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Square eyed? Artist's impression of an array station, part of the Square Kilometer Array. Credit: CSIRO

Small might be beautiful, but to astronomers bigger is often better. This is certainly true in radio astronomy, where international teams have now approved two giant arrays — the Atacama Large Millimeter Array (ALMA) and the Square Kilometer Array (SKA).

ALMA, scheduled to be built by the end of this decade on a high plateau in Chile's Atacama Desert, will have 64 12-metre antennas tuned to millimetre wavelengths, making it the world's largest ground-based telescope. But this will be dwarfed by SKA, a centimetre-wave radio array with a collecting area of a million square metres.

SKA, long a dream of radio astronomers, came a step closer to reality last week. At a meeting of the International Astronomical Union in Manchester, England, representatives from Australia, Canada, China, India, the Netherlands and the United States signed an agreement to bring SKA online by the middle of the next decade. The plan calls for choosing a design by 2005 and starting construction in 2010.

Why the continual push for bigger telescopes? “This project can by justified by the rule of thumb that says you need something ten times bigger to make major discoveries in astronomy,” says Barry Turner of the National Radio Astronomy Observatory in Charlottesville, Virginia. “Three times bigger can get you better data, but ten times bigger might lead to important new findings.”

Many challenges, including fund-raising, lie ahead. “There is no pile of money sitting on the table,” says University of Manchester astronomer Peter Wilkinson, an early champion of SKA. “What we have is a resolve to pool our resources and a consensus that [after ALMA] this is the next big project in radio astronomy.”

The ultimate location of the array is uncertain, with sites in Australia, Chile and China being considered. The success of the project will depend on whether researchers can find ways to reduce the cost by a factor of ten or more compared with conventional approaches, bringing the total to roughly a billion dollars.

There are several potential designs. Backers of the Allen Telescope Array, a one-hectare array named after the Microsoft billionaire Paul Allen who is giving it substantial funding (see Nature 406 , 551; 2000), hope it will be a model for SKA. This telescope, which will link between 500 and 1,000 commercial radio dishes, is being developed by the SETI Institute and the University of California, Berkeley, for operation in 2005.

Australian researchers are investigating the use of ‘refracting’ telescopes called Luneburg lenses that would be deployed in 100 stations, each consisting of 400 lenses. The Canadian proposal is an antenna with a diameter of 200 metres that would reflect radio waves to a receiver suspended from a balloon 500 metres above the surface.

Chinese astronomers are contemplating a network of 30 radio antennas similar to the Arecibo telescope in Puerto Rico, a dish with a 300-metre diameter that sits in a natural depression. Researchers in the Netherlands favour a so-called phased-array approach, consisting of millions of small antennas embedded in flat panels.

Given the technical and logistical uncertainties, Turner, for one, is sceptical of the timetable set for SKA at the Manchester meeting. “For every step of the way, there are so many choices — the size of the dish, the target frequency range — all of which are subjects of sharp debate. They could get it all sorted out by 2005, but I wouldn't count on it.”

But many in the field are confident that SKA will be worth the wait, even if it does not stick precisely to the schedule. For example, scientists are keen to spot irregularities in the distribution of the primordial hydrogen that gave rise to early galaxies.

And the telescope will also aid the search for extraterrestrial intelligence, enabling astronomers to examine more than a million Sun-like stars while listening for ‘leakage’ — random TV and radio signals — as well as messages sent intentionally.