 Why is the Earth worse than the aliens imagine?
Earth was so far away that it wasn't visible. But this vast distance did not mean that isolation could endure forever. A meteor shower could pass over and their clumsy instruments could interpret it as an all-out enemy attack." "And how long can Earth last? They went much closer to Earth, not intending to commit themselves. Atmospheric analysis showed that as far away as we were." "What's worse is that I now find they also have missiles, range one thousand miles and upward. The southern plains were green, coursed with rivers; the oceans were blue; and much of the northern hemisphere was glistening white. Ragged clouds covered the pole, and a dirty pall spread over the mid-regions of the north.
 The Earth has missiles and is close to space travel.
 The Earth has missiles and is close to space travel.  The humans are rough and desperate.  A meteor shower could be interpreted as an enemy attack by the humans' clumsy instruments.  The humans don't like aliens.