The Pioneer 10 (also known as Pioneer F) spacecraft, launched in 1972 and now on a very long journey towards Taurus, has a plaque mounted on it which is designed to inform alien civilizations about the spacecraft’s origin. The plaque contains a diagram of our solar system, the trajectory of the spacecraft, a drawing of a man and woman, and groups of vertical and horizontal strokes — you guessed it, binary code — that gives information about how to find us:
Disclaimer: I’m a computer scientist, not a physicist or astronomer.
The binary information in the diagram is encoded as groups of vertical strokes (|) and horizontal strokes (—). | means 1, and — means 0. For example, |—|— is 1010. Each group of strokes represents a binary integer, and there are binary integers associated with three entities: pulsars, planets, and people. I’ve annotated the original diagram and highlighted their encodings in gray:
Hydrogen As a Yardstick
The hydrogen atom, indicated by the two circles at the top left of the diagram, serves as a universal reference for time and distance. Specifically, the atom’s hyperfine transition, which is about 1420 MHz, is the base of measure. A frequency of 1420 MHz gives a period of about 7.04 * 10-10 seconds, and a wavelength of about 8.3 inches. These numbers are implied as units in other parts of the diagram.
There are 14 radial lines in the diagram, representing the positions of 14 pulsars relative to the sun. Pulsars emit radio waves at a different frequencies, which helps identify them. The waves emanate across much of the galaxy, so presumably an alien culture could detect them and locate our solar system. Also, since a pulsar’s frequency decreases with time, aliens could use that to determine when the spacecraft was launched.
Each pulsar line is labeled with an encoded binary integer, which is read in the direction of the gray arrows I’ve drawn. Starting at the unlabeled line (appearing at 3:00 in the diagram) and heading clockwise, those numbers and their decimal equivalents are:
|Pulsar Number||In Binary||In Decimal|
These numbers represent the period of the pulsar’s radiation, counted in units of 7.04 * 10-10 seconds. Multiply each integer by 7.04 * 10-10 to get the period.
The binary code for each planet indicates its average distance from the sun. (Pluto is included because it was still considered a planet in 1972.) The distances are relative to Mercury’s distance from the sun, which is given as 10 units. For example, Saturn is 24.7 times farther from the sun than Mercury. Confusingly, the wavelength derived from the hydrogen atom plays no role in these values.
|Planet||In Binary||In Decimal|
The height of the woman in the diagram is encoded by the |— — —, which is 1000, or 8 in decimal (nevermind that it’s written from the bottom up and one of the horizontal strokes has a defect in it). If you multiply this by the reference unit of length, 8.3 inches, you get the height of the woman — about 5 feet, 6 inches.
Will aliens know how to read binary code? Will they know that the binary code represents binary integers? Will they understand that the units of distance and time associated with those integers, in most cases, are derived from properties of the hydrogen atom? Will they make numerous other assumptions required to understand that plaque? Not likely. But the scientists wanted to make an attempt at communication, and having chosen binary is a testament to its simplicity and universality.
- A funny, yet informative, article about the plaque.
- Details on the science of pulsars.
- The Pioneer 11, also known as Pioneer G, was launched in 1973, with the same plaque.