On March 10, 1972, an Atlas-Centaur rocket blasted off from pad LC-36A at Cape Canaveral, Florida, carrying the unmanned probe Pioneer 10 on humanity’s first mission to the planet Jupiter. Along with cameras and various scientific instruments to study the gas giant planet and its moons, Pioneer 10 also carried a curious object. Bolted to the struts of the probe’s high-gain antenna was a small 9×6-inch aluminium plate bearing the images of a naked man and woman, a diagram of the solar system, and other mysterious symbols. This was the Pioneer Plaque, a message to the stars on behalf of all mankind – our first attempt to communicate with extraterrestrial beings.

The idea of sending a message into the cosmos was first proposed in 1971 by British science journalist Eric Burgess. On a visit to NASA’s Jet Propulsion Laboratory in California, Burgess discovered that on completion of their missions to Jupiter and Saturn, Pioneer 10 and its sister probe Pioneer 11 would fly off into interstellar space, becoming the first manmade objects to leave the solar system. Inspired, Burgess contacted Dr. Carl Sagan of Cornell University – a prominent figure in the Search for Extraterrestrial Intelligence or SETI community – and pitched the idea of attaching a message to the probes in case they were ever discovered by alien beings. Intrigued by the idea, Sagan contacted JPL, who, to his surprise, immediately agreed to the proposal. But there were two conditions. First, the message had to fit onto a 9 x 6 x 0.05 inch gold-anodized aluminium plaque – all that could be accommodated without throwing off the delicate balance of the spacecraft. And second, it had to be ready in three weeks, when the probe was due to launch. Wasting no time, Sagan assembled a team consisting of himself, his wife Linda, and fellow astronomer Frank Drake and set about the daunting task of creating a universal greeting on behalf of all humanity.

The team’s first challenge was how to indicate where the plaque – and the spacecraft it was attached to –  came from. This was accomplished through the use of pulsars, rapidly-spinning neutron stars that emit regular pulses of radio energy. The starburst figure on the left of the plaque is actually a map, each line representing a different pulsar with its frequency and distance from the sun written in binary. While only three pulsars are needed to triangulate the position of the sun, the team included a total of 14 to provide as much data as possible without cluttering the plaque. The use of pulsars also conveniently allowed the launch date of the spacecraft to be estimated. Originally it had been planned to electroplate onto the plaque a sample of uranium, the radioactive decay of which could be used to determine how much time had elapsed since the probe was launched. However, time ran out before this could be done. Instead, as the frequencies of pulsars decay at a measurable rate, comparing the current frequency of a given pulsar with that listed on the plaque would give a good estimate of how long the probe had been travelling. Once an alien had figured out the location of the sun, a diagram of the solar system at the bottom of the plaque would allow them to determine which planet the probe had been launched from.

But all these measurements were useless without a standard unit of time and distance on which to base them. At first the team proposed using the dimensions of the plaque itself as a reference, but it was soon realized that over millions of years of bombardment by cosmic dust this figure would grow increasingly inaccurate. It was thus decided to base the measurements not on a physical object but rather a physical constant: the hydrogen hyperfine transition. When the electron of a Hydrogen atom at its lowest energy state flips from spin-up to spin-down, it emits a photon with a wavelength of 21.106 centimetres and a period of 0.704 nanoseconds. This property, represented by a pair of circular diagrams in the upper left-hand corner of the plaque, provides a standard unit of length and time for the rest of the message.

At this point you are probably wondering how extraterrestrial beings are supposed to figure out any of this. When asked this question, Carl Sagan pointed out that given the vast distances between stars, it was unlikely Pioneer would ever crash-land on or even come anywhere near a planet, meaning only advanced civilizations capable of interstellar travel would be able to recover and examine it.

With the scientific component of the plaque complete, Linda Sagan set about creating an artistic  representation of humanity. Drawing upon ideas of universal proportion pioneered by ancient Greek and Renaissance artists, Sagan drew the figures of a man and a woman with their other hands raised in greeting. In order to represent the diverse peoples of earth, she gave them a mixture of racial features such as almond eyes on the woman and a broad nose and afro haircut on the man. These diagrams would go through several revisions before the plaque was finalized. The figures, originally holding hands, were separated so they would not be interpreted as one large organism, while the woman’s arm was also lowered so as not to give the impression that humans always had their hands raised. Finally,  the man’s hairstyle was changed from an afro to a curly mediterranean style as the former proved hard to depict using engraved lines. Then, with the addition of the outline of the spacecraft for scale, the Pioneer Plaque was at last complete. It was a remarkable achievement: in less than three weeks the trio had succeeded in crafting an elegant and poignant greeting to the cosmos. All that remained was to get NASA’s approval.

But here the project immediately ran into trouble. While they liked the rest of the plaque, NASA was uncomfortable with the inclusion of the two nude figures – especially the inclusion of a single line depicting the woman’s vulva. Though believing these criticisms to be overly prudish, knowing that there was little time left until the launch Sagan and his team caved to NASA’s demands and removed the line, leaving the female figure without genitals save for the outline of the Mons Pubis. The finalized design was engraved by Precision Engravers of San Carlos, California to a depth of 0.015 inches and fixed to the antenna truss of the spacecraft to protect it from erosion by cosmic dust. Pioneer 10 was launched on March 10, 1972, reaching Jupiter in November 1973. Its sister ship, Pioneer 11, was launched on April 6, 1974, reaching Jupiter in December of that year and Saturn in September 1979.

When the Pioneer Plaque was revealed to the American public, it ignited a storm of controversy. Unsurprisingly, much of the criticism centred on the two nude human figures, with many accusing NASA of using taxpayer dollars to launch ‘smut’ into space. Others questioned the plaque’s depiction of humanity, claiming that the figures looked too caucasian to be truly representative of all races. One editorial cartoonist went so far as to give the figures speech bubbles announcing “Hi, we’re from Orange County!” In addition to the erasure of the woman’s genitals, feminists took issue with the figures’ poses, questioning why the male figure was depicted boldly greeting the universe while the woman stood submissively at his side. Finally, some took issue with the purpose of the plaque itself, questioning the wisdom of announcing the earth’s location to potentially hostile alien beings.

One major criticism which had nothing to do with the human figures concerned the one symbol Sagan and his team had taken most for granted: the arrow showing the Pioneer probe leaving the earth. To humans, shaped as we are by our hunter-gatherer pasts, arrows evoke motion and direction, but to an extraterrestrial this symbol might be utterly meaningless.

Yet despite this backlash, NASA was impressed by Sagan’s work on the Pioneer Plaque, and in 1977 they invited him to create another cosmic message to place aboard the Voyager 1 and Voyager 2 planetary probes. This would eventually lead to the creation of the famous Voyager Golden Records – and for more on this please see our video “The Celestial Message in a Bottle.”

On February 25, 1990, Pioneer 11 crossed the orbit of Neptune, becoming the first manmade object to travel beyond the planets. As of this recording, it is currently around 101 astronomical units or 9.3 billion miles away while Pioneer 10  is around 123 au or 11.4 billion miles away from earth. However, the Pioneers are not the most distant manmade objects in the universe, having been overtaken by the much faster Voyager 1 probe in February 1988. It is estimated that in 928,000 years Pioneer 11 will pass within 0.8 lightyears of the star TYC 992-192-1,  while in only 90,000 years Pioneer 10 will pass within 0.75 lightyears of star HIP 117795 – the closest flyby of any of the Pioneer or Voyager probes. By this time humanity may very well be no more, but in the unlikely event that extraterrestrial beings ever stumble across these derelict craft drifting through the void, on board they will find a greeting from an alien race that once lived on a distant planet long ago: a cosmic postcard, bearing a simple message of peace and friendship from a cross the cosmos – wish you were here.

If you liked this article, you might also enjoy our new popular podcast, The BrainFood Show (iTunes, Spotify, Google Play Music, Feed), as well as:

• Voyager’s Golden Record
• That Time the U.S. Military Launched a Half a Billion Needles to Space for Reasons…
• There and Back Again- The Story of Able and Miss Baker… In Space
• Why Do Radio Signals Travel Farther at Night than in the Day?

Bonus Fact

The Pioneer Plaques and Voyager Golden Records were not the only cosmic messages conceived by Carl Sagan and his colleagues. On May 4, 1976, NASA launched the Laser Geometric Environmental Observation Survey – or LAGEOS – satellite into Medium Earth Orbit. This was an entirely passive satellite consisting of a 60-centimetre-diameter aluminium-plated brass sphere covered 426 retro-reflectors, giving it the appearance of a giant disco ball. Pulsed laser beams are fired from the ground and bounce back off the reflectors, allowing the position of the satellites to be determined with extreme precision. This in turn provides a benchmark for geodynamic measurements of the earth, and allows the exact shape and gravity distribution of the planet to be measured.

LAGEOS’ 5,900-kilometer orbit is incredibly stable, meaning that the satellite is not expected to reenter the earth’s atmosphere for 8.4 million years. As such, NASA asked Carl Sagan to design a message for any future humans who might come across it. Sagan’s message consists of an aluminium plaque enclosed within the sphere, engraved with three images of the earth’s continents as they appeared at the time of launch, 268 million years in the past, and 8.4 million years in the future. This would hopefully allow future humans to determine when exactly the satellites were launched. To further aid in the interpretation of the message, the plaque also includes the numbers 1-10 in binary and a diagram of the earth orbiting the sun representing 1 year – the basic unit of time used on the rest of the plaque.

A second, identical LAGEOS satellite was launched in 1992. Both are still in active use to this day, and – given that they are essentially indestructible – are likely to remain so for quite some time to come.

Two years earlier, Sagan’s colleague Frank Drake devised an even more ambitious message to the cosmos. This was intended to celebrate the reopening of the Arecibo Observatory in Puerto Rico – then the world’s largest radio telescope – which had just undergone a major upgrade. The message consisted of 1,679 binary digits to be transmitted by the Arecibo dish at a frequency of 2,380 MHz, the frequency being modulated by 10Hz to indicate ones and zeroes. Drake was counting on any sufficiently advanced alien civilization recognizing 1,679 as being a semiprime number – that is, the product of the prime numbers 73 and 23 – and arranging the binary digits into 73 rows and 23 columns..

The pictorial message that emerges is divided into seven parts. The first is the numbers 1-10 in binary, an easily recognizable pattern intended to help with the interpretation of the rest of the message. Then comes the  atomic numbers of the elements hydrogen, carbon, nitrogen, oxygen, and phosphorus – the basic building blocks of life on earth – followed by the chemical formulas of the nucleotides and sugar phosphate components of DNA. Below this is an image of the DNA double-helix molecule along with the number of nucleotides in the human genome. Next comes a crude figure of a human, along with average human height and the population of earth – around 4.3 billion – at the time of transmission. And finally, closing out the message are a diagram of the solar system with the earth – the origin of the signal – highlighted, and a depiction of the Arecibo dish transmitting the signal along with its diameter.

The Arecibo message was transmitted during the observatory’s dedication ceremony on November 16, 1974, in the direction of the globular star cluster M13, some 25,000 light-years away. Unfortunately, due to the relative motion of the earth, the M13 cluster will no longer be in the same place when the message finally arrives, meaning the signal is unlikely to be picked up anywhere. Not that it was expected to be in the first place; like the Pioneer Plaques and Voyager Golden Records, the Arecibo Message was largely intended as a symbolic gesture on behalf of humanity rather than a serious effort to communicate with extraterrestrial beings.

But if all this seems just a bit familiar to you, almost the exact same process occurs in reverse in Carl Sagan’s 1985 novel Contact and the 1997 film of the same name. In the book and film, aliens from the Vega star system use prime numbers to transmit a message containing diagrams to humanity. But instead of basic scientific information about the aliens’ homeworld, the message contains blueprints for an interstellar transport vehicle. The film also features an appearance by the Arecibo dish, which is perhaps even most famous for its inclusion in the 1995 James Bond movie and 1997 video game Goldeneye.