NASA picked Columbus Day of this year to turn on the most advanced radio receivers ever built for SETI--the Search for Extraterrestrial Intelligence. Why radio? Because space travel is very expensive, and the distances between stars are vast. Radio waves travel at the speed of light and are far cheaper than spaceships. So on the 500th anniversary of Columbus’s voyage, NASA is embarking on a radio exploration of the galaxy, looking for faint unnatural signals from the Milky Way.
The scientists involved in the search are pretty sure something is out there. A lot of numbers, some high and some low, are thrown around to express the probability of intelligent life. Here are some figures that are middle-of-the-road: There are an estimated 400 billion stars in the Milky Way. Planets may be fairly common, so you can figure one out of every ten of these stars has planets, which equals 40 billion stars with planets. If every such star has ten planets, that’s 400 billion planets. But how many of these places might be suitable for life? If life elsewhere is similar to our own form of life, then we need a planet that’s not too hot or too cold, with an atmosphere and with water. In our solar system only Earth qualifies, though Mars and Venus come close. So let’s be conservative and estimate that only one planet in each stellar system will do. That’s 40 billion habitable planets.
How many of them actually develop life? That estimate depends on how difficult you think life is to start. We can take a one-in-ten estimate, a fairly high degree of difficulty. That’s 4 billion planets with life.
The key question, though, is how many of them have intelligent life? That’s even tougher to pin a number on. There’s no scientific yardstick. Let’s say it happens one in 100 times. That’s 40 million civilizations. How many of these civilizations are technologically advanced enough to communicate over great distances? If they don’t have radio, SETI won’t find them. But the laws of physics are evidently the same all over the universe, and intelligent creatures will presumably discover some of the same laws we have and build devices, such as radio, to exploit them. Let’s say one civilization in ten does this. That’s 4 million groups we could communicate with.
But it’s an awfully old universe, and how long do these communicative civilizations last? Suppose the average age of a planet is 10 billion years (our young sphere is a bit less than half that) and its civilization is communicative for a thousandth of that time--10 million years. That would mean a thousandth of the advanced civilizations, or 4,000 worlds, could be detectable right now.
To find them, NASA has devised a two-pronged strategy, with efforts divided between its Ames Research Center in northern California and the Jet Propulsion Laboratory in Pasadena. Ames will look at about 1,000 stars similar to our sun--presumably the most hospitable places for planets with life--on 14 million radio channels. Using the giant Arecibo radiotelescope in Puerto Rico, they will focus on one star at a time, to get maximum sensitivity.
The Pasadena group will scan the whole sky, covering the entire galaxy, but with less sensitivity. They’ll use an antenna in Goldstone, California, that is part of the Deep Space Network used for communicating with spacecraft. They’ll start with 2 million channels but eventually work up to 32 million, covering the microwave spectrum from 1,000 to 10,000 megahertz.
The universe is a noisy place, with cosmic rays, quasars, pulsars, and gas clouds all emitting signals for radioastronomers to study. Mother Nature’s signals, however, are smeared out over many kilohertz of frequency. SETI generally assumes that extraterrestrials are transmitting signals much narrower than this, beacons that would stand out distinctly as artificial.
What happens if something is detected? Michael Klein, the program manager for SETI at the Jet Propulsion Laboratory, says the system contains a lot of highly automated filters. There’s lots of checks in there to say: Is this the kind of signal we’re looking for? Is it possibly interference? Is it a satellite? Is it somebody’s microwave oven?
Our own noise is indeed a major headache, a din of radio signals from airplanes, car phones, and military communications, among other sources. JPL has made the most extensive study ever of the radio noise of this planet, to help develop computer programs for automatically throwing out such signals.
Some candidates will still get through this weeding-out process, and then humans come into the act. You get kind of excited, says Klein. But, he continues, you’ve got to make sure that this is the real thing and not be fooled, either by a satellite or by somebody trying to hoax you. The signal will have to be confirmed by another observatory to prove that it is really from a cosmic source.
One day, Klein hopes, they will get a verified signal. And then, he says, NASA headquarters will tell the president.
The NASA system will not be the only one operating, points out Klein. Other advanced SETI systems complementing it will be searching as well, including the Planetary Society’s Project META (the Megachannel Extraterrestrial Assay) at Harvard and the Argentine Institute of Radio Astronomy, and SERENDIP III (the Search for Extraterrestrial Radio Emissions from Nearby Developed Intelligent Populations) of the University of California at Berkeley, operating at Arecibo.
The implications of success are stunning to contemplate. As John Billingham, chief of the Ames SETI office, says, It would change our view of ourselves, our civilization, and our own planet in something like the way that the Copernican revolution of the Renaissance overthrew the view that the Earth was the center of the universe.
