Published online 20 June 2007 | Nature | doi:10.1038/news070618-11

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Really? A telescope? Made from liquid? On the Moon?

Scientists have found a liquid that they say is perfect for building a telescope on the Moon — once we go back there and build a base, of course.

Can you really make a liquid telescope?

Mirror mirror on the Moon? NASA

Sure. We already have some down here on Earth. These are usually made from liquid mercury, which makes a lovely parabola shape when spun in a container. The quicker the container spins, the deeper the parabola. Liquid mirror telescopes offer a cheaper alternative to glass mirrors because they are easier to make on a large scale, and the surface has fewer imperfections.

"The real advantage of the liquid mirror is its simplicity," says Ermanno Borra from Laval University, Quebec, Canada. "Fundamental forces of nature conspire to give the liquid surface the right shape." The gravity on the Moon is about one-sixth of that on Earth, so he notes that a liquid mirror there could spin more slowly than on Earth to get the right shape.

So are they going to shoot a load of mercury to the Moon?

For the chilly, atmosphere-less Moon, Borra's plan is to instead use a layer of reflective silver on top of a special liquid known as an ionic liquid.

What's that?

An ionic liquid is a salt that exists as liquid at room temperature or lower. Because they are salts, and so made up of positively and negatively charged components, ionic liquids don't evaporate easily—if a negative ion leaves the system, a positive gap is left, which then sucks the negative ion back into itself. This is great for the Moon, where the lack of air makes things more prone to evaporation.

Does it work?

Borra has managed to deposit a layer of silver on top of a millimetre-thick layer of an ionic liquid, creating the mirrored surface he was after. This is the first time an ionic liquid has been used like this, so the work done so far is just a proof of principle.

And a telescope on the Moon — is that realistic?

In theory. NASA's lunar programme isn't science-driven, but it is science enabling, says Ben Bussey, from the Johns Hopkins University Applied Physics Laboratory and part of a team looking for a suitable place for a lunar base. "Scientists are thinking about what science they could do once they get to the Moon," he says. And the liquid mirror telescope is an excellent example of that kind of project, he adds. Plenty of other lunar telescopes have been proposed, but Borra says his could be cheaper and better for some purposes.

What would they use it for?

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A telescope outside the Earth's atmosphere is always going to get a better view of things because it doesn't have to peer through kilometres of gases that make up that atmosphere. A telescope on the Moon would be able to stare long and hard at far-off objects — and maybe get some information about the early Universe.

According to Borra, the ionic liquid/silver mirror will be able to give the high optical quality and the large diameter (they are hoping for between 20 and 100 metres) needed to see these dim, distant objects.

At the moment the telescope would, however, have to point straight up to keep its shape. There are plans in the works to make it tiltable.

Will it happen?

Does Borra think his telescope will be built? "I really do," he says, "but first we have to go back to the Moon."

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