Good science fiction is, ultimately, good literature. Arthur C. Clarke, Isaac Asimov, and Poul Anderson have been responsible for a large share of the genre's best. These three novels continue the tradition of offering stunning visions of our common future - with varying degrees of literary success.

THE POSITRONIC MAN, by Isaac Asimov and Robert Silverberg (Doubleday, 259 pp., $22.50). Decades ago, the late Isaac Asimov laid down the law - the laws that govern all robots who serve humanity. Asimov's famous Three Laws of Robotics formed the groundwork for a series of novels featuring robot characters and the strange situations into which they calculated themselves. ``The Positronic Man,'' an expansion of a 1976 Nebula award-winning short story, ``The Bicentennial Man,'' raises some sticky questions about the distinction between man and sophisticated machine.

Robot NDR-113, a.k.a. Andrew Martin, is a fuzzy-logic-based expert system on steroids. As the Martin family's all-purpose assistant, the robot is capable of doing most anything its masters ask. But when Andrew develops a talent for art, the shrewd business sense to profit from his creations, and an unusual desire for self-improvement, it becomes clear that ``he'' is no ordinary droid. ``The Positronic Man'' becomes a mediation on what the term human implies as Andrew lobbies the World Legislature for civil rights amid irrational robophobia - irrational because of the unshakability of his programmed constraints - and aspires, ultimately, for a slot in the human race. Although Andrew often displays a force of will that the authors never convincingly reconcile with the Three Laws, and while a rather rushed plot makes this read, in spots, more like a thought experiment than a novel, Asimov and Silverberg treat us to a moral discourse with implications that anyone from a teenage science-fiction initiate to a professor of philosophy will appreciate.

HARVEST OF STARS, by Poul Anderson (Tor Books, 395 pp., $22.95). Aiming for the imaginative sweep of Asmiov's Foundation series, the intellectual edge of Philip K. Dick, and the political intrigue of Frank Herbert, Poul Anderson never quite manages to find his voice in this slow, directionless novel about the struggle for hegemony between two futuristic factions, the Avantists, an inscrutable society of social engineers, and Fireball, a giant transportation, mining, and scientific concern whose tentacles reach into most every corner of society.

The novel hinges around the idea of individual identity. The Avantists manage to download onto a computer the mind of their principal nemesis, Fireball's legendary director Anson Guthrie, and strive to use it against him. But instead of allowing Guthrie and his simulacrum to square off directly, Anderson wheels out talking head after talking head to discuss obliquely the various conspiratorial machinations at work and solemnly recount the history of their rivalry.

The presence of an extensive dramatis personae at the front of the book should be taken as a warning sign of what's to come. Anderson, imagining as fast as he can, leaves the reader in the dust.

THE HAMMER OF GOD, by Arthur C. Clarke (Bantam Spectra, 226 pp., $19.95). If Trump's art is the deal, Arthur Clarke's is the concept. And what more compelling concept than that of his latest fictional flight: The looming obliteration of civilization by a stray asteroid and the efforts of a bold few to avert it?

Apocalypse has been the theme of many popular entertainments. What distinguishes ``The Hammer of God'' is both the scale and the plausibility - no, the mathematical near certainty - of the potential disaster Clarke depicts.

Blind to the vast shooting gallery that is our solar system, we scarcely recognize how fortunate we have been since the dinosaurs were snuffed out some 650 centuries ago. In 1972, an asteroid the size of a small house grazed the earth's wispy atmosphere over Oregon at a speed of 30,000 miles per hour. Had it struck, the explosion would have had five times the force of the Hiroshima bomb. Clarke's novel gives life to a profoundly chilling notion: that our home planet is little more than a cosmic clay pigeon whizzing through a range of haphazardly aimed interplanetary buckshot.

But by the standards of superior fiction, ``The Hammer of God'' misses the anvil. A mere 40,000 words long, it never achieves the depth or complexity of a truly satisfying speculative novel. When the comet, named Kali for the Hindu goddess of destruction, is first detected beyond Saturn by a Mars-based amateur astronomer, the year is 2109 and the world is a very different place. Christianity and Islam have merged under the name Chrislam. The Superquake has leveled California. The United States has dissolved into a commonwealth. Tantalizing tidbits, but they're just a cardboard backdrop to a plot line Clarke follows linearly to its conclusion: the heroic and ingenious efforts of Capt. Robert Singh to deflect Kali from its target.

Clarke's descriptions of feats of the technological imagination have often been thrilling (witness the orbital slingshot maneuver from ``2001''). Here he outlines without pacing, tension, or true inspiration Singh's schemes to save the earth. In the end - and to give away the ending would be criminal, except to say it is not wholly what one would predict - a reader looking for more than just a concept to dine out on will be disappointed. Perhaps because of its literary flaws, this book's conceptual importance may not be realized until a 2,000-foot tsunami, spawn of an asteroid strike, has washed away every copy in existence.

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