Asteroids mostly contain the same naturally radioactive elements we find in Earth's rocks - mainly potassium, uranium, and thorium in very small amounts. When they hit Earth, they don't typically create radiation hazards. Scientists have checked out famous impact sites like Meteor Crater in Arizona and found normal radiation levels. While impacts can briefly create some radioactive isotopes through the collision process, it's really the impact's explosive force that does the real damage, not radiation.