China building world’s biggest radio telescope

The largest and most famous radio telescope in the world – the Arecibo Observatory in Puerto Rico – is about to be upstaged. In a remote part of Guizhou province in southern China, construction has begun on a true behemoth of engineering, the Five-hundred-metre Aperture Spherical radio Telescope (FAST), an instrument that promises to transform radio astronomy.

“FAST is an awesome project,” says Subramaniam Ananthakrishnan of the National Centre for Radio Astrophysics in Pune, India. When completed, its 500-metre diameter single dish will make it the largest and most sensitive radio telescope in the world. What’s more, although FAST’s dish will be fixed in its crater-like setting, a series of large motors will be able to change the shape of its reflective surface, allowing it to scan large swathes of the sky. FAST will be able to peer three times further into the universe than Arecibo. Astronomers expect it to uncover thousands of new galaxies and deep-sky objects up to 7 billion light years away.

Much like the Puerto Rican landscape that is home to Arecibo, Guizhou province is pocked with dramatic karst depressions, sinkholes formed by aeons of water eating away at limestone bedrock. Using a combination of satellite imagery and aerial surveys, astronomers led by Rendong Nan of the National Astronomical Observatories (NAO) in Beijing settled on a remote 800-metre-wide karst ringed by mountains, which is far enough away from population centres to be free of radio frequency interference.

But it was not entirely unpopulated. At the bottom of the karst was a 90-year-old village of about 80 people – all members of the same family – who lived in traditional wooden homes with farm animals housed at ground level and people upstairs. Several of the children would hike out of the karst to go to a nearby school. “They would climb up every day,” says NAO astronomer Di Li. “It’s quite a climb to go over the ridge.”

The villagers were relocated to the nearest town before construction started in March, and it is scheduled to finish by September 2016. Workers will excavate a million cubic metres of soil to get the karst into the hemispherical shape to support the dish antenna, which will be made of 4400 triangular aluminium panels. The panels will be interconnected at nodes, which can be moved up and down via a cable or motor system to change the shape of the dish’s surface.

Though it was inspired by Arecibo, FAST has key differences. The Arecibo dish has a fixed spherical curvature. This means that the incoming radio waves are focused to a line above the dish. Hanging overhead are secondary and tertiary mirrors that focus the line into a point, which can then be processed by instruments. At any given time, only 221 metres of the 305-metre dish are being used to study the sky.

For FAST’s 500-metre-wide dish, similar overhanging mirrors would weigh 10,000 tonnes. So FAST engineers decided to use the dish itself to focus the signal. To do this, a subset of panels on FAST’s surface will shift to form a parabolic mirror 300 metres across – the size of the full Arecibo dish. This smaller dish can be formed anywhere on the larger 500-metre surface, allowing FAST to track objects and study different parts of the sky across a much wider field of view. “This would make FAST one of the most powerful radio astronomy instruments in the world,” says Adrian Tiplady, project scientist for Johannesburg-based SKA South Africa, which is vying for the chance to build another large radio telescope, the Square Kilometre Array.

Hanging above the FAST dish will be a receiver that will collect the focused signal and study 19 regions of the sky simultaneously in different bands of the radio spectrum. Arecibo can only study seven regions at a time so FAST will be able to survey radio sources far faster than any existing radio telescope.

Even though FAST has a slew of science objectives (see boxout “FAST to join search for ET” below), it is the unexpected that excites radio astronomer Seth Shostak, of the SETI Institute in Mountain View, California. “There’s a phenomenon in radio astronomy that’s somewhat singular: namely, that the biggest instrument usually makes a large fraction of the important new discoveries. So FAST is surely going to be making some dramatic finds,” says Shostak. “When it comes to studying the universe at radio wavelengths, bigger truly is better.”

FAST to join search for ET

China’s FAST radio telescope is expected to discover thousands of objects that will help us understand the universe better. Observations of pulsars, remnants of stars gone supernova, will help fine-tune Einstein’s general theory of relativity.

Tens of thousands of new galaxies – invisible to optical telescopes – will come into focus as FAST picks up on faint radio emissions from the neutral hydrogen gas they contain. This will give clues to the nature of dark matter and galactic evolution.

Closer to home, FAST will join the Search for Extraterrestrial Intelligence. It will be able to study 5000 sun-like stars for alien transmissions. “FAST could detect a transmitter like the radar on the Arecibo dish at a distance of more than 1000 light years,” says Seth Shostak of the SETI Institute.

Anil Ananthaswamy, New Scientist

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