If a nuclear-powered space shuttle heading to, say, Mars, exploded in space, what would be the effect on Earth? At what distance would the radiation reach Earth?
Interesting question! It depends where the spacecraft is when it explodes and how catastrophic the explosion is. I'll go through what I think would happen in a couple scenarios. In each case, I'll have to make some assumptions to simplify things.
First, I'm guessing we are talking about true fission reactors here, since radioisotope generators like those used on deep space probes don't provide enough power to launch a rocket to Mars, and because fusion reactors are a long way off.
If the spacecraft crashed but the reactor did not explode (maybe some other part of the spacecraft failed, causing it to fall back to earth) then there would probably be no danger. NASA has built many probes that have radioactive power sources, and in a few cases, these radioactive materials fell back to Earth -- safely.
If the reactor had a melt-down and caused a crash, AND if the containment somehow was breached, radioactive material would be dumped into the atmosphere and the effect might be similar to Chernobyl. However, I'm guessing the reactor on a spacecraft would be smaller and the radioactivity would be more widely spread, so it might not be so disastrous.
If the reactor went critical and exploded like an atomic bomb in the atmosphere, then we can turn to high-altitude bomb tests from the 60s. These tests showed that atomic explosions can cause damage to electronics on the ground through an ElectroMagnetic Pulse (EMP). They also released particles into earth's magnetic field, where they were trapped and damaged satellites. In some cases, the light from the flash caused eye damage. I don't know the details of what radioactive debris reached the surface, but this paper seems to have some more information.
If the reactor exploded like an atomic bomb farther out in space, the effect on earth would be negligible. The explosion would mostly produce x-rays and gamma rays, which would be absorbed by our atmosphere. The sun produces x-rays and other events in the universe produce gamma rays, both of which hit our atmosphere all the time. The amount of x or gamma rays from the explosion might be larger than normal, but I don't think much would happen.
Nuclear power can be dangerous, but it is still being seriously considered because it can provide lots of efficient power without the need for huge solar panels,and it works even in the outer solar system where solar power is not practical. Modern technology has come a long way to make nuclear power fail-safe, but many people still have a knee-jerk opposition to anything nuclear.
Some of these answers involve speculation on my part, but I've tried to imagine what would probably happen. I hope this helps!
Page last updated on June 22, 2015.