Two of science-fiction's premier novelists team up to produce a dazzlingly inventive but chaotic and overlong comet-chaser. In the middle of the 21st century, as Halley's comet retreats from the sun after yet another apparition, a determined band of colonists--some human-normal Orthos, the rest genetically enchanced Percells--struggle to build tunnels and refuges inside the comet (the whys and wherefores of this remain vague, however). The story boasts three narrators: physician-researcher Saul (he tends to mutter in Yiddish), immature space-engineer Carl, and the woman that both men love, computer-whiz Virginia. Among the incessant problems and crises besetting the cometonauts are: Halley's own indigenous life-forms, which rapidly invade the colonists' domain, causing illness and sometimes death; the increasingly hostile attitude of Earth; political tensions between the Percells and the Orthos, culminating in several pitched battles on and in the comet; and disputes over what the comet's ultimate destination should be. Saul, by studying his own immune system--somehow it has accommodated itself to the infusion of Halley life-forms--is able to save the colony: humans and Halley-forms effectively create a symbiosis; and, by cloning himself, Saul becomes personally immortal. Carl takes over as expedition leader and herds the surviving cometonauts to a mutually acceptable solution (they'll take refuge in the cometary Oort cloud, far from Earth and the sun). And Virginia, dying after one of the battles, transfers her personality into her supercomputer. An uneasy collaboration, then, bulging with ideas but with all the seams and patches showing: promising work that cries out for a thoroughgoing edit and rewrite.