SEVENEVES, by Neal Stephenson. William Morrow, 867 pp., $35.

Neal Stephenson's new novel, "Seveneves," is long, though some parts are longer than others. Fans of the late Tom Clancy will understand me immediately -- Stephenson, who shares Clancy's penchant for technical detail, has let that predisposition overwhelm huge chunks of his new book. For its first several hundred pages it is less a novel about mankind's desperate attempt to escape the death of all life on earth in a rain of fire than a manual for how to do same.

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This is admittedly a fascinating idea, filled with obstacles that must be overcome by a clever and representative sampling of humanity. Perhaps the most interesting and sympathetic of Stephenson's characters is Dubois Jerome Xavier Harris, PhD, "Doc Dubois," Doob to his friends, a figure a lot like superstar astrophysicist Neil deGrasse Tyson. He's the one called on to explain to humanity the event that will end life on earth: the explosion of the moon. There's also Julia Bliss Flaherty, J.B.F., informally, the president of the United States and a dead ringer for Hillary Clinton.

Doob, Flaherty and several others hatch a desperate scheme to remove themselves and as many as they can from the planet: the Cloud Ark, a formation of little spaceships that will keep disaster from wiping out humanity over the 5,000 years that the earth's surface is ablaze. Its former residents will have to remain in orbit.

Stephenson, author of "Anathem," "Cryptonomicon" and other titles, offers insights into engineering that are astounding and improbably beautiful; his insights into human nature are less so. The virtuous characters in "Seveneves" tend to get introduced with lists of accomplishments: "Raised in a dodgy part of London, she'd gone to a posh school on scholarship and went on to earn a biology degree at Oxford. She had gone to Harvard for her PhD, working with a project there on de-extinction. . . . She had done TED talks and other public appearances," etc., etc. It's not so much irritating as exhausting, a tic that functions as shorthand for intelligence and capability, as though every character had to pass a job interview before Stephenson could allow them to take part in the novel.

But then Flaherty returns, and she is evil in a very interesting, irrational, human way. The book's final section, set 5,000 years in the future, when mankind is finally returning to earth, falls back into the over-explaining, but the middle section is a fantastic ride, filled with betrayal, reversals, spaceships, robots and a great big comet. Ultimately, "Seveneves" is everything that makes science fiction fun. It's also everything that makes science fiction tedious and insufferable. These qualities are all bound up with the conceptual weightlifting that make Stephenson a worthwhile writer. The book's price isn't just in dollars -- it's in the hours of homework needed grasp the unequaled bigness of his ideas.