© 1992, Astronomical Society of the Pacific, 390 Ashton Avenue, San Francisco, CA 94112
by Sally Stephens, Astronomical Society of the Pacific
Is anybody out there? On Oct. 9, 1992, the 500th anniversary of Columbus' arrival in North America, NASA will inaugurate a special program designed to answer that age old question. The program will look for messages from any technological civilizations on planets around other stars, the most comprehensive Search for Extra-Terrestrial Intelligence (SETI) ever attempted.
We know that life evolved here on Earth, a product of volcanic gases, organic chemicals brought by impacting comets and meteorites, and naturally occurring chemical reactions. We also know, from our robot spacecraft, that the other planets and satellites in our Solar System are unlikely habitats for complex life forms. Certain conditions apparently must be met, such as the amount of warmth obtained from the parent star, in order for life to begin. If there are planets orbiting other stars, as seems likely (see The Universe in the Classroom, no. 19), perhaps conditions on some of them are suitable for life as we know it to develop. But unless we undertake a search, we may never know if other intelligent life forms exist.
However, it is entirely possible to communicate with other civilizations by using waves that naturally travel through space at the speed of light. Among the many different ways we might try to communicate, radio waves — especially those called microwaves — are the most efficient at carrying messages. They can come through our planet's atmosphere and are less likely to be absorbed by the dust scattered among the stars; thus they can travel farther than other wavelengths. And there is very little background interference for radio waves, either from man-made or galactic sources.
One region of the spectrum, in particular, has attracted a lot of attention: the range of relatively noise-free frequencies (or channels) where hydrogen (H) and a hydroxyl molecule (OH) give off or absorb characteristic radiation. Because these are the ingredients of water, this area in the radio spectrum has become known as "the water hole.'' Scientists speculate that a species for whom water is important — like us — might relish the symbolism of using this quiet region of the radio spectrum to broadcast its message and "meet'' other life forms. It is important to remember that radio communication with another civilization does not necessarily have to begin with two-way conversations. Depending on how far away the other civilization is, it could take tens, hundreds or even thousands of years for radio waves to make the round-trip between question and answer. But if there are communicative civilizations "out there,'' they may already be sending out messages for their own purposes or to inform others of their existence. These are the types of messages SETI programs are designed to find. One interesting speculation your students might enjoy goes like this: If there are more advanced civilizations out there, for them, sending out messages to "beginner civilizations'' like ours might be the sort of interesting "science fair'' project a high school class would undertake.
1. What star does it originate from?
Our Milky Way Galaxy alone contains an estimated 200 billion stars. However not all of these will have an equal likelihood of having an Earth-like planet.
2. What channel or channels are they using?
Here on Earth, when you want to receive a message from your favorite radio station, you tune to the channel (or frequency) they have been assigned in the spectrum. For extraterrestrial messages, we have no idea what channel they might be using or how "wide'' their message might be (how many frequencies they are sending it out on). In other words, are they "narrow"-casting or "broad"-casting?
3. How faint is their message?
We all know that for receiving radio messages on Earth, the power of the sender is often a crucial consideration. On a car trip, the weak radio stations fade out long before the really strong ones. We may similarly miss an extraterrestrial message even while pointing our antenna in the right direction if our receiving equipment is not sensitive enough to pick it up.
4. What method have they used to code information into the radio waves they are sending?
It would be wonderful if we could rely on all alien civilizations knowing Morse code, for example, but this is not very likely. We need to be prepared to examine a variety of ways in which messages might be coded (so that we can recognize a signal when we receive it).
In past searches, astronomers have had to guess at many of these message characteristics, a process that can be compared to poking your hand into the haystack at random, hoping you've guessed right and will feel the elusive needle. The importance of the upcoming NASA search is that it will search a much broader range of possibilities than has ever been attempted.
Some popular accounts of SETI have joked that one reason that extraterrestrials are not visiting us is that they have listened to our radio and television broadcasting, and, so far, see no sign of intelligent life on Earth. To be precise, however, while our neighborhood may appear brighter in radio waves that it would naturally, as a result of our broadcasting technology, it is unlikely that any program content would be decipherable many light years away.
A few (mostly symbolic) messages have also been sent intentionally. In 1974, Frank Drake and his colleagues used the gigantic radio telescope at Arecibo, Puerto Rico to beam an elaborate coded message in the direction of a globular star cluster (a cluster of millions of stars) called M13. However, because of the cluster's great distance, it will take the message 25,000 years to get there. The message, coded in the binary notation of ones and zeros, contained 1679 bits of information (that is,1679 ones and zeros). 1679 is the product of two prime numbers 23 and 73, which should suggest to an alien to break the message up into some combination of those two numbers. If the message is arranged in 73 columns of 23 bits each, no discernible pattern results. But if the message is arranged in 23 columns of 73 bits each, and the zeros and ones are replaced by white squares and black squares, respectively, an interesting pattern emerges (see figure). Coded into this pattern are (from the top down): binary representations of the numbers 1- 10, atomic numbers of the five elements essential to terrestrial life, the chemical formula of the DNA molecule, numbers for the average human height and the world's human population, images of the human form, the solar system (with Earth displaced to indicate it is the planet from which the signal originated), and the transmitting radio telescope, with its diameter indicated. Although it will be tens of thousands of years before the message reaches the target cluster, its transmission did serve to remind us of the kind of information an interstellar message can contain.
A key element of the program will be its ability to search more than ten million channels simultaneously over a broad range of frequency and to use computer software that has only become feasible in the last few years to pinpoint a variety of complex signals that would not be readily apparent to the human eye or ear. Among the signals the program will be able to find are those that "drift'' in frequency; this is important because we expect that we and the senders are each moving in orbit around a star, and that radio signals would thus tend to shift in frequency due to the Doppler effect (which also causes the familiar drop in pitch of a police siren as it comes toward, passes and then begins to move away from you).
The program, called the Microwave Observing Project (MOP), consists of two separate surveys that will run simultaneously. A targeted search will listen for signals from 800 stars like our Sun within 80 light years of Earth at more than a billion separate radio frequencies. Radio telescopes will look at each star for long periods of time, making the targeted search billions of times more comprehensive than any previous attempts. A second "all sky'' survey will search for signals with less sensitivity than the targeted search, but will cover the entire sky. Scientists have tested their new equipment by searching for the faint signals from the Voyager and Pioneer spacecrafts, which, over the past decade or more, visited Jupiter and Saturn and are now billions of kilometers away, heading out of the Solar System. The NASA systems were able to acquire the signals and successfully recognize that the signals were technological, not natural, in origin.
On Oct. 9, 1992, NASA will "turn on'' the two surveys during special ceremonies at the Arecibo (Puerto Rico) radio telescope and the Goldstone (California) antenna of the Deep Space Network. The program is expected to last ten years, at a cost of roughly $10 million per year. That works out to less than a nickel from each American each year. Not a bad investment if it can help us answer one of the most fundamental questions we, as a society, can ask — is there anybody else out there?
It is conceivable that Pioneer 10 could eventually pass near intelligent life forms living in other parts of our galaxy. On the chance that such an encounter could occur, a message has been placed on Pioneer. The message is carefully designed so that aliens might decipher what we have to say, even though they will not speak our language or necessarily use the same units of measurement that we do. In the best case, aliens might even learn from the message where the spacecraft came from, when it was launched, and even a little bit about the creatures who sent it.
Procedure (students can work in teams of 3 to 5):
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