From out of space…a postcard?
This week, begin to decipher the cryptic symbols engraved on the face of the plaque which accompanies Pioneer 10 out of the familiar solar system and into undiscovered space
History and Composition
In 1973, one year after it was launched on February 27, 1972, the Pioneer 10 spacecraft became the first manmade object to leave the Earth’s solar system. The spacecraft was created to study Jupiter where the spacecraft would then use the immense momentum created by the gas giant to propel itself out of the solar system. Pioneer 10 is a small spacecraft, measuring in at 2.9 meters (9.5 feet) tall, only slightly taller than any unfortunate chrononaut that may find it floating in interstellar space, and it is relatively light at 258 kilograms (570 pounds). Since 1972, the spacecraft has trekked faithfully through the empty expanse of space towards the star Aldebaran—the eye of Taurus—about 65 light years away from Earth, powered by four radioisotope thermoelectric generators. While it is traveling around 11.5 km/second (about 25,700 mph) Pioneer 10 will take more than 2 million years to reach the celestial neighbor. Intended as a scientific instrument, it was known during its construction that Pioneer 10 would become the first manmade object to leave the familiar solar system for destinations unknown. In a spark of inspiration, it was agreed that in the unlikely chance it would be discovered by intelligent life, the spacecraft would be equipped with a simple greeting card from humanity. The engraving was designed in 3 short weeks by Carl Sagan, Frank Drake (of Drake Equation fame), and Linda Salzman Sagan. The 6-inch-by-9-inch (15.24 by 22.86 cm) plate is mounted externally on the antenna support structure, behind the ARC plasma experimental package. The Pioneer Plaque is 0.05 inches thick made of 6061 T6 gold-anodized aluminum plate, intended to be durable enough to endure the rough interstellar voyage. The message on the plaque is etched ~10-2 cm deep, so the symbols would remain readable even after the accumulated damage barrage of micrometeorites over the travel distance of ~10 parsecs* (32.6156 light years), and possibly as far as 100 parsecs (326.156 light years)
Hyperfine Transition of Hydrogen
To begin deciphering the Pioneer Plaque, the top left diagram is the most important as it represents the legend and “universal yardstick” that will be used throughout the diagram. Rather than any specific language, the Pioneer Plaque uses mathematics as a universal language. It could be assumed that any technologically-advanced intelligence capable of catching the spacecraft would also be familiar with some common scientific features in space, most importantly, the characteristics of hydrogen, as it is the most abundant element in the universe. The two circles represent the same hydrogen atom in different states. This symbol represents the 21-cm hyperfine transition of neutral atomic hydrogen.
The hyperfine transition in neutral hydrogen atoms occurs when the atoms change energy states and electromagnetic radiation energy is released at a precise wavelength. The transition of these states is represented by the inverted symbols on the top of circles displaying the antiparallel to parallel nuclear and electronic spins. As a wavelength, the change in energy states can represent two potential values: a unit of length (21 cm) or a unit of time (1450 hz). This creates a legend for the hydrogen-based cipher that is used throughout the diagram. The transition between each state is connected by a line in the diagram with the binary digit for 1 below to represent this transition as a binary value. The binary values used throughout the diagram represent either the length of time value of the hydrogen cipher
Binary was chosen as an interstellar language on the plaque since it is the simplest representation of numbers and can survive long periods of time under the predicted erosion caused by micrometeorites in its path along the interstellar journey.
Binary is a base two system, where a value can either be 0 or 1. It is read from left to right, where the position of the value (n) represents the value 2n. The values for each position are multiplied by 2n and finally added together to get the final value. For example, the binary value below is 100111 represents the integer value 39:
Each string of binary values on the diagram can be assumed to be read in either direction, making the true translation slightly obscured at first glance. However, since the string of binary values are displayed in the order of most significant digits, they all start with 1 and end in 1 or 0.
For example, on the far right center of the diagram, next to the women’s figure is the value either 0001 or 1000 in binary. Since the string must start with 1, the most significant digit, the value in binary is ‘1000’ representing the integer value of 8.
Once translated from binary to an integer value, the value is translated to its intended length value by multiplying it by the hyperfine transition of hydrogen value of 21-cm.
8 (in binary) x 21 (cm) = 168 (cm)
The value is intended to represent the average height of women at around 168 cm (or about 5-foot-6 inches)
Solar System Diagram
The bottom diagram represents the Earth’s solar system and is perhaps the most recognizable diagram for any terrestrial travelers. The nine planets (and the Sun) present at the time of the Pioneer’s launch are represented from left to right: the Sun, Mercury, Venus, the weary and familiar sight of Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.
While each planet’s size and distance between each other is not to scale, they are accompanied by additional binary values that represent their relative distance from the Sun. These binary symbols are different from the binary values used throughout the plaque since they do not use the hydrogen cipher. This difference is indicated by the binary values being displayed as serif letters instead of sans-serif. After it has been translated from binary, the values represent a multiple of 1/10th the semimajor axis of the orbit of Mercury or 0.0387 A.U:
integer value from binary X 0.0387 (AU) = orbital distance from the Sun
These values on the diagram would be difficult to resolve without knowing the orbit of Mercury. However, even if the distances of the solar system’s planet cannot be deciphered by an alien life, the representation of the rings of Saturn should help to distinguish the Earth’s solar system from the remaining thousands of the nearest stars that the pulsars could indicate. In addition, the small schematic of the Pioneer spacecraft indicates which planet in the diagram the craft was launched from as well the path it took around Jupiter and out of the solar system. The antenna is also positioned to point back at Earth to further clarify its point of origin.
With the combination of the 10 visible fingers and toes on each human and the use of the binary value of 10 to represent the orbit of Mercury it is also possible that this could be correctly interpreted as an indication of the Earth’s development of a base-10 counting system
How to Read a Pulsar Map with the Galactic Center
The most prominent and cryptic symbols of the plaque are the radial display on the left. This represents a display of stellar landmarks that give the position and frequency of 14 pulsars relative to the center of the map’s home star, the Earth’s Sun. Pulsars are rotating neutron stars that emit strong directional jets of high energy particles. Since the stream of electromagnetic radiation can only be observed when it is pointed in the direction of the observer, as the star rotates it acts much like a cosmic lighthouse. Pulsars can be identified by their unique and predictable rotational period.
Frank Drake identified the 14 pulsars with short periods of rotation and the greatest longevity and luminosity to make them easy to identify even after long stretches of time to serve as the prominent landmarks. The pulsars are displayed on the plaque as an axial radial map with the Earth’s Sun at the center, connected by 15 solid lines that end in tick marks. Each of the 14 radial lines represent a unique pulsar’s relative distance from the Sun. The 15th line is the longest horizontal line and runs behind the diagrams of the man and woman. This line has no binary representation and represents the galactic plane and the distance the Sun sits from the center of the galaxy. This is a reference line for all the other pulsar’s distances. The length of a pulsar’s line can be compared to the length of the 15th line to gauge their relative distances compared to the distance between the Sun and the center of the galaxy.
If the Pioneer craft is intercepted within the next few tens of millions of years—even after it has travelled hundreds of parsecs—it is likely that all 14 pulsars will still be detectable. Even if they are not all as visible, only a few pulsars are needed to triangulate the Pioneer’s point of origin. In addition, two of the Pulsars (0950 and 1929) are particularly close to the Earth’s sun, allowing the position of the Earth to be approximated to within 1 in 103 stars.
The characteristic frequency of each pulsar is represented in binary along each axial line, however since the hydrogen cipher can represent both a distance and a time, there is no direct indication which is used for the pulsars. However, since the binary values are exceptionally large, they can be assumed to be a time, rather than distance, although a smart traveler (with ample time) could try both.
integer value from binary x 1450 (hz) = Period (in units of hyperfine transition of Hydrogen)
Along the axial spokes, the string of binary numbers represent the unique frequency of the pulsar at the time of the Pioneer launch. Pulsars radiate at a regular frequency emitting electromagnetic radiation at predictable intervals and over time that frequency decreases, allowing the exact period of the pulsar to serve as a timestamp. The pulsars allow an intelligent civilization with historical records of pulsars to determine the time elapsed since the Pioneer was launched, acting as a landmark of time and space.
The tick marks near the end of each axial spoke line represent the z-coordinate relative to the galactic plane. They are also used to determine the distance from the galactic center for each pulsar. The angle between the line representing the galactic plane and the tick mark of a pulsar represents the direction angle from the galactic center.
The distance each pulsar is shown on the pulsar map as a relative distance. The distance for each pulsar is from the center of the map to the tick mark on the line for each pulsar. The length of the 15th line for the galactic plane can be labelled as 100, so that when the length is compared to the distances for each pulsar they lie between 1 and 100.
While the map is a flat two-dimensional object, it can be extrapolated based on the polar coordinates (r, θ) of each pulsar. As a coordinate, r is the distance a pulsar is from the Sun and θ is the angle between the galactic plane and the tick mark on the end of the axial spoke. Pulsars above the horizontal line of the galactic plane are stars found above the Sun’s relative position in the galaxy (+) and those beneath it sit below the Sun along the galactic disc (-)
The plaque includes an artistic diagram of two humans to represent the starry-eyed creatures that created and launched the plaque and spacecraft. The figures, a man and a woman, were intended as a universal greeting and representatives of all mankind.
Any meaning from the stances or features of the figures would likely be lost on any alien life, but among humans the man’s upraised hand is considered a universal symbol of goodwill and was included for want of a better symbol. However, it has the additional advantage of clearly displaying the human hand and the opposable thumb. With the ten visible fingers and toes, an optimistic writer could hope that this could potentially be a clue to humanity’s arboreal lineage and base-10 counting system. The woman stands in a different, more relaxed pose than the man, with her body weight shifted to display the flexible and mobile nature of the human body as well as the diversity among different human body types. Originally they were envisioned to be holding hands in a gesture of goodwill, but were eventually decided to be separated to avoid accidentally implying that they are a single organism. Both figures are positioned in front of a large schematic of the Pioneer spacecraft to put them in scale with the spacecraft. Whether or not the Pioneer Plaque will be discovered by alien life or curious and lost chrononauts, the plaque serves as a message to the universe from a time when humanity had first begun to take bold strides into the stars and beyond
Pioneer 10 was sent with an intergalactic return address, but no stamps