What is the Arecibo Message?

On November 16th, 1974, a coded radio message was broadcast from the Arecibo Observatory in Puerto Rico. The message contained information on mathematics, humanity, the Solar System, DNA, and the Observatory itself. The destination for this message was Messier 13 (NGC 6205 or “The Great Hercules Cluster”), a globular star cluster located about 25,000 light-years from Earth in the constellation of Hercules.

This historic signal was the Arecibo Message, humanity’s first attempt at Messaging Extraterrestrial Intelligence (METI). Almost fifty years later, the Message remains a focal point in the Search for Extraterrestrial Intelligence (SETI), the ethics of messaging, and why we haven’t heard from any extraterrestrial civilization (the Fermi Paradox). What’s more, a growing movement today would like to see more METI efforts mounted in the future.

Part of what makes the Arecibo Message such a significant event is that it was not only the first METI effort ever but the only one ever made. And like all major historic events, the Arecibo Message has a significant background that must be considered. This includes the earliest radio detection efforts, the emergence of SETI as a scientific discipline, and the researchers and scientists who played a vital role.

The first known attempts to detect extraterrestrial radio signals were conducted in the late 19th and early 20th century by Nikola Tesla and the U.S. Naval Observatory (USNO). However, these efforts aimed to communicate with Mars, which many believed to be inhabited at the time. The efforts were modest and (predictably) unsuccessful but helped pave the way for future surveys aimed at distant stars.

In 1957, Ohio State University built the first dedicated radio observatory with grants from the National Science Foundation (NSF). The resulting Ohio State University Radio Observatory consisted of a flat-plane radio telescope equipped with a parabolic reflector, known as “Big Ear.” Later, the observatory launched the Ohio State University SETI program, the world’s first continuous SETI program, and would also be responsible for detecting the strongest candidate SETI signal to date – the famous “WOW! Signal.”

Drake and Arecibo

Born in Chicago in 1930, Frank Drake demonstrated an aptitude for electronics and chemistry and claimed to have been fascinated by the possibility of extraterrestrial life at the age of eight. After graduating high school, he enrolled at Cornell University on a Navy Reserve Officer Training Corps (NROTC) scholarship, where he began to study astronomy. After briefly serving aboard a U.S. naval destroyer as an electronics officer, he began his graduate studies in astronomy at Harvard University.

In 1951, Drake’s interest in extraterrestrial life was reinforced by a lecture by a Russian-American astronomer Otto Struve, one of the most prolific astronomers of the mid-20th century. Struve was noted for this belief that life and intelligence were widespread in the Universe, which was based partly on his research of slow-rotating stars. Like our Sun, Struve noted that many stars spin slower than predicted by then-current theories of stellar evolution, which he attributed to the presence of planetary systems.

Frank Drake and the Drake Equation. Credit: SETI Institute

After graduating, Drake began his career at the National Radio Astronomy Observatory (NRAO) in Green Bank, West Virginia, and the NASA Jet Propulsion Laboratory. As a researcher, Drake was responsible for much of the pioneering work on pulsars (short for pulsating radio source), a class of highly-magnetized rotating neutron stars. These stars were not officially discovered until 1967 by Irish-British astrophysicist Jocelyn Bell.

In 1960, he spearheaded the first modern SETI experiment using the radio telescope at the National Radio Astronomy Observatory in Green Bank, West Virginia. Known as Project Ozma, this experiment listened to two nearby Sun-like – Tau Ceti and Epsilon Eridani – for radio signals in the 1420 MHz range, which corresponds to the frequency of cold hydrogen gas in interstellar space. While the project failed to detect anything beyond radio static, Ozma paved the way for far more ambitious SETI (and METI) efforts.

Construction on the Arecibo Observatory began in the mid-1950s, and the telescope became operational by 1963. Originally, the facility was commissioned by the U.S. Department of Defense (DoD) and the Advanced Research Project Agency (ARPA) as a dual-use radio telescope. On the one hand, it was meant to detect incoming ballistic missiles as they traveled through the ionosphere while also studying Earth’s ionosphere.

At this point, Drake was a professor at Cornell University and the Director of the National Astronomy and Ionosphere Center (NAIC) – the formal name for the Arecibo facility. During the 1960s, he spearheaded the conversion of the Observatory from a dual-use radio telescope into an astronomical facility dedicated to the search for extraterrestrial intelligence via radio communications.

In 1961, a year after Project Ozma, Drake arranged a summit at the Green Bank facility, where the subject of extraterrestrial intelligence and SETI was discussed for the first time. In preparation, Drake formulated an equation that summarized the challenges of communicating with ETI that would come to bear his name – the Drake Equation. The meeting was attended by many luminaries who would play an important role in SETI research, including Carl Sagan.

The Arecibo radio telescope in Puerto Rico and the Arecibo Message (inset). Credit: Seth Shostak/SETI Institute/AP

The Signal

In the early 1970s, Frank Drake organized the first campaign to compose a message destined for space. The effort would utilize Arecibo’s megawatt transmitter attached to its 305-meter (1000- foot) antenna. The resulting emission was equivalent to a 20-gigawatt omnidirectional broadcast, meaning that it would be detectable any radio antenna in the galaxy (provided it was similar in size to Arecibo).

The purpose of the message was to demonstrate human technological abilities, scientific knowledge, and information about humanity’s location in the galaxy to a possible extraterrestrial intelligence rather than an invitation to converse. Drake composed the message with the assistance of Sagan and other prominent astronomers, which was transmitted on November 16th, 1974, at a frequency of 2380 MHz and an effective bandwidth of 10 Hz towards the globular cluster M13.

This cluster is made up of 300,000 stars and occupies a region of space that measures 145 light-years in diameter. Because of the number of stars and the cluster’s age (11.65 billion years), it was considered a likely place for an extraterrestrial civilization. The total broadcast lasted less than three minutes.


The message consisted of a 1679-binary digit picture (210 bytes), which is the product of two prime numbers, arranged rectangularly into 73 lines of 23 characters per line (also prime numbers). The decision to use prime numbers was deliberate since it would likely make the message easier for an alien civilization to decode. The binary digits (ones and zeroes) were transmitted by frequency-shifting at the rate of 10 bits per second. They conveyed a series of scientific, geographical, biological, and astronomical information in different colors. These included:

  • A counting scheme of 1 to 10 (white)
  • The atomic numbers for hydrogen, carbon, nitrogen, oxygen, and phosphorus, which make up DNA (purple)
  • The chemical formula of the four purines and pyrimidine bases that make up DNA (green)
  • An image of the DNA double helix and ab estimate of the number of nucleotides (blue and white, respectively)
  • A stick-figure of a human being (red) our average dimensions (blue/white), and the human population of Earth (white)
  • A depiction of the Solar System, indicating that the message is coming from the third planet (yellow)
  • A schematic of the Arecibo Observatory and its dimensions (purple/white and blue)

Response Hoax

In 2001, an imprint was found in a crop field near the Chilbolton radio telescope in Hampshire, UK, which portrayed a response to the Arecibo Message. The “Arecibo Answer,” as it was called, was a near-replica of the original and used the same 73 x 23 grid pattern, and most of the chemical data remained the same. However, the section dealing with chemical elements swaps carbon with silicon (implying silicon-based life), and the diagram of DNA was rewritten.

Similarly, the picture of a human stick figure was replaced with a large, bulbous-headed alien. The Arecibo telescope was replaced with a replica of a crop circle that appeared in the same field a year before. The SETI Institute officially stated that the crop formation was a hoax, undoubtedly perpetrated by the same people who had made crop circles in the same area during the previous two years.

METI Controversy

The Arecibo Message remains a matter of some controversy in light of renewed efforts to attempt messaging extraterrestrials. Whereas SETI is generally viewed as a “passive effort,” the act of messaging potential civilizations is referred to as “active SETI.” Russian scientist Alexander Zaitsev coined the term METI in 2006 to establish a proper distinction between the two methods. As he described it in a paper on the subject:

“The science known as SETI deals with searching for messages from aliens. METI science deals with the creation of messages to aliens. Thus, SETI and METI proponents have quite different perspectives. SETI scientists are in a position to address only the local question “does Active SETI make sense?”

In other words, would it be reasonable, for SETI success, to transmit with the object of attracting ETI’s attention? In contrast to Active SETI, METI pursues not a local and lucrative impulse, but a more global and unselfish one – to overcome the Great Silence in the Universe, bringing to our extraterrestrial neighbors the long-expected annunciation ‘You are not alone!’”

Should we be listening or broadcasting in the search for ETI? Credit: UCLA SETI Group/Yuri Beletsky, Carnegie Las Campanas Observatory

Many notable authors, such as David Brin, Stephen Hawking, Seth Shostak, and Milan M. Cirkovic, have expressed criticism for this approach. According to these authors, the very act of announcing humanity’s existence to potential alien civilizations (some of whom may be malevolent) represents an existential risk to humanity. In addition, the SETI Institute has taken a hard line against METI efforts in recent years in response to the emergence of new and privately-funded efforts.

In 2015, in conjunction with other members of the SETI community, SETI@home issued a statement regarding beaming messages into space:

“METI programs carry unknown and potentially enormous implications and consequences. We feel the decision whether or not to transmit must be based upon a worldwide consensus, and not a decision based upon the wishes of a few individuals with access to powerful communications equipment. We strongly encourage vigorous international debate by a broadly representative body prior to engaging further in this activity…

“Intentionally signaling other civilizations in the Milky Way Galaxy raises concerns from all the people of Earth, about both the message and the consequences of contact. A worldwide scientific, political and humanitarian discussion must occur before any message is sent.”


Nevertheless, groups like METI International and others continue to advocate for future follow-up efforts on the Arecibo Message. Rather than attempting these efforts on its own, METI International states that its goals are to “[f]oster multidisciplinary research on the design and transmission of interstellar messages, building a global community of scholars from the natural sciences, social sciences, humanities, and arts.”

In particular, they emphasize the need for public outreach to emphasize the importance of SETI and METI research and promote international cooperation in the field of astrobiology. In short, this organization and others like it are committed to fostering a global consensus among the scientific community that will consider the ethics and possible implications of messaging extraterrestrials before any efforts are made.

Another notable effort is the NAIC’s Arecibo Message Global Challenge (aka. The NEW Arecibo Message), which launched in 2019 to coincide with the 45th anniversary of the iconic Arecibo Message. The challenge is described as an education and public outreach (E/PO) activity that challenges students of all ages to:

“[D]efine an updated message that would represent the most smart and safe way to say hello to our possible galactic neighbors, promoting the peaceful uses of Space and exploring the cutting-edge technology and scientific knowledge available nowadays.”

Fifty years later, the Arecibo Message remains a fixture in the minds of scientists and astrobiologists. Its timing coincided with the Space Age when human efforts to explore the cosmos were mirrored by similar interests in finding extraterrestrial intelligence. As we enter the “New Space Age,” where the interest in space exploration is rapidly growing (thanks to many exciting new missions), there is renewed interest in SETI and METI projects.

As with any major effort, there must be considerable debate before any actions are taken. For instance, do we keep listening, or do we break the “Great Silence” by broadcasting our existence? And if we do decide to message, who will speak for Earth, what will they say, and how would we make sure we’re not putting Earth in danger? These questions will undoubtedly inspire a lot of interest as the 21st century continues.


3 Replies to “What is the Arecibo Message?”

  1. Drakes equation is niave. Written in the optimistic sixties it has no parameter for those intelligent alien civilizations who have decided that it is safer to keep quiet in the universe for fear of invasion. My estimate is about 99.9%
    Me -I’m all for keeping quiet.

    1. Me, I doubt if it makes much of a difference if we keep quiet or not. The Universe is pretty big, and I have no doubt that there are many highly advanced alien civilizations out there. Whether they are truly spacefaring is a different matter. They may ultimately decide that interstellar travel is more trouble than its worth. We may come to the same realization.

    2. Actually, Drake never meant for his equation to be a prediction or an appraisal of finding life. This is what most people misunderstand about it. It was a thought experiment that summarized the challenges of SETI – i.e., we have no idea what the statistical likelihood of intelligent life emerging from basic life is. We also have no idea how long a species could communicate, or even if they’d want to. Also, we can’t possibly estimate how many civilizations will want to remain quiet, that’s just another parameter with a huge degree of uncertainty.

      Still, that would be very useful to include in a revised Drake Equation. There are efforts on to do this, perhaps it should be included. Call it the “Dark Forest” parameter, represented by fq (q for “quiet”).

Comments are closed.