orbit - What would happen if an astronaut orbiting Earth was exposed to radiation?

I guess that depends on how much radiation they're getting. I should also point out that astronauts are not 100% protected from radiation, as discussed in this question. One of the most common affects on actual astronauts is the development of cataracts. They can also receive damage to their nervous system over time, among other effects.

But it appears that you're asking what would happen to a person who was exposed to a huge amount of radiation, such as if they went into space with a suit that did little to protect from radiation. I think to answer that, we just have to look at cases where humans have actually been exposed to huge amounts of radiation.

One interesting example that comes to mind is Anatoli Bugorski who was a Russian particle physicist who accidentally had his head inside a particle accelerator when it was on. His head was subjected to huge amounts of radiation. The results of this are described as

The left half of Bugorski's face swelled up beyond recognition and,
over the next several days, started peeling off, revealing the path
that the proton beam (moving near the speed of light) had burned
through parts of his face, his bone and the brain tissue underneath.
As it was believed that he had received far in excess of a fatal dose
of radiation, Bugorski was taken to a clinic in Moscow where the
doctors could observe his expected demise. However, Bugorski survived
and even completed his Ph.D. There was virtually no damage to his
intellectual capacity, but the fatigue of mental work increased
markedly. Bugorski completely lost hearing in the left ear and only
a constant, unpleasant internal noise remained. The left half of his
face was paralyzed due to the destruction of nerves.1 He was able to
function well, except for the fact that he had occasional complex
partial seizures and rare tonic-clonic seizures.

Now imagine what could happen if a person were to be exposed in a similar fashion to highly energetic particles in space, but throughout their body and not just their face. I expect that a person could not survive such an exposure and that their body would be destroyed. Even if they survived, it is highly likely they would live with crippling medical conditions and very likely develop cancer later in life.