Wheelers

In the year 2270, with travel to the nearby planets well established, a bizarre discovery is made on Callisto, the eighth moon of Jupiter. Dozens upon dozens of strange wheeled artifacts ("wheelers"), hundreds of thousands of years old, are found buried beneath the icy surface. What were they used for, and who left them in our solar system? At the same time, it is discovered that the moons of Jupiter have moved from their expected positions. An expedition discovers that Jupiter is inhabited by a race of balloon-like aliens, who defend their world against comet strikes by moving the moons using gravitational technology. In this case, though, their re-direction is aiming an incoming comet directly at Earth!

Communication at first proves impossible, but an Earth child who has an intuitive understanding of alien behavior becomes to key to contacting the aliens -- and joining forces with them to save our world.