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Nancy Kress began her career writing fantasy, but starting with An Alien Light in 1988, has established herself as a fine writer of hardcore science fiction. Best known for her series of novels beginning with Beggars in Spain, Kress is at the forefront of a generation of women SF writers who are proving that technology and science are no longer the sole province of men. Probability Moon, which combines a space adventure plot with a mystery involving the inhabitants of a planet known simply as World, will do nothing to detract from her growing reputation as a writer capable of exploring advanced technologies while never losing sight of the people whose lives that technology affects.

In Probability Moon, human beings have expanded into space, and in the process discovered an interstellar transportation system left behind by some unknown alien race. Several other civilizations have been discovered, all humanoid and seemingly related to homo sapiens. The one exception is the Fallers, who have been unrelentingly hostile from first contact on, and who have engaged humanity in a war that humans are having a tough time winning. When it is discovered that that one of the moons of World is an artifact, possibly a weapon, possibly left behind by the same aliens that built the transportation system, the race is on to figure out just what the moon is before the Fallers can get there.

The hitch is that World is inhabited. And while the humans in orbit deal with the artifact, most of Probability Moon focuses on the Worlders and the team of scientists who have been sent to study them.

The Worlders are humanoid, and related to humans, with one unique distinguishing trait: They practice a concept known as shared reality, which means that once the majority has accepted something as true, everyone accepts it as true. To deviate from shared reality causes physical pain in the form of punishing headaches. Someone who strays too far is deemed to be unreal, and unreal persons, with few exceptions, are not allowed to live.

The scientists, of course, are interested in shared reality and the mechanism that enforces it. A team of four, they are all experienced veterans, with the exception of David Allen, a young researcher fresh out of school. David is the kind of person who tends to believe that his version of reality is so self-evidently correct that it should be shared by the others, and when David's reality conflicts with the Worlders', the Terrans run the risk of being declared unreal. This conflict, along with the struggles of the military to understand and control the artifact, form the core of the story in Probability Moon.

Probability Moon is a text-book example of how to use two plot strands to feed off and support each other. The tension grows throughout the novel as the two stories slowly come together in a way that by the end of the book feels both inevitable and natural. The mystery of the artificial moon, and the struggles of the scientists to understand and survive in the Worlders' culture of shared reality are in the end intertwined in such a way as to explain both how the society of the Worlders came to be and what the moon does -- all of it tied together by the physics of probabilities.

Probability Moon is a deftly written novel with believable characters who make mistakes and learn to live with the consequences. The prose is straightforward and never gets in the way of the story that Nancy Kress has to tell. It's a story that readers of science fiction should find to be engaging, thoughtful, and a pleasure to read.

Reviewer Greg L. Johnson lives in Minneapolis, Minnesota, where the approaching winter is a shared reality that none can deny. His reviews also appear in Black Gate and the The New York Review of Science Fiction.