Radiation environment
The charged particles and energetic photons a spacecraft is exposed to in orbit or in deep space, and which its electronics and crew must survive.
Radiation is a design constraint before it is a hazard. Ordinary electronics degrade or glitch when struck by energetic particles, so hardware meant to last must be radiation-hardened - which usually means older, slower, far more expensive chips than the commercial equivalent. That single trade-off explains much of why spacecraft computing lags consumer technology by years. Above low orbit the exposure rises sharply, which is why a lunar mission is a different engineering problem from a satellite that never leaves the shelter of the magnetosphere.
The step change matters for where we are heading: the 5 lunar missions in our tracker leave the protection that keeps the 144 low-orbit missions relatively sheltered, and that shift shows up as harder requirements on hardware long before it shows up as a headline.
Electromagnetic radiation in the astronomical sense - the light telescopes collect. The same word covers both, and in our own archive the astrophysical sense is the more common one, which is why this term is defined here but never auto-linked in article text.