The Pioneer Plaque: Our Calling Card to the Cosmos

In 1972 and 1973, Pioneer 10 and 11, respectively, left planet Earth with one-way tickets out of the Solar System. These two pioneers (heh) explored Jupiter, Saturn, and their associated moons before heading out into the great unknown on an uncharted interstellar voyage. Each of them carried a plaque, dubbed the Pioneer Plaques, and that’s what this story is about.

Eric Burgess, science correspondent for the Christian Science Monitor, recognized that by being the first spacecraft designed to leave our Solar System, it too would be planet Earth’s emissary to the stars. He believed the Pioneers should contain a message from its creators, one that could serve as an introduction and greeting from any being that might make contact with the Pioneers thousands or millions or more years from now. This thought spawned the idea for what became the Pioneer plaques. Burgess approached Carl Sagan, who was at NASA’s Jet Propulsion Laboratory in Pasadena, CA, working in connection with the Mariner 9 program. Sagan was thrilled with the idea and agreed to promote the idea with NASA officials.

Two identical plaques were made–one for Pioneer 10 and one for Pioneer 11. They are 9 inches by 6 inches, .05 inches thick, and constructed of gold-anodized aluminum. They were constructed and engraved by Precision Engravers of California, a company that is still in business today and sells replica plaques. The design itself was created by Carl Sagan and Frank Drake, with the artistic help of Sagan’s then-wife Linda Salzman Sagan. NASA accepted the idea and their design, and received approval to have them flown aboard Pioneer 10 and 11. They would be attached to the craft’s antenna supports, positioned such that they would be protected from erosion caused by interstellar dust.

The design consists of a few different elements symbolizing humanity’s place within the galaxy, and information about our species.

The Pioneer Plaque

Beginning in the top-left is a schematic representing the hyperfine transition of  neutral hydrogen.Hyperfine transition of neutral hydrogen extracted from the Pioneer plaque

Wait! Don’t go! Give me a chance to try and unpack that gobbledygook for you. 

This piece of the plaque is actually kind of important, because it serves as a reference for the other elements of the plaque. For this explanation, consider that the electrons in atoms exist in one of two states: spin up and spin down. Hydrogen was chosen for the diagram due to it being the most abundant element in the Universe as well as one of the simplest, containing a single electron. Basically, the magnetic field of an electron can either be oriented parallel to the magnetic field of the atom’s nucleus, or it can be oriented in the opposite direction. These are the two states I referred to. The diagram shows both of these phases connected by a line that represents the transition–a hyperfine transition I might add–between these two states. When this occurs, a photon is emitted with a specific wavelength of about 21 centimeters and a frequency of 1420 MHz. A being that might one day come into contact with the plaque would hopefully understand the distance and frequency represented, for if they could they would then be able to use it as a reference for the other diagrams on the plaque.

Like, for example, the diagram of us.

Depiction of humans on the Pioneer plaque


Here, the plaque depicts a nude male and female human. To the right of the woman figure are hash marks indicating the top and bottom of her height. Between those marks is the symbol “| – – -“, which is the binary symbol for 8. The woman is 8 tall. 8 what, you’re asking? 8 feet? 8 inches? Remember when we created our scale using the hydrogen transition thingamajig, and came up with 21 centimeters? That’s right, the woman is 8 x 21 cm, which equals 168 cm (just a skosh over 5′ 6”). Make sense?

There have been claims made that the original drawing had the man and woman holding hands, but that a conscious decision was made to separate the two out of concern that an alien gazing upon the plaque would think of the two humans as a single being. There are also rumors that the original design included a more anatomically-correct woman body, but that single extra line needed to be erased to garner top NASA official authorization.

What a wonderful time to have been around JPL for those discussions. There’s a lot we can learn about ourselves within a debate on how to present ourselves to alien beings thousands or millions of years into the future.

Moving on…

Silhouette of the Pioneer spacecraft relative to the size of the humans.Behind us (the humans), there’s a silhouette of the Pioneer spacecraft, showing the relative size of humans to the craft. I guess this is there in case the aliens are too lazy to do the hydrogen transition conversion thing we just talked about.

At the bottom of the plaque, we have a depiction of our solar system and where Pioneer came from. Also, more hash marks. I hope the aliens realize that this time they’re supposed to be multiplying by 1/10th of the distance of Mercury’s orbit from the Sun, and not 21 cm like they were to do with the human models. If not, they’ll have a hard time finding us if they’re looking for tiny planets that have orbits mere hundreds of centimeters from their star. I really hope aliens enjoy puzzles.


The Solar System with the trajectory of the Pioneer spacecraft.


I also hope that by the time they see this part of the plaque that word hasn’t gotten to them about Pluto being downgraded to dwarf planet….

But ours is only one of millions of solar systems within our corner of the galaxy. Providing a map of our solar system won’t help them if they have no way to find it to begin with. That brings us to the next part of the plaque:


This schematic shows the location of Sol (our sun) relative to the center of the Milky Way and 14 pulsars. I’m going to spare you the technical details and give you the bare bones version. The length of the lines indicate the relative distance between the Sun and the various pulsars. The long binary numbers give the periods of the pulsars, basically their signature. One thing worth noting about the periods of the pulsars, is that their frequency will change over time. Knowing this, a being deciphering this part of the plaque would be able to not only figure out where in the galaxy the Pioneers originated from, but also when they left Earth. Depending on where the plaque is encountered, only some of the pulsars might be visible thus the redundancy of including 14. This should be enough to allow for triangulation back to us. There’s a 15th line coming out of the center of the figure (which, if you haven’t guessed already is where the Sun is located); it’s the long one pointing to the right. It shows the relative distance from the Sun to the center of the Milky Way galaxy.

So there you have it. The Pioneer Plaque: a representation of humans and their size, a celestial map to the place and time the craft and its plaque originated from, and a tool to use as a standard unit of measure to decode all of the details.

If only we put so much effort into the selfies we post of ourselves on Facebook.