The only radio telescope dedicated to looking for extraterrestrial civilizations will be going dark this week unless the institute that runs it can raise millions of dollars.

The Allen Telescope Array is a group of radio telescopes at the Hat Creek Radio Observatory, designed for a project called the search for extra-terrestrial intelligence, or SETI. Unlike any other radio telescopes in the world, the antennas are specifically built to look for alien civilizations and do other kinds of basic research at the same time.

There are 42 antennas in the array, each 6 meters (19.68 feet) in diameter. It began operations in 2007, with $25 million in seed money from Microsoft co-founder Paul Allen (for whom it is named). Since its inception it has been a joint project of the University of California, Berkeley and the SETI Institute, a non-profit that backs the search for alien life.

By building dozens of smaller antennas rather than one big one, costs can be kept down, as cheaper electronics mean that the processing gets less expensive. A group of 10 small antennas that are 6 meters across is as good as a single one of 600-meters, because the signal can be combined to duplicate the larger antenna's sensitivity. The smaller antennas can also be moved far apart, and which increases their combined resolution.

That could come to an end, though, if a lot of money doesn't come through relatively soon. The chief executive officer of the SETI Institute, Tom Pierson, sent out an email appealing to supporters for assistance.

The facility costs about $1.5 million per year to run, said Jill Tarter, director of the SETI Institute's Center for SETI Research. The SETI Institute had planned a two-year, $5 million project that would point the antennas at the worlds the Kepler Space telescope had found, and see if any of the more than 1,200 of them showed signs of artificial signals.

But the budget cuts to the University of California and the National Science Foundation came, and the Allen Array was forced to cut back. The radio telescopes will be kept in a state of hibernation, with parts of the signal processing equipment kept cool (they have to maintain temperatures approaching -324 degrees Fahrenheit to function), waiting to be turned on again.  

The SETI Institute hopes that the public will step in. Tarter noted there is a proposal to use the antennas to help the U.S. Air Force track and map space debris in orbit. The bits of flotsam and jetsam in low Earth orbit can be dangerous to satellites and to people working on the International Space Station. Even a tiny piece of metal or plastic is dangerous when travelling at thousands of kilometers per hour - faster than bullets - and tracking them would be a great benefit to satellite operators as well as a steady source of funds.

The Allen telescopes have been part of several research projects that aren't directly related to listening for extraterrestrials. Among them are measuring the hydrogen content of galaxies, measuring the magnetic field of the Milky Way and detecting star formation. But all this enables the antennas to listen at certain frequencies for an artificial signal.

To find the signal there are two approaches the Allen Array uses. One is to listen to 1,000 stars within a 1,000 light years of earth, at a frequency of 1 GHz to 10 GHz. The other is to listen to all the stars in the plane of our galaxy - 100 billion of them - at 1420 MHz to 1720 MHz. That's the frequency at which hydrogen and hydroxyl emit radio signals. It is also the frequency at which it is easiest to hear anything. Much below that and the noise from the cosmic background radiation, stars, and the galaxy itself combine to make it too loud to hear an alien civilization. Too far above it, and the atmosphere gets in the way.

For now, no new projects will be started. But Tarter is hopeful that something will come through, especially for the graduate students who were doing their research on the Array. A big part of the antenna is training the next generation of radio astronomers, she said.