You should read this page and this page for about 95% of the answer. The rest of the answer is that Apollo wasn't the first object to enter the Van Allen belts. I can't remember, offhand, the name of the probe, but I'm pretty sure I remember Jay mentioning it in the Van Allen Belt page somewhere.

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In Fallout 3, 'happiness' is a warm junkyard dog and a loaded gun. It's mostly the loaded gun.
- Moose's one-line review.

"your going to regret that one. You are now a colonoscope...
- Chrissy, corrupting PraedSt's wish.