Relativistic science doesn't leave much room for faster-than-light drives, but Roger MacBride Allen turns that limitation into an opportunity in his new book, The Depths of Time (Bantam Doubleday Dell, $13.95).

More than 3,000 years from now, humanity has built an interstellar culture without benefit of FTL technology. The fastest starships in human space can only reach a tenth of the speed of light, but instead travel through wormholes to arrive at their destinations in only a few weeks of objective time.

While the ship travels for decades of subjective time, the crew remains in coldsleep, aging only a few days during the journey.

It's a workable but clumsy transportation system, depending on several different technologies working together. Of more immediate relevance to Allen's plot, these time-traveling spaceships present several risks to the spacetime continuum.

For example, there is no physical reason ships can't travel forward in time through the wormholes, or manipulate their travel times so as to arrive in and change their own pasts.

Only the constant vigilance of the Chronologic Patrol, which guards both ends of known wormholes, prevents a temporal crisis.

The responsibility of doing whatever is necessary to protect history rests heavy on the shoulders of the book's protagonist, Anton Koffield -- especially when an attempted time incursion forces him to destroy the wormhole he guards just as a vital supply convoy is passing through it.

His performance of his duty makes him a hero, but the many lives lost due to his actions make him a pariah. After being promoted to admiral and shuffled off to a desk job, he finds himself drifting into the orbit of Oskar DeSilvio, a legendary expert on terraforming.

From there, Allen opens out his story, exploring the finer points of his vision of space and time travel.

Time, it seems, can play a variety of roles in space exploration. The same cryogenic sleep that protects a starship crew can be a crucial aid to terraforming.

By freezing a team of terraforming experts and reviving them as needed, the centuries-long work of making a world habitable can be conducted in the course of a single -- albeit stretched -- lifetime.

The secret to mastering space, then, may lie in controlling time.

The trouble is, as Allen points out, that even massively long-lived humans are still just humans, and humans are not omniscient. Prediction and interpretation -- observing conditions and applying knowledge to them -- weave throughout the book as a crucial theme.

Koffield himself is a master observer, while Allen also spends several chapters following the viewpoints of two sharp-eyed young officers. Significantly, the villains of the novel, meanwhile, are not so much evil as willfully unaware.

The novel's climax is not so much the resolution of a problem as the discovery that there's more at stake than the heroes thought. While this primarily serves to set up a sequel, the emphasis on knowledge makes this revelation a satisfying ending in its own right.

It also sets up a strong moral lesson. The people of 5339 have powerful tools for altering their environment and even predicting the consequences -- but predictions are only as good as the data they are built on, and it's hard to be certain you've taken all the data into account.