The universe is an awfully big place. How can you best search the huge sky for a radio signal from ET? There are three basic dilemmas:
- How to search such a large area of sky
- Where to look on the radio dial for ET
- How to make the best use of the limited radio-telescope resources available for SETI
Large vs. Small Areas of Sky
Because the sky is so big, there two basic approaches to SETI searches:
- Wide-field search - In this method, you survey large chunks of the sky, one at a time, for signals. A wide-field search allows the entire sky to be searched at a low resolution in a short period of time. However, if a signal is detected, it would be difficult to pinpoint the exact source without a subsequent high-resolution search.
- Targeted search - In this method, you make intensive investigations of a limited number (1,000 to 2,000) of sun-like stars for ET signals. The targeted-search allows for more detailed investigations of small areas that we think might be probable locations of ET, such as stars with planets and conditions favorable for life as we know it. However, this approach ignores large portions of the sky and might yield nothing if the guesswork is wrong.
What's the Frequency?
When you're in an unfamiliar area and want to find a station on your car radio, you have to turn the dial until you pick something up, or press the "search" or "scan" button if your radio has these features. Well, the question is, where might ET broadcast? This is perhaps the biggest challenge for SETI researchers because there are so many frequencies -- "billions and billions," to quote Carl Sagan. The universe is filled with radio noise from naturally occurring phenomena, much like a summer night is filled with the sounds of crickets and other insects. Fortunately, nature does provide a "window" in the radio spectrum where the background noise is low.
In the 1- to 10-gigahertz (GHz) range of frequencies, there is a sharp drop in background noise. In this region, there are two frequencies that are caused by excited atoms or molecules: 1.42 GHz, caused by hydrogen atoms, and 1.65 GHz, caused by hydroxyl ions. Because hydrogen and hydroxyl ions are the components of water, this area has been called the water hole. Many SETI researchers reason that ET would know about this region of frequencies and deliberately broadcast there because of the low noise. So, most SETI search protocols include this area of the spectrum. Although other "magical" frequencies have been proposed, SETI researchers have not reached a consensus on which of these frequencies to search.
Another approach does not limit the search to any one, small range of frequencies, but instead builds large, multichannel-bandwidth signal processors that can scan millions or billions of frequencies simultaneously. Many SETI projects use this approach.
Limited Radio-telescope Resources
The number of radio telescopes in the world is limited, and SETI researchers must compete with other radio astronomers for time on these instruments. There are three possible solutions to this problem:
- Conduct limited observing runs on existing radio telescopes
- Conduct SETI analyses of radio data acquired by other radio astronomers (piggyback or parasite searches)
- Build new radio telescopes that are entirely dedicated to SETI research
Much of SETI research has been done by "renting" time on existing radio telescopes. This is the way it was done in the movie "Contact." In the real world, Project Phoenix (the only targeted SETI search) has rented time on the Parkes radio telescope in Australia, the 140-meter telescope in Green Bank, West Virginia and the Arecibo radio telescope in Puerto Rico. Project Phoenix has a tractor-trailer full of signal-analysis equipment that it attaches to the telescope for the search.
The SERENDIP Project piggybacks an extra receiver onto a radio telescope (Arecibo) that is used by someone else. The SERENDIP researchers then analyze the signals acquired from the target of interest. Project SERENDIP takes advantage of large amounts of telescope time, but its researchers do not have control over which targets are studied and cannot conduct follow-up studies to confirm a possible ET signal.
The Allen Telescope Array is a new radio telescope being built by the SETI Institute. Located northeast of San Francisco, in the "radio quiet area" of the University of California at Berkeley's Hat Creek Observatory, the array will be dedicated entirely to SETI, using hundreds or perhaps thousands of backyard-type satellite dishes to collect radio signals by interferometry (see the section Dishes for the Sky for information on radio telescopes). The Allen Telescope Array is projected to cost about $26-million.
Several SETI projects have been conducted since 1960. Some of the major ones are:
- Project Ozma - The first SETI search, conducted by astronomer Frank Drake in 1960
- Ohio State Big Ear SETI Project - Launched in 1973, detected a brief but unconfirmed signal called the WOW! signal in 1977 and was shut down in 1997 to make way for a golf course
- Project SERENDIP - Launched by the University of California at Berkeley in 1979
- NASA HRMS (High-resolution Microwave Survey) - Launched by NASA in 1982 and discontinued in 1993 when the U.S. Congress cut its funding
- Project META (Mega-channel Extraterrestrial Assay) - Launched at Harvard University in 1985 to search 8.4 million 0.5-Hz channels
- COSETI (Columbus Optical SETI) - Launched in 1990 as the first optical SETI search for laser signals from ET
- Project BETA (Billion-channel Extraterrestrial Assay) - Launched at Harvard University in 1995 to search billions of channels
- Project Phoenix - Launched in 1995, SETI Institute's continuation of the NASA SETI effort
- Project Argus - Launched in 1996, SETI League's all-sky survey project
- Southern SERENDIP - Launched in Australia in 1998, piggyback project to search the southern sky
- SETI@home - Available as of 1999, screensaver program for analyzing SETI data using home computers
For details on these and other SETI projects, see the Links section at the end of the article.