Drake, along with Carl Sagan, designed plaques that were carried on the Pioneer 10 and Pioneer 11 spacecraft. The Pioneer plaques depicted symbolic messages for any aliens the spacecraft might encounter as they travel outside our solar system. Drake also worked with Sagan on the Voyager Golden Record. Containing sounds and images of life on Earth, the record was sent on both the Voyager 1 and Voyager 2 spacecraft.

Besides pursuing his interest in alien life, over the course of his career Drake conducted radio studies of the planets in our solar system, discovering the radiation belt of Jupiter and showing that Venus had a very high surface temperature. He also studied pulsars, neutron stars that spin rapidly and thus send out flashes of electromagnetic energy much like a lighthouse beacon. Drake was the director of the Arecibo Radio Telescope Observatory for 12 years, and taught at Cornell University and the University of California, Santa Cruz. Now retired from teaching, he runs the Carl Sagan Center for the Study of Life in the Universe at the SETI Institute.

"There are currently two scientific programs at the SETI Institute," says Drake, "one for radio SETI and one for astrobiology. I tell people that the first program deals with radio and optical searches, and then I take care of the rest of the universe."

Frank Drake sat down with Astrobiology Magazine's Leslie Mullen to discuss sending a message to the aliens from Arecibo, the Drake Equation, and the search for alien life.

Astrobiology Magazine (AM): In 1974 you sent a message from the Arecibo radio telescope into space. What was the message composed of entirely?

Frank Drake (FD): It had a number system. It had a group of five numbers which were the atomic numbers of the five elements in DNA. And then there were actual schematics of the DNA molecule itself, with the bases and the deoxyribose frame. There was a sketch of the solar system. There was a diagram of a telescope focusing rays to a point, with the size of the Arecibo telescope given because that's what sent the message, and that indicated the maximum level of intelligent technology on Earth. There was a sketch of a human being, our population was given, and planet three in the solar system sketch was displaced towards the human to indicate that's where we lived.

AM: Did Carl Sagan collaborate with you on designing the message?

FD: Carl Sagan wasn't part of that, actually. It was constructed by several of the scientists on the staff at Arecibo, but mainly by me. But Carl Sagan did play a role after it was constructed. I took him to lunch one day and presented him with the message already decoded. I asked, "Can you understand this?" This was a test to see if the message was understandable to a very knowledgeable Earth scientist. We eventually learned that nobody could interpret all of the message; each scientist only could interpret the part relevant to their discipline. So Carl got the numbers right and the planetary system right, but he didn't get the DNA chemistry.

AM: The message was sent to the globular cluster Messier 13. Was that to increase the odds of someone receiving the message, because more planets should be in that area of so many stars?

FD: That's right. The message would come to the maximum number of stars.

AM: I've heard that life may be less likely in globular clusters because of the intense gravitational forces and high radiation environment.

FD: That's true, although more important is the lower abundance of the chemical elements of life there. It would have been better to send the message to the galactic center, but the Arecibo telescope can't point towards to the center of the galaxy because of its mechanical limitations. The dish is fixed in the ground, and it can't look south of minus 2 degrees. If we could send the message again we'd send it from a telescope that could point to the center of the galaxy. There's a 100-meter telescope at Green Bank that could do that.

AM: How long will it take the Arecibo message to reach the globular cluster?

FD: 25,000 years. We sent it in 1974, so it's 33 light years out.

AM: Has it reached any stars at all during that time?

FD: No. It'll come to 30 stars along the way, but it hasn't reached any of them yet.

AM: Your "Drake Equation" sets constraints on the possible number of intelligent civilizations in the galaxy. I see the equation as establishing which questions scientists should ask in the search for alien life.

FD: That's a good way to look at it, as a table of contents of what we have to study.

AM: Do you feel much progress has been made in filling in the values of the equation? FD: Oh, enormous progress has been made. Most important was the detection of the extrasolar planetary systems. Before, we had theories but no direct knowledge. There's also been increased understanding on the origins of life, showing it may be an easy process. We've also improved our understanding of what makes a planet habitable, and how many habitable planets there might be.

AM: The extrasolar planets found so far are thought to be not habitable.

FD: But they may be. Determining habitability is complicated. In our own solar system we've found a potentially habitable planet in Jupiter's satellite Europa, which is outside the solar system's habitable zone based on, as we now know, an oversimplified theory. I think the habitable zone actually extends out almost to infinity from a star, because, if a planet has an insulating layer, it can have temperatures suitable for liquid water. In the case of Europa, that insulating layer is ice. On Jupiter there are layers in the atmosphere at room temperature, and also on Saturn, Uranus and Neptune. For those planets, the insulating layer is gas. In the case of Mars, the insulating layer is probably soil. There could well be life deep under the surface. So when there's an insulating layer, a planet can be much farther from the star and still be warm enough.

AM: Do you think a lot of the models searching for life are too limiting?

FD: Yes, very much. Some of the models take no account for the fact that a greenhouse effect can cause the habitable zone to be much farther out from the star. We could put Venus much farther from the Sun, and with its massive atmosphere it would have liquid water on the surface. Of course, there's the habitable zone for any kind of life, and a habitable zone for intelligent creatures. They're probably not the same. Microbial life is going to be everywhere, but intelligent life won't be as widespread.

AM: What do you think of the idea that life, once it does get started on a planet, will inevitably advance to higher complexity, even if it's not in forms we may recognize?

FD: Sure, Darwinian evolution will proceed.

AM: Although we have no evidence for intelligent life in our own solar system other than Earth.

FD: But that's meaningless. Probably every planet can produce more than one intelligent species eventually. But they do it at different rates. So on every suitable planet in very many planetary systems, there may be many intelligent species about to appear, but one is always first. And the first one looks around and says, "We're the only smart ones!" It is the only way it can be, and this is greatly misunderstood. This inevitable situation does not say that a planet can produce only one intelligent species. This fact says nothing about the probability of intelligent life or the possible eventual number of intelligent civilizations. AM: Have you ever felt the need to make any changes to the Drake Equation over the past 47 years?

FD: No. I do get letters all the time suggesting we should add more factors, like the role of politicians. But all of that is a part of the already existing factors, so there's been no need to change it. It's held up well. The numbers may change, but not the equation itself. One rapidly changing factor in the equation is the typical number of planets in the habitable zone. Well, that number is changing all over the place, but that just reflects a healthy march of science towards the real truth.

AM: There are a number of missions in the near future that could have an impact on your equation. Are there any in particular you are looking forward to?

FD: A mission to Europa. I think that's the best chance of finding life elsewhere in the solar system, if they can drill through the ice. But that mission doesn't exist yet.

AM: Is there anything in the science of astrobiology in general that strikes you as being particularly interesting?

FD: Subsurface life on Mars is important to explore, and that's relatively easy. Also, searching for potential life on Titan, but that's more problematic because it's so cold there.

AM: What about in the field of SETI?

FD: The funds are drying up. SETI has always been dependent on private gifts, and those have become very limited. The price of SETI is not a lot, only a few million dollars a year. People ask me, "How long will it take for you to succeed?" I say, "It all depends on how much money people give."

AM: Success depends on more than money, though. Dan Werthimer, who runs the SETI@Home project, says the success of that project is highly dependent on how much computer processing they can get done. They need millions more volunteer computers crunching the numbers, analyzing the data.

FD: Dan's statement is very true. A lot of people think SETI has failed, but we just haven't looked at nearly enough stars or radio frequencies in different ways. The signals may not be there all the time, and we may have to look at the same star in different frequencies over and over in order to be listening at the right time.

AM: Listening at the right time, at the right star that has a planet with life at the same point of evolution as us - the chance of that seems so small. FD: Small, yes, but we live in a galaxy very rich in stars suitable for life. My estimate is we'll find existing intelligent life in one in ten million stars.

AM: Speaking of listening at the right time, what are your thoughts about the "Wow signal" received by the Big Ear telescope at Ohio State University in 1977?

FD: It's an unsolved mystery. It could have been an alien signal, or it could have been a human signal inadvertently picked up, or something else, perhaps an equipment failure. Hundreds of people have looked for that signal over the years, but it's never been repeated.

There have been some other tantalizing candidate signals. The long Harvard search of Horowitz and Sagan observed more than thirty signals that had the earmark of an extraterrestrial signal. The SETI@Home program has observed more than a hundred such signals. Both of these programs are automated, though, so no one was there at the time to do immediate follow-up observations. Researchers later tried to detect these signals, but, as with the Wow signal, they've been unsuccessful. So the origin of these signals is an open question.

Project Phoenix of the SETI Institute also has found many good candidates, but that program could immediately determine the origin of the signal and all of them turned out to be of human origin. It may be that all the potential signals detected so far were generated by humans. But for now they remain a mystery, and that gives hope to those of us who search for alien signals.

Article is courtesy of NASA's Astrobiology Magazine team at Ames Research Center. This article is public domain and available for reprint with appropriate credit.

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