Columbia Accident
When I was a Senior in high school, the Challenger was lost. It was a very emotional experience for me and I had hoped that it would be the one and only shuttle accident... obviously it wasn't. After Challenger I wanted to learn as much about the accident as possible (and hoped everyone at NASA would do the same) in the belief that if we learn from our mistakes we can avoid repeating them. While the Columbia wasn't lost for the same technical reasons, it was lost for the same social reasons.