This is a long post, but it doesn't feel like it once you get into it: it's lucid and readable and educational and fun!

I have a question about Tyson's alien visitation example. Since our atmosphere has 'stuff' too, like oxygen, carbon dioxide, hydrogen, etc., won't that mean that the alien, since he will have been in contact with this stuff on his way here, will have exploded long before he reached our front lawn?