies about what aliens should look like. They could be slipping past us all the time, and we'd not notice.