25 Jun 2007
A team of Canadian and American scientists has made a liquid mirror that could one day be used to build a large moon-based telescope.
The research team, led by Ermanno Borra of the University Laval in Quebec, Canada, fabricated the mirror from ionic liquids, a class of materials that are non-volatile and exist as liquids at room temperature, which are then coated with chromium and silver ions.
According to Borra, this is the first time that anyone has tried to deposit metals on a liquid. "The key here was to use a hydrophilic ionic liquid and coat tiny chromium and silver nanoparticles on its surface," said Borra, a pioneer in the development of liquid mirrors. The resultant mirror had a reflectivity of about 91%, a low freezing point of –98 °C, and it also remained stable throughout the six months of study.
Liquid mirrors have already been used in ground-based telescopes, since they are extremely light, cost much less than conventional glass mirrors, and can be transported over long distances without the risk of breakage. Here on earth, liquid-mirror telescopes exploit mercury, which is placed in a circular container that rotates at a constant speed to cause the liquid to settle into a parabolic surface.
However, the imaging ability of ground-based telescopes is limited by interference from atmospheric air molecules and man-made light sources. In contrast, a moon-based telescope would be capable of detecting light from the earliest stars in the universe.
For a liquid mirror to be used on the Moon, the base material must remain liquid at ultracold temperatures (up to –150 °C) and have a very high reflectivity. Mercury fails on both counts because it has a freezing point of –38 °C and a reflectivity of about 80%.
Borra and his team want to see a telescope the size of a football field on the Moon's surface. "It could revolutionize astronomy," he said. And with the US government working on plans to set up a manned base on the moon, ideas like this are getting increasing attention from space agencies worldwide. Even a 20 m telescope would be able to image objects at least 100 times fainter than those resolvable by the James Webb Space Telescope, a $3.5 bn replacement for the Hubble telescope that is slated for launch in 2013.
While this particular ionic liquid mirror is not yet suitable for a future lunar telescope, it has shown the way forward. "We have merely scratched the surface," said Borra. With more than a million pure ionic liquids and trillions of ternary variants, research is only just beginning to discover which chemicals could prove to be ideal candidates for telescopes on the Moon.
The biggest drawback of liquid mirrors is the requirement to be kept horizontal all of the time, making them incapable of tracking an object across the sky. But this is not a concern for cosmologists, who look at a fixed region of the sky for light from the earliest galaxies.