One of them, New York University physicist Martin Hoffert, has resurrected a notion broached during the first Arab oil crisis: orbiting solar collectors in space, where the sun appears eight times brighter, and beaming it to Earth via microwaves ("probably no stronger than your cell phone's"). In 1978, the concept involved a mirror the size of Manhattan; today the idea is smaller reflectors - - possibly balloons made of shiny Mylar -- strung around the Earth. David Criswell and John Lewis, of the universities of Houston and Arizona, respectively, set their sights higher: on the moon, where reflectors could be made from silicates and metals mined on site, rather than hauled expensively into orbit. The moon might also hold the key to practical, clean nuclear fusion, still elusive on Earth but reportedly more promising if He-3, a helium isotope found on the lunar surface and in the atmospheres of Jupiter and Saturn, is used.

Or, they write, if we can't wean ourselves from coal, then seed our own atmosphere with sulfate particles, which would form an artificial cloud cover to counteract greenhouse warming. Or hang a 2,000-kilometer-wide screen in space, which, like a permanent sunspot, might block enough solar flux to compensate for a doubling of carbon dioxide in the atmosphere. Or try to somehow harness the explosive, fleeting potential energy of antimatter. The idea, Hoffert says, is to imagine everything, however outlandish, in hopes that something proves possible. At Chicago's 1893 World's Columbian Exposition, he notes, technology exhibits for the coming century failed to predict airplanes or television.

Last July, Hoffert and his coauthors gathered in Aspen, Colo., with other scientists to brainstorm. Discussions included a proposal by high-altitude-wind specialist David Shepard for suspending turbines on giant kites at 30,000 feet, where jet-stream power is enormous. UC Irvine physicist and science fiction novelist Gregory Benford had a low-tech, low-cost plan: Instead of using crop wastes for biomass energy, we'd save even more carbon buildup in the atmosphere by simply burying them at sea. Much talk involved revolutionizing the electrical grid, possibly with superconductors, or by connecting the entire world so the off-peak side could power the half in shadow, as Buckminster Fuller once proposed.