The Pioneer 10 and Pioneer 11 launched March 2, 1972 and April 5, 1973 respectively:
Each spacecraft carried identical plaques:
NASA gave Carl Sagan permission to send a message with the Pioneer spacecraft, so that if the spacecraft were to be intercepted by extraterrestrial life they could learn something about the human race. Together with Frank Drake the plaque designed. Sagan’s wife at the time, Linda Salzman Sagan, did the artwork.
They wanted to tell ET where we were, what we looked like, how tall we were, etc. But how do you tell ET something about you when you haven’t got much in common? They won’t have any common units of measurement. They won’t know how a long a second lasts or how long one metre is.
Well, we both live in the Universe for starters. We can use universal measurements. This is where the two circles in the top left hand corner of the plaque represent. They depict hydrogen (the most common element in the Universe) and a certain electron energy transition. This transition produces an important spectral line in astronomy: the “21 cm hydrogen spectral line“. As the name suggests, the line corresponds to a wavelength of 21 cm and it also represents a frequency of 1420 MHz (which is a unit of time).
[For more information, I've written a few blog posts about the fascinating subject of spectroscopy. We can learn a lot about distant objects using spectroscopy]
Now we have communicated some information about length, we can discuss the pictures of the humans. The dots to the right of the female represent in binary 1000, which is 8. 8 x 21 cm = 168 cm: giving a height of a typical human. Incidentally, Sagan originally wanted the man and woman to be holding hands but then thought that ET would think the picture represented a single creature with two heads!
Moving onto the radial star-like pattern. This is where the information about time is used:
This part of the picture (which was also etched onto the Voyager Golden Record) is a “we are here” map. There are 15 lines in this diagram, with 14 representing pulsars. Pulsars are a type of dead star – they are rapidly rotating neutron stars and were first discovered in 1967 by Jocelyn Bell Burnell. “Jets” of radio radiation are emitted from the magnetic poles of the star. Each time a pole points at Earth, we see a pulse of radio waves. We see some pulsars emit radiation at incredibly regular intervals – so regular they can be used for precision timing. The frequency of the pulses can also be very rapid – the fastest-spinning pulsar currently known spins at about 716 times a second!
Given how bright and useful pulsars are, pulsars are used to show where we are. The lengths of the lines show the relative distances of the pulsars to the Sun. A tick mark at the end of each line shows how far the pulsar is from the plane of the Milky Way. The “notches” on each line are binary numbers giving the frequency of pulses in the units provided by the 21 cm hydrogen line. The non-pulsar line (it extends from the centre of the radial map to behind the human figures) indicates the Sun’s relative distance to the centre of the galaxy.
The bottom of the plaque shows the Solar System with the large Sun on the left:
The sizes of the planets are to scale, but obviously the distances are not. The notches above/below each planet represent in binary the relative distance to the Sun.
Notice the arrow showing the trajectory of the Pioneer spacecraft. An article in Scientific American criticized the use of an arrow as ET may find the arrow symbol meaningless. Also, though Pioneer took the shown trajectory (via Jupiter), Pioneer 11 actually exited the plane of the Solar System via Saturn.
Also notice there are 9 planets: Pluto was still classed as a planet back then!Follow @kashfarooq