For a century, we’ve been listening for extraterrestrial signals and heard nothing. So, where are they?

On a chilly April morning in 1960, astronomer Frank Drake began using the giant radio telescope at Green Bank, West Virginia, to scan two nearby stars, Tau Ceti and Epsilon Eridani.

Drake billed his efforts as “Project Ozma,” taken from the name of the Queen of Oz—a character in the fantasy novels of Frank Baum. The 29-year-old astronomer settled on the name because Oz was a “land far away populated by strange and exotic beings”—and Drake was hoping to find beings living many light years from Earth.

The more technical name for Drake’s project was the Search for Extraterrestrial Coherent Transmissions—or, as it became known, the Search for Extraterrestrial Intelligence (SETI).

Radio had been invented at the turn of the century, and astronomers such as Drake believed the technology had finally reached the point that it could be used to distinguish alien transmissions from natural electromagnetic background noise.

Drake was not the first person who thought he could detect a message from space. Decades earlier, peculiar signals had been discovered by radio enthusiasts, including Guglielmo Marconi, the inventor of modern radio. In 1921, Marconi informed the world he had detected radio wavelengths that seemed far too long to be natural electromagnetic phenomena. Mars was trying to talk to us, Marconi suggested.

And so began the first coordinated search for radio signals from an extraterrestrial civilization—not at the beginning of SETI in 1960 as many recall, but 100 years ago, in August 1924, when Mars was making its closest approach to Earth in centuries.

The 1924 search was organized by 69-year-old David Todd—often described as one of the unluckiest astronomers of his era. The heavily publicized project involved military radio-telegraph stations around the world, commercial broadcasters in the United States, an early television pioneer, and America’s leading cryptographer. The goal was to detect and decipher transmissions from what many believed was a dying Martian civilization. This fanciful view of Mars had been popularized by Percival Lowell, a Boston businessman who had devoted his fortune to astronomy. Lowell had mapped a vast network of what he thought were canals carved to shunt water from Mars’ polar ice caps to the parched inhabitants of the red desert planet. “This was really the first time people purposefully listened for signs of life” says Samantha Thompson, the Phoebe Waterman Haas Astronomy Curator at the National Air and Space Museum.

Alas, Mars was quiet, as were Drake’s stars and all other planets since then.

Undaunted by the silence, scientists continue to scout the universe for evidence of other civilizations. Researchers are conducting the largest radio search in history while expanding their repertoire of techniques to detect the heat of alien megastructures, telltale chemical traces in planetary atmospheres, and even the nighttime glow of lights from extraterrestrial cities.

At the same time, astronomers are pondering bigger questions: Is anyone out there? If so, why haven’t we heard from them? Is it even possible an intelligent civilization is close enough in both development and distance to make communication feasible?

“Whether we’re alone in the universe is one of the oldest questions in science,” says Adam Frank, an astrophysicist at the University of Rochester and author of The Little Book of Aliens (Harper, 2023).“I can see the Greeks yelling at each other about this 2,500 years ago.”

Astronomers are searching with renewed vigor thanks to advances in technology and the discovery that planets are common throughout our Milky Way galaxy, with potentially billions of them suitable for life. “We’re entering a new golden age in the field,” says Steve Croft, project scientist for Breakthrough Listen, which is using some the world’s largest radio telescopes to scan the stars for signals.

Tuning In to Mars

On the muggy night of August 22, 1924, the Georgetown University observatory in Washington, D.C., was packed. Dozens of guests, including the Japanese chargé d’affaires, gathered to look at Mars, which was passing less than 34.7 million miles from Earth and shining like a brilliant orange beacon—“the opportunity of a couple of lifetimes,” according to the Chicago Tribune. While a quartet played “Song to the Evening Star” and other celestial compositions, guests observed Mars through the observatory’s 12-inch telescope, and David Todd talked about possible life on the planet.

Todd was the retired director of Amherst College Observatory in Massachusetts. He had specialized in studying the sun, journeying around the world to observe total solar eclipses (often unsuccessfully). For a 1907 eclipse, Todd had teamed with Percival Lowell. He accepted Lowell’s belief that Mars was inhabited and that Martians would try to contact Earth during especially close approaches—with 1924 offering the closest in centuries.

Todd persuaded the U.S. Army and U.S. Navy to instruct their radio operators to listen for broadcasts from Mars on August 22–25—“probably the first time the U.S. government got involved in SETI,” says Steven J. Dick, a former NASA chief historian. The U.S. War Department also made its chief cryptographer, William Friedman, available to interpret putative Martian messages.

Todd asked broadcast stations to go quiet for five minutes every hour during the chosen August dates to reduce interference with possible extraterrestrial transmissions. WRC in Washington, D.C., complied, although other stations demurred.

Todd teamed with radio pioneer Charles Jenkins—who had developed a way to transmit pictures via radio—to record radio waves on long strips of photographic paper. Jenkins’ SE-950 receiver had been designed to communicate with aircraft during World War I, says Kristen Gallerneaux. She is curator of communications and information technology at The Henry Ford, a museum in Dearborn, Michigan, which acquired the receiver around 1940. Despite his participation, “Jenkins was skeptical from the get-go,” says Gallerneaux.

So were most astronomers. “Within scientific circles, not many thought life on Mars could be very complicated,” says Matthew Shindell, curator of Earth and planetary sciences at the National Air and Space Museum. Indeed, the newspapers of the day quoted numerous naysayers. “The idea of getting signals from [Mars] is ridiculous,” said the University of Chicago’s Edwin Frost. “Absolute nonsense,” concurred Britain’s Arthur Eddington, one of the world’s leading astronomers.

Todd pressed on, however. Several radio operators reported unusual signals. Technicians at WOR in Newark, New Jersey, who were maintaining an all-night Mars-listening vigil, heard signals that lasted for two hours, but “no known code could be distinguished.”

Jenkins’ recordings showed some squiggles, which “people interpreted in different ways,” says Gallerneaux. “Some said they could see a little face in there. But people were projecting what they expected to see.” If Martians did exist, they didn’t have much to say.

Worlds of Possibilities

During lunch with several fellow physicists in 1950, Nobel laureate Enrico Fermi posed a simple question: If there’s intelligent life elsewhere in the universe, where is it? 

Our home galaxy, the Milky Way, contains hundreds of billions of stars, the oldest of which are almost three times the age of our sun. With billions of stars that are billions of years older than our own, Fermi reasoned, there had been plenty of time for more than one civilization to evolve and establish itself throughout the galaxy, or at least a large swath of it. Instead, we haven’t heard a peep. Known as the Fermi Paradox, it’s vexed searchers ever since.

The silence suggests several possibilities: There’s never been any other intelligent life, civilizations don’t survive long, or it’s impossible for a society to propagate far from its home world. Or perhaps civilizations are common when measured by the great sweep of cosmic time, but it’s unlikely that two would be at the same stage of technological development simultaneously. “On Earth, we’ve only had [radio] technology for the last hundred years or so,” says Shindell. “That’s a very, very small fraction of the history of the universe or the history of this planet. If anybody was watching Earth 500 years ago, they wouldn’t have picked up any signals.”

“It’s incredibly unlikely that, if there’s intelligent life out there, it would be at the same stage of development we are,” says Ross Irwin, chair of the National Air and Space Museum’s Center for Earth and Planetary Studies. “It would either be much farther behind, in which case it wouldn’t be able to communicate with us, or much farther ahead, in which case it wouldn’t be a level playing field.”

Others acknowledge such a possibility, but say it’s still worth tuning in—just in case. Besides, they say the field has barely gotten started. SETI pioneer Jill Tarter “once said that the amount of SETI we’ve done is like dipping a glass into the ocean to find out if it has any fish,” says Jacob Haqq Misra, director of the Blue Marble Space Institute of Science in Seattle. “Since then, we’ve expanded it to maybe a bathtub of ocean, but there’s still a lot of oceans left to sample.”

“My preferred answer to the Fermi paradox is that we just haven’t looked hard enough,” says Breakthrough Listen’s Croft. “We haven’t done a comprehensive job, but today we’re casting a wider net.”

That net includes Breakthrough Listen. Launched in 2016 with a $100 million gift from internet billionaire Yuri Milner, the project utilizes an expanding network of large radio telescopes in the United States, Australia, and elsewhere to scan one million stars in our region of the galaxy. Breakthrough Listen is also monitoring the center of the Milky Way—and about 100 other galaxies. 

Advances in computer technology have enabled astronomers to be more ambitious in their searches. Early SETI efforts required scientists to manually review their observations, usually on paper printouts. Such analyses were difficult enough when SETI projects studied only a few targets at a time, at a limited range of wavelengths. Today, Breakthrough Listen scans thousands of stars at once, at millions of wavelengths, amassing a petabyte of data or more per year, with much larger caches likely in the years ahead. Analyzing the harvested data then requires lots of computing power and sophisticated software.

Yet even mass digital searches can miss some intriguing signals, so researchers are incorporating artificial intelligence techniques, particularly machine learning, to make sure they don’t overlook anything. Such systems can teach themselves to weed out background radio fluff—GPS, smartphones, even the crackle of nearby microwave ovens—to isolate possible extraterrestrial messages. “This will enable us to do bigger and better searches than ever before,” says Croft.

During a test in early 2023, a machine learning system identified three million signals from 820 stars in the Breakthrough Listen database, including many promising ones that conventional software had missed. (Again, though, nothing has been heard in follow-up observations.)

In the years ahead, AI and other types of software will be scanning much more than observations by radio telescopes. Scientists are expanding the search for alien civilizations by using techniques that look for technosignatures—evidence of alien civilizations that don’t manifest as radio waves.

“I think of it as a cosmic stakeout,” says the University of Rochester’s Frank, who is a member of a NASA-funded team that is cataloguing possible technosignatures for current and future searches. “We can watch these planets like detectives, with their cold cups of coffee and their crappy doughnuts.”

The list of possible technosignatures includes the glow of space-based megastructures designed to capture enormous amounts of energy from a star (a few early candidates turned out to be natural phenomena); lasers beamed into space to capture the attention of other civilizations or to propel interstellar solar sails (so far, no flashing); and the nighttime glow of sprawling cities on heavily populated planets.

Perhaps most important, though, researchers are looking for signs of life, including technologically advanced life, in planetary atmospheres. And there are plenty of atmospheres to examine. Astronomers have discovered more than 5,000 confirmed exoplanets, with thousands more candidate worlds awaiting confirmation. “We’re finding planets everywhere,” says Frank. “That’s changed everything.”

Most of the planets were discovered during transits, when they passed in front of their parent star, causing the star to grow slightly fainter. Repeated passages, along with additional measurements, enable astronomers to determine a planet’s size, mass, distance from its star, and other valuable information. From that dossier and knowledge of the star, scientists then determine if a planet resides in the star’s habitable zone, which is the distance from the star that’s most suitable for Earth-like life.

Starlight shining through the atmosphere during a transit can reveal the fingerprints of chemical elements and compounds in the atmosphere—some of which might offer hints of whether the planet is home to an extraterrestrial civilization. For instance, high levels of pollutants such as chlorofluorocarbons, which have damaged Earth’s ozone layer, could reveal the presence of an industrialized society. Such compounds would be difficult to detect with today’s technology—“it’s at the ragged edge of what James Webb Space Telescope could do,” says Frank. However, future space telescopes, such as NASA’s Habitable Worlds Observatory (slated to launch by 2027), would be designed to conduct high-resolution measurements of exoplanet atmospheres.

“The field is really exploding,” says Frank. “We’re on the edge of finding something. In 10, 20, 30 years, we’ll have actual data to address the question of extraterrestrial life. It’s damned exciting, and it’s great to be a part of it.”

Don’t Yell at the Jaguar

Frank Drake might not have been the first to listen for signs of extraterrestrial intelligence, but there’s no doubt he was the first to try to talk to the stars (a field known today as messaging extraterrestrial intelligence, or METI). In November 1974, Drake used the 1,000-foot Arecibo radio telescope in Puerto Rico to beam a pictogram toward Messier 13, a dense, ancient star cluster roughly 25,000 light-years from Earth. (As the radio signal spreads out, it encounters many closer stars along the way.)

The message was intended primarily to publicize upgrades to Arecibo—then the world’s most powerful radio telescope—including a new transmitter that served as a solar system “radar,” enabling astronomers to study the surfaces of nearby planets and passing asteroids.

“They only transmitted the message once, which isn’t something you’d do if you really wanted an answer—you’d transmit it over and over again,” says Rich Isaacman, a member of the team that crafted the message. “It was very much a symbolic gesture.”

The message consisted of 1,679 bits, arranged in 73 lines of 23 characters each. It took just 168 seconds to transmit. It had seven graphical sections, depicting the numbers 1 through 10; a partial description of the structure of DNA; a stick figure of a human being; a diagram of the solar system pinpointing Earth as the origin of the message; and the Arecibo dish.

“The Arecibo message was historically significant, but when I look at it today, I think it’s difficult to understand,” says Misra. “But it’s a good effort at asking how we might communicate with an extraterrestrial society.”

“It’s far-fetched to think that anyone who intercepts the message could decrypt it,” says Croft. “But it’s certainly possible that we could recognize a signal as artificial even if we can’t understand it. It’s like listening to a radio broadcast. You might not understand the language, but you know they’re encoding information, you know their intent.”

Soon after Drake dispatched the message, Carl Sagan, a colleague at Cornell University, tried to demonstrate the feasibility of decoding an interstellar message. As part of a BBC documentary, he summoned Isaacman and five other graduate students on a Friday afternoon and gave them the weekend to decipher a more complicated missive: 19 printed pages containing 29,791 characters.

The students converted the numbers to a two-dimensional diagram that resembled Mickey Mouse. When they tried three dimensions, however, part of the message looked like the structure of a formaldehyde molecule, says Linda French, one of the students and now professor emeritus at Illinois Wesleyan University. 

The students decided the message was instructing them to tune to the wavelength at which formaldehyde emits radio waves to find more extensive broadcasts. “It’s like a blinking sign that says ‘Eat’ but doesn’t give you the menu,” says French.

Despite that success, many scientists say it would be difficult for the intelligence of one world to decode a message crafted by another. “The original Arecibo message was based on 10 digits, but aliens might not have 10 digits—they might have six, for example,” says Jonathan Jiang, a senior research scientist at NASA’s Jet Propulsion Laboratory and leader of a team that’s developing a next-generation message. “For the new message, we tried to think, ‘hey, if we received this, how would we decode it?’”

Jiang says his team would beam the signal to several locations within the galaxy, particularly the “galactic habitable zone”—a wide band outside the core where conditions are most conducive for habitable planets. The team has no plans to transmit the message yet, however, in part because of the debates over who has the right to compose and send it, and the wisdom of announcing our presence to the galaxy at all.

“Most people in SETI today think we should be listening first and maybe transmitting down the line,” says Croft. “If you’re walking through a dark forest and you don’t know if there’s something out there that wants to eat you, then banging pots and pans isn’t smart. A jaguar might already know that you’re there, but you might not want to go up to it and start yelling at it.”

Some argue that there’s a certain “shutting the barn door after the horse is gone” aspect to trying to remain anonymous because Earth has been yelling into the cosmic forest for more than a century. Most of our radio and television transmissions are too weak to be detected at any great distance. Some, however, could carry much farther, such as military radar, broadcasts to spacecraft by NASA’s Deep Space Network, and even the radar beams from Arecibo (the Arecibo telescope experienced a structural collapse in 2020).

Since the Arecibo message, groups of scientists and others have transmitted more than 30 other messages into the void, ranging from mathematical formulas to a Beatles song. There are also a few “messages in a bottle” waiting to be found, such as the Voyager spacecrafts’ Golden Record, a 12-inch gold-plated copper disk containing sounds and images that portray the diversity of life and culture on Earth.

Some scientists have pointed out that the great distances between stars preclude a large-scale alien invasion of Earth. Even talking to another civilization across such immense gulfs would be a challenge, says Frank. “If we send a message to a civilization just a hundred light-years away, it would take a hundred years for the message to get there and another hundred years to get a reply. That’s a pretty boring, slow conversation. What it’s really about now is just finding life and watching it—trying to pull out every bit of information we can.”

So, scientists will keep their eyes and ears on the stars, still trying to figure out if we’re alone in the universe. For now, Shindell remains optimistic: “Just because SETI hasn’t produced any positive results doesn’t mean that no one is out there.” 

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Damond Benningfield is a science writer and audio producer in Austin, Texas. He is a frequent contributor to Air & Space Quarterly.

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This article is from the Summer 2024 issue of Air & Space Quarterly, the National Air and Space Museum's signature magazine that explores topics in aviation and space, from the earliest moments of flight to today. Explore the full issue.

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