The Pioneer Plaque
The Pioneer 10 and the Pioneer 11 were the first spacecraft to have missions to leave the solar system. Although they are much less famous than the latter Voyager missions these two were, well, the pioneers. In 1972 Carl Sagan got in touch with NASA and convinced them to include plaques displaying humanity in case they were found by aliens, or, more likely, by humans in the far future. An advert or an time capsule. He talked through the ideas with Frank Drake, of the Drake equation, and then had his wife Linda Sagan illustrate it. Here's a representation of the plaque:
Although many of the shapes would hopefully be recognised, one of the problems that both this and the similar Aricibo Message faced was conveying a sense of scale. How can you talk about the length of space or time without referring to something personal to your species like the size of your planet or how long it takes to go around its star? The answer is that you can link it to atoms that you were able to find everywhere. The most abundant element in the universe is Hydrogen, so that was a good place to start.
The picture above shows a hydrogen atom flipping between two hyperfine spin-flip transitions. I don't know what this means, but what matters is that it is something that happens with a constant time period and is very abundant and fundamental across the universe. The period is about 0.704 ns and light with the same frequency was a wavelength of 21.106 cm. There is a little dash under the central line to indicate that this is the unit length and time used for the rest of the plaque. All other dimensions are given as binary representations of these base units. With that established we then proceed to tell them about us:
These humans have a scale beside them in binary indicating a distance of 8 units (8x21.1=169ish cm). These pictures came under a lot of fire for appearing to portray Caucasian humans (they were aiming for an amalgamation of humanity, but things like the afro got cut at the last minute), for having the male as the dominant figure greeting aliens and for drawing the male genitalia in detail while leaving out the details for the female parts. They argued that they were going for a classical statue look (which did the same thing) and that they were afraid that NASA would treat any more detail on a female as obscene. Sigh.
To locate our home system from amongst the other stars in the galaxy any finders of the spacecraft would need to have some recognisable waypoints. Most collections of stars are very indistinct, but there is a space phenomena which is more unique. Pulsars are very rapidly spinning former stars that let out bursts of energy every time they spin. They are fairly rare with distinct spinning frequencies, so a plaque finder should be able to identify them by their requencies alone. Each line in the picture above is a binary representation of the time period of a pulsar and the length of the line indicates how close it is to our solar system. Finally a number at the end of the line gives the distance above or below the plane of the galaxy.
These should mean that even if you are unable to see some of pulsars you should still be able to triangulate to find our location. On top of the redundancy that comes from these 14 pulsars, the final long line to the right indicates the location of the centre of the galaxy (which helps to picture where they all are). One of the lines has a slight error in it. Sorry aliens.
Finally the map of our solar system. We have learned a lot more about it since the 1970s, so having Saturn with rings but not the other gas giants and having Pluto but not Eris or Ceres is admissible. The route of the Pioneer 10 is pictured, with its gravity assist around Mars. The distance from the sun is down as multiples of Mercury's orbit. The use of arrows has been criticised as they are very human centric and come from our hunter gatherer upbringings. Not even all human tribes find arrows as intuitive as I would have assumed.