 What is the aliens' mission?
But this vast distance did not mean that isolation could endure forever. Instruments within the ship intercepted radio broadcasts and, within the hour, early TV signals. The history of the planet was tabulated as facts became available. "It's not what they've done," said Bal, the second alien. A meteor shower could pass over and their clumsy instruments could interpret it as an all-out enemy attack." There's nothing in this region of space our people want," said Ethaniel. "And how long can Earth last? They went much closer to Earth, not intending to commit themselves. For a day they circled the planet, avoiding radar detection, which for them was not difficult, testing, and sampling.
 Bal and Ethaniel are on a mission to Willafours.
 Bal and Ethaniel are on a mission to Earth to set up an interstellar trade route.  Bal and Ethaniel are on a mission to Willafours.  Bal and Ethaniel only have one week to save the Earth, but that is their mission.  Bal and Ethaniel are on a mission to steal the big bomb from the humans.