What causes radio blackouts to occur when, for example, a manned space capsule is descending through the Earth's atmosphere? If the atmosphere itself is in some way responsible for these blackouts (as surely it must be)* then how exactly? NB. Surely it can't be due to the ionosphere. I suggest this because footage of Apollo 13 reveal that its crew were in a state of radio incommunicado with Mission Control during the latter stages of their descent to Earth - that is, until their capsule was seen to emerge from the base of what appears to have been a fairly low cloudbank. Somehow, I can't quite believe the ironosphere extends to such depths. Just an observation. * I don't recall this sort of thing happening during the actual Moon-landings, for example.