Probability Moon — Nancy Kress

A Desperate Bid to Save Humanity

Humankind has expanded out into interstellar space using star gates--technological remnants left behind by an ancient, long-vanished race. But the technology comes with a price. Among the stars, humanity encountered the Fallers, a strange alien race bent on nothing short of genocide. It's all-out war, and humanity is losing.

In this fragile situation, a new planet is discovered, inhabited by a pre-industrial race who experience "shared reality"--they're literally compelled to share the same worldview. A team of human scientists is dispatched--but what they don't know is that their mission of first contact is actually a covert military operation.

For one of the planet's moons is really a huge mysterious artifact of the same origin as the star gates ... and it just may be the key to winning the war.