This article is from the In-Depth Report The 40th Anniversary of Apollo 8's Journey to the Moon

Mirror, Mirror, on the Moon...

Small dish of silver-coated liquid may be first step to giant liquid telescope on the moon
liquid mirror

It sounds like the premise for a Jules Verne story: a football field–wide pool of silvery liquid on the moon that gathers faint light from the most ancient galaxies in the universe. Although still years from constructing such a thing, researchers say they have taken the first step toward a lunar looking glass by laying a smooth coating of silver on a small dish of so-called ionic liquid. Similar to a liquid version of table salt, ionic liquids do not evaporate, and they remain liquid at frosty temperatures such as those found on the moon.

Add a silver lining and the coated liquid becomes highly reflective too—fulfilling the criteria for a lunar liquid mirror telescope, says astrophysicist Ermanno Borra of Laval University in Quebec. The silvered fluid was 80 percent reflective—shy of the 95 percent or more needed for a lunar telescope—and freezes well above lunar temperatures of 130 kelvins (–225 degrees Fahrenheit). However, Borra, a longtime advocate of liquid mirrors, says these hitches are minor compared with applying the silver at all. "It was very far from obvious it could be done," he reflects. "It sounds kind of crazy."

The idea of a moon-based liquid telescope is controversial, but it could help astronomers solve the central problem in constructing more powerful telescopes—the need for larger focusing mirrors. The Hubble Space Telescope has a 2.4-meter- (94-inch-) wide mirror, and its proposed successor, the James Webb Space Telescope, would have a 6.5-meter mirror. The largest mirrors, which reside in the twin telescopes of Hawaii's Keck Observatory, each measure 10 meters across.

But astronomers would need a mirror up to 100 meters wide, or the length of a football field, to observe galaxies born nearly 13 billion years ago, when the universe was only one billion years old. Getting such an object into space would be a monumental undertaking, Borra says. "With a liquid mirror," he notes, "basically you carry it in a jug." Spinning the liquid in a large dish causes it to curve like a crater and focus light to a sharp image.

Researchers have built a few liquid telescopes on Earth from mercury [see image above], but they absorb too much light to focus faint images of ancient objects, Borra says. Looking for a shinier version, he and colleagues let silver vapor collect on a dish of ionic liquid, forming a 30-nanometer-thick layer. Although the liquid freezes when chilled below 175 kelvins (–144 degrees F), there are many other ionic liquids to try, the group reports in a paper published online today in Nature.

Researchers are still decades from being able to build a lunar liquid telescope, let alone get it into space, but the concept already has strong detractors who would prefer to see future telescopes remain in space, says astrophysicist Arlin Crotts of Columbia University, who has worked on liquid telescopes.

Unlike regular telescopes, the liquid kind only points in one direction. A lunar version would also have to contend with dust speckling its surface and the moon's interfering glow, Crotts says. "Some people think it's like you [would] spend all this money to go out [to the moon] and then you halfway ruin the thing," he says.

Crotts agrees the idea is risky but says it is worth pursuing. Given the faint images it could detect, he says, "it would be revolutionary."

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