When Frank Drake began his astronomy career in the late 1950s, this was an incredibly exciting time for the field. Humanity was beginning to unlock the secrets of the Universe using ever more powerful radio frequency and optical telescopes, including the tantalizing prospect of space-based telescopes. Amidst the ramping up Space Race between the US and USSR, there was an ever-growing excitement about humankind’s future among the stars.

As concrete plans for landings and colonies on the Moon, Venus and Mars were proposed and put into action, it also brought to the forefront many existing and new questions about humanity’s place in the Universe. During Frank Drake’s 92 years on planet Earth – until his passing on September 2nd of this year – he was one of the driving forces behind the search for extraterrestrial intelligence (SETI), along with other legends like Carl Sagan.

Although to the average person the acronym SETI is most likely to bring to mind popcorn movies about little grey – or green – men, Drake’s Project Ozma, as well as the SETI Institution and the ongoing Breakthrough Listen project are just some of the attempts made by Drake and his colleagues over the decades to answer that one question that may affect the very course of humankind’s future: are we alone in the Universe?

Intelligent Life As A Fluke

In a Universe that contains billions upon billions of stars and planets, what is the chance that life will form on any of these planets? Of this life, what percentage will possess a level of intelligence that enables complex societies in which scientific inquiry and technological development can be sustained? Out of these societies, how many will acquire the means to reach out beyond the limits of their planet?

Although the speculation about extraterrestrial life has been around for hundreds, if not thousands of years, it hasn’t been until the development of more advanced means of observation that humanity has gained the ability to put these speculations to the test. As commonplace as we consider lifeforms – whether intelligent or not – to exist within the Earth’s biosphere, we know at this point in time that of all planets and moons in our Solar System, only the Earth is capable of supporting life, never mind an advanced society.

In the 1930s, rocket scientist Konstantin Tsiolkovsky mentioned his doubts about alien intelligent life in an unpublished work, with physicist Enrico Fermi becoming associated in the 1950s with a formal definition of these doubts, commonly referred to as Fermi’s Paradox. Essentially this paradox entails the conflict between the likelihood of a significant number of alien civilizations, and the clear absence of these civilizations.

The search for extraterrestrial intelligence thus seeks to resolve this paradox. Are we wrong about the likelihood of intelligent life forming, or are there other factors that we may be missing? In 1961 Drake would formalize these factors in what is called the Drake Equation, which is:

N = R* · fp · ne · fl · fi · fc · L

Here N is the number of civilizations within our galaxy with whom communication may be possible.

  • R* is the average rate of star formation.
  • fp is the fraction of stars with planets.
  • ne is the fraction of these planets that can support life.
  • f1 is the fraction of planets that actually develop life at some point.
  • fi is is the fraction of planets with life that develop intelligent life (civilizations).
  • fc is the fraction of these civilizations with technology that allows them to be detected through e.g. radio transmissions.
  • L is the length of time during which these detectable signals will be emitted.

Unsurprisingly, the values one can assign these factors will range wildly, making N to be of questionable use, but it is a useful aid in showing the many underlying questions to be answered before the larger question of whether Earth life in general and human civilization in particular is a cosmic fluke, somewhat rare or actually commonplace.

In Carl Sagan’s Cosmos PBS television series – as well as the book with the same title – these same questions are also asked and considered from many angles. In science fiction works such as Star Trek, Babylon 5, etc. the uncomfortable questions are avoided as these feature a Galaxy brimming with thousands upon thousands of civilizations. Based on the available scientific evidence one might ask the question of whether we are perhaps afraid of being all alone in the Galaxy.

What if we do travel out there in faster-than-light spaceships, but find a Galaxy utterly devoid of life and habitable planets?

The Wow! Signal

The Wow! signal represented as "6EQUJ5" with Jerry R. Ehman's handwritten comment.
The Wow! signal represented as “6EQUJ5” with Jerry R. Ehman’s handwritten comment.

Would we recognize another civilization if we came across it? This is one of the questions posed in Carl Sagan’s Cosmos, as he describes a hypothetical scenario in which a probe like the Voyager 1 & 2 approaches Earth, and its operators try to determine whether Earth has an active biosphere, and maybe a civilization. Based on the atmospheric levels of organic molecules like methane and photosynthesis indications the former seems likely, while the Earth’s surface shows signs of structures, but would they be signs of an intelligence?

A significant amount of attention with SETI has been directed towards radio frequency (RF) communication, as RF signals can travel significant distances through space, and are at least for human civilization a common communication method that also liberally gets broadcast into space. If Earth has been lit up like a proverbial RF beacon for about a hundred years, surely this would be the case for other inhabited planets too.

This assumption was one of the reasons why in 1977 a narrowband RF signal received by the Ohio State University’s Big Ear radio telescope got a surge of attention, as it seemed to be the surest sign of extraterrestrial intelligence. This so-called ‘Wow! signal‘ came from the direction of the Sagittarius constellation and lasted beyond the 72 second observation window by Big Ear. Unfortunately no modulation was detected in the 1420 MHz signal, and so far the signal has not been repeated, making it likely that it was an astronomical phenomenon.

On the 35th anniversary of the Wow! signal, the National Geographic Channel sponsored a promotion for one of its shows by transmitting a digital stream encoding thousands of Twitter messages to the presumed origin of the 1977 signal via the Arecibo Observatory’s radio telescope dish. To this day we have either received no response, missed the response, or our message ended up in someone’s spam folder.

A Matter Of Time

What is perhaps one of the most humbling aspects of astronomy is the sheer, mind-boggling scale of the Cosmos. Not just in terms of space and distances, but also in terms of time. Much of the electromagnetic radiation that is now being captured by the newly launched James Webb Space Telescope was sent out by their sources millions to billions of years ago. Our own Milky Way Galaxy is approximately 87,000 light years in diameter, with an estimated 100 – 400 billion stars. The light from the furthest stars in the Milky Way relative to Earth’s position originate from a time when humanity was still living as hunter-gatherers on a wild Earth.

Map of the Milky Way Galaxy with the constellations that cross the galactic plane in each direction and the known most prominent components annotated including main arms, spurs, bar, nucleus/bulge, and notable nebulae. (Credit: Pablo Carlos Budassi)
Map of the Milky Way Galaxy with the constellations that cross the galactic plane in each direction and the known most prominent components annotated including main arms, spurs, bar, nucleus/bulge, and notable nebulae. (Credit: Pablo Carlos Budassi)

This notion is perhaps the most difficult one that Frank Drake and his colleagues had to contest with when it comes to SETI projects. It feels that all we can do is keep listening, even if the likelihood is vanishingly small that there is anything to receive. This did however not prevent the SETI@home project from attracting over a million users who dedicated part of their computer resources to running a distributed super computer that processed data from the Arecibo and Green Bank radio telescopes.

Even though the SETI@home project is currently dormant after no conclusive findings, the Berkeley SETI Research Center behind the project still has other ongoing projects, of which the most notable one is the Breakthrough Listen project. With $100 million in funding, the project began in 2016 and is expected to run for 10 years, providing the most comprehensive search to date using both radio and visible light telescopes.

Differentiating Intelligence

The plaque attached to Pioneer 10.
The plaque attached to Pioneer 10.

The most vexing aspect of SETI projects is that although there are plenty of signals coming in that bear a closer look, who is to say what signal ‘definitely’ comes from an advanced civilization, and which ones are from natural phenomena? The Cosmos is after all a rather noisy place in the electromagnetic spectrum, which significantly increases the burden of proof.

Over the past decades, humankind has sent out messages directed at potential alien civilizations. These have ranged from RF transmissions to physical items, such as the Pioneer plaque attached to the Pioneer 10 and 11 spacecraft that was designed by Drake and Sagan. A few years later the Voyager 1 and 2 spacecraft would be launched, each with a Voyager Golden Record attached to it.

At this point in time, Pioneer 10 and 11, as well as Voyager 1 and 2 have left the reach of Earth’s star system and are travelling through interstellar space. Even though only Voyager 1 and 2 are still actively gathering sensor data and communicating with Earth, the messages these spacecraft carry should last long enough for another civilization to find them and perhaps manage to decode them.

Barring faster-than-light travel or another means of transport which humanity has not yet conceived of, such an event would take place many thousands to millions of years into the future of Earth. Even the radio and television broadcasts we have sent out for decades now will take thousands of years to reach the more distant parts of the Milky Way, and possibly vice versa, making SETI one of the longest endurance games imaginable.

Lightspeed, Mr. Drake

Regardless of what humankind’s future will look like, Frank Drake’s legacy along with that of Carl Sagan and other great minds of the recent past, should endure for many more decades and centuries to come. Perhaps the most impactful aspect of their teachings is that how they taught us to take the time now and then find to ourselves outside at night. To find a spot without any artificial light and to look up, so that we can take in the enormity and beauty of the Milky Way and the countless stars which we can perceive even with the naked eye.

By allowing us to see even just a bit more of this one small Galaxy and to allow our minds to wander on the question what – and who – we may find among all those stars, humanity is better prepared to deal with the challenges and possible discoveries than before, regardless of what the final Drake Equation ends up looking like. May you find peace among the stars, Mr. Drake.


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