LONDON - You don't have to be the Astronomer Royal to understand a few basic rules about building telescopes: They should have a good view of the sky, they should be easily accessible and it's probably best to point them upwards.
Think again. Astronomers are completing plans for a telescope buried under the South Pole. One cubic kilometre large, it will point down through the centre of the Earth toward the northern skies above Europe, Asia, North America and the Arctic.
This is the IceCube, the strangest, most isolated and certainly coldest terrestrial telescope ever planned. The IceCube is a neutrino detector created to seek out ghostly, high-energy sub-atomic particles from the farthest reaches of space. It will be made by drilling 2.5-kilometre-deep holes in the ice with hot water, then dropping in strings of light detectors assembled like pearls on a thread.
These plans were given a boost this month with the publication in Nature of the first results from the IceCube's sister, Amanda, or the Antarctic Muon and Neutrino Detector Array. Those results are being discussed this week at the U.K. National Astronomical Meeting in Cambridge.
Neutrinos are the most elusive particles known. They have almost no mass, no charge and can flit through planets -- or the centre of galaxies -- at the speed of light. They travel the largest imaginable distances unabsorbed by matter or radiation and so are unhindered by stars, planets or magnetic fields.
Most neutrinos detected on Earth come from the atmosphere, where they are generated by cosmic rays hitting air molecules, or from the sun. But high-energy neutrinos are also produced in collisions in some of the most violent places in the universe.
By detecting the particles and following their trails back to their point of origin, physicists hope to get unparalleled insight into colliding black holes, gamma-ray bursts -- the mysterious explosions that light up the cosmos once a day -- the wreckage of exploded stars and the violent cores of distant galaxies.
To investigate neutrinos, physicists need a dense shield that filters out other less persistent particles. In a laboratory, that is usually made from lead. But when it comes to looking at cosmic neutrinos, a far bigger and more sensitive detector is needed. One solution is to use Earth itself as a shield and adapt the properties of the clear ice of the South Pole.
The principle has been shown to work with Amanda, built in the 1990s at the U.S. Amundsen-Scott South Pole Station. It is designed to look down to the sky in the Northern hemisphere using the Earth to block out everything else.
Francis Halzen, a physicist at the University of Wisconsin-Madison who led the study, says: "We have proved the technique. We have a unique probe with a sensitivity well beyond other experiments, and the neutrinos we've seen are of a higher energy than has been seen before."
Amanda is made from 677 glass modules, each the size of a bowling ball, strung out on electrical cables beneath the ice and arranged in a cylinder 500 metres tall and 120 metres in diameter. The array detects the one in 10 billion neutrinos travelling the distance of Earth's diameter that happens to collide with a proton.
The collision produces another type of sub-atomic particle known as a muon. These particles travel close to the speed of light in a vacuum but exceed the speed of light in water or ice. The result is Cherenkov radiation, the optical equivalent of a sonic boom. The muon leaves behind a trail of blue light identical to the path of the neutrino. Amanda's modules, or photomultiplier tubes, work like light bulbs in reverse by capturing these faint streaks and sending information about their direction and energy levels to the surface.
The streaks can also be detected in water and have been picked up by a smaller telescope in the clear seas of Hawaii. Proposals have been made to create another detector in the Mediterranean that will measure neutrinos coming in from the southern sky. Ice scatters light more, but absorbs it less. The ice detectors will gauge energy levels more accurately, while deep-sea detectors will be better at tracking direction.
The team has a sample of a few hundred neutrinos detected by Amanda. "All other properties of these neutrinos agree with their atmospheric origin," says Halzen. "We actually show that no more than 15% of the sample could possibly have a different origin. At present, we are looking for cosmic neutrinos, for instance those from gamma-ray bursts and active galaxies.
The IceCube will be built around Amanda.
The $65-million array will be made from 4,800 optical sensors on 80 strings. Each string will be 125 metres apart and have 60 modules spaced at 16m intervals.
"It is not only bigger; it is a much better instrument than Amanda," says Halzen. "It can identify neutrino flavours, or types, and has far superior resolution."
The uppermost modules will be more than 1 kilometre beneath the surface.
The array should be big enough to pick up astronomical neutrinos moving up through the ice. Those moving down will be mostly muons produced by cosmic rays in the atmosphere. The array will also be able to reveal the neutrino type, or flavour.
The hope is that the IceCube will answer questions about gamma-ray bursts, as well as about neutrinos. Two years ago, a team of 100 Japanese and American physicists studying neutrinos in a vast tank of water inside a mountain found compelling evidence that the particles have mass.
They believe that though, individually, a neutrino's mass is negligible, collectively they may account for a significant fraction of the missing mass of the universe -- the substance known as "dark matter."
The findings came from Super-Kamiokande, a detector made from a 60-million-litre cavity carved beneath mountains in Japan and lined with steel. The tank contained 13,000 light detectors that could spot Cherenkov radiation. The detector showed that muon neutrinos vanished and reappeared as they passed through the Earth.
It seems unlikely that energy and momentum are really disappearing from the universe. A more plausible explanation is that neutrinos are changing as they pass through space into types that cannot be detected. And this, the researchers concluded, was possible only if the neutrino had mass.
The IceCube is still at the planning stage, but could be in place within five or six years. If all goes well, Halzen and his colleagues believe they will soon be embarking on a new era of astronomy.
