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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And you, to whom adversity has dealt the final blow
With smiling [faces] lyin' to ye' everywhere ye' go
Turn to, and put out all your strength of arm and heart and brain
And like the Mary Ellen Carter, rise again.