Molten Mirrors

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Making a large, perfectly smooth, concave optical surface out of glass is an involved and expensive process. Very tiny flaws in the glass can make a mirror unusable. The containers that hold liquid mirrors, says Borra, don't need precisely smooth surfaces and would be much cheaper to manufacture. Telescopes that rely on liquid mirrors would cost about 100 times less than glass-mirror telescopes of comparable size, says Borra.

"The forces of nature conspire to give the right shape," Borra says of liquid mirrors, which need only be rotated to form a flawless reflective surface. As the mirror spins, centrifugal force and gravity pull the liquid into a smooth parabola. Unlike with a glass mirror, if the liquid is perturbed, it can move right back into shape.

Borra expects that a liquid-mirror telescope would be assembled on the moon by robotics. "A container holding the liquid will be sent to the moon and opened up like an umbrella," he says of an imagined future system. A liquid-mirror telescope could not be put into orbit because gravity is necessary to form the optical surface--and because it would spill.

"There's a tremendous amount of research to be done" to fine-tune the mirror, cautions Robin D. Rogers, a chemistry professor at the University of Alabama. He points out that there are hundreds of other ionic liquids that might have a better set of properties than those used in the Laval mirror.

"It may take 20 years before it's built," Borra says of his telescope. If it does come about, however, such a system could help cosmologists observe faint signals from when the universe was only a billion years old, "at that time when matter first assembled into stars, stars into galaxies," says Borra.