Pioneer plaque

The Pioneer plaques were placed on board of the Pioneer 10 and 11 spacecrafts, sent into space in 1972 and 1973. They are designed to educate aliens about the origins of the spacecrafts.

The top left symbol depicts the spin-flip transition of a hydrogen atom's electron, which occurs with a frequency of about 1420 MHz. Light at this frequency has a wavelength of about 21 cm, which is used as a base unit in the rest of the diagram.

The hand of the male figure is raised as a sign of good will - it also demonstrates the opposable thumb and how humans limbs can be moved. It has been criticized that this gesture could be misinterpreted as aggressive.

The left part shows the location of the sun in relation to the center of the galaxy and 14 pulsars. The long binary numbers represent the pulsars' frequencies, and the tick marks on the lines give the relative distances perpendicular to the galactic plane.

At the bottom, there's a diagram of the Solar System, and an arrow shows there the spacecraft came from. In the background, there's the silhouette of the Pioneer spacecrafts, hinting at the relative size of the humans.