Why do Ships Lose Contact When They're Entering the Atmosphere?

If you have a high speed entry, you create a pocket of ionised gas around your probe. The gas is heated up so much, it becomes ionised - it's stripped of its electrons. That is not transparent to radio waves, so you have a radio black out. The same thing happens with vehicles re-entering the Earth's atmosphere. It used to happen with the shuttle I think and the Apollo vehicles. So that's the main reason. It's the state of the gas around a high speed probe.