Every Thursday evening during the fall of 1979, I'd cycle across the Charles River from Boston's North End to the Museum of Science for a series of free lectures. Carl Sagan, Lynn Margulis and Philip Morrison entertained and inspired, but none more so than Frank Drake. He's best known for the "Drake Equation," a back-of-the-envelope attempt at estimating the number of extraterrestrial civilizations in our galaxy with whom we might communicate. He came up with the idea of such an estimate shortly after making the first Search for Extraterrestrial Intelligence (SETI) scan of two comparatively nearby stars with a radio telescope in West Virginia in 1960, dubbed Project Ozma (after Princess Ozma, ruler of Frank Baum's land of Oz). As I told him many years later — he kindly wrote the forward to my first published book, The Wrong-Way Comet — his talk that evening hooked me on science in general and astronomy in particular.

In the 40 years since, radio astronomers have made ever more sensitive and extensive searches for alien signals, with, of course, zero results. Researchers have, however, improved the estimates of several parameters of Drake's original equation. One of these is the percentage of stars that have planets, while another is the average number of such planets that can potentially support life. Both of which are looking pretty good in the light of findings over the last 30 years.

For example, we now know that the molecular clouds in which solar systems form, along with ancient comets and meteoroids, are rife with organic precursors of life, including polycyclic aromatic hydrocarbons (PAHs), amino acids and polyphosphates. Buoyed by the finding that life probably began here on Earth right after our planet had sufficiently cooled some 4.5 billion years ago, most researchers are now comfortable with the idea that life will start in virtually any suitable environment. And since we've recently discovered that solar systems are the norm around stable, long-lived stars, we can be fairly confident that life is common throughout the Milky Way.

Life, yes, but intelligent life? Life smart enough, and curious enough, to build radio telescopes? Or with the chutzpah to beam out microwave or laser signals announcing "We are here!" to their neighbors? That's something else. Based on Earth's evolutionary history, it seems unlikely that intelligence is inevitable once life gets started. Assume, for instance, that something like mammals, with our complicated nervous systems, are needed for cosmic-messaging intelligence. For virtually all the 200-odd million years they've been around, mammals have done just fine without developing technology. Our humanoid ancestors only figured out stone tool use a couple million years ago (a blink in cosmic time), metalworking much more recently, and the ability to send and receive interstellar signals just a generation or two back. While life may be common, technologically intelligent life may well be a rare accident.

And — to really put a damper on all this speculation — how long will we be able to continue to communicate until we do ourselves in? Drake's final parameter is L, the average length of time that a civilization releases detectable signals into space. For what it's worth, and based on the duration of 60 historical civilizations here on Earth, skeptic Michael Shermer estimates L as 420 years. (Although others have argued that if we can get over our current self-destructive hump, our civilization will be essentially immortal.)

The real objection to any form of communication with aliens, however, is the fact of the sheer distances involved, which translate into wait-times between messages. Even if civilizations are commonplace in the galaxy, we can reasonably assume that centuries would pass between sending a message and receiving a reply. As SETI pioneer Philip Morrison observed, contacting aliens might be like communicating with the ancient Greeks.

Bottom line: ETs are probably out there but we'll never know for sure.

Barry Evans (barryevans9@yahoo.com) believes that our future on Mars and beyond hinges on bioengineering ourselves.