1. Pioneer 10 and Pioneer 11 ↑
Pioneer 10 became the first spacecraft to travel through the asteroid belt in 1972, and the first to observe Jupiter in 1973[1b]. Pioneer 10 is currently about 116 AU away[2a] (one AU is the distance between the Earth and the Sun). This is over three times the distance to Neptune, and well beyond the Kuiper Belt and scattered disc. It will reach the Oort Cloud in the next few hundred years, although NASA lost contact with it in 2003.
Pioneer 11 reached Jupiter in 1974 and became the first probe to reach Saturn, five years later[4a]. Pioneer 11 is currently about 95 AU away, also beyond the Kuiper Belt[2b]. Pioneer 11 is moving in the opposite direction to Pioneer 10, towards the centre of the Galaxy, although NASA lost contact with it in 1995[4b].
1.1 The Pioneer plaque ↑
British journalist Eric Burgess was the first to suggest that the Pioneer probes should contain a message from Earth, and he approached American astronomer Carl Sagan with the idea. Sagan was already involved in the SETI (Search for Extra Terrestrial Intelligence) program, which actively searches for signs of extraterrestrial intelligence.
Pioneer 10. Image credit: NASA on The Commons/Public domain.
The first SETI meeting was held at the Green Bank radio observatory in 1961, and it was here that American astronomer Frank Drake first discussed the Drake equation. The Drake equation states that the number of extraterrestrial civilizations in our galaxy that we may be able to communicate with, N, can be calculated as follows:
N = R* × fp × ne × fl × fi × fc × L, where
R* = the average rate of star formation per year in our galaxy,
fp = the fraction of those stars that have planets,
ne = the average number of planets that can potentially support life, per star that has planets,
fl = the fraction of the above that go on to develop life,
fi = the fraction of the above that go on to develop intelligent life,
fc = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space,
L = the length of time such civilizations release detectable signals into space.
Although we can only guess the answers to the last four factors, in 1961 only the average rate of star formation rate in our galaxy could be calculated with any degree of accuracy.
Sagan liked Burgess' idea, and took it to NASA, who gave him three weeks to design the message. Sagan enlisted the help of Drake, and they designed a picture to be engraved on two 23 cm wide aluminium plaques, with Linda Salzman Sagan providing the artwork[8a].
The Pioneer plaque. Image credit: NASA Ames/Public domain.
The two circles at the top of the plaque represent a hydrogen atom undergoing a change in the spin state of its electron. This produces a photon with a wavelength of 21 cm and a frequency of 1420 MHz, and these units are used to provide a scale[8b].
The right side of the plaque contains the image of a naked man and woman in front of one of the Pioneer probes. They are all drawn to the same scale, so that the size of the people can be deduced by measuring the probe. The binary representation of the number eight is given on the far right, this, multiplied by 21 cm, gives the height of the woman. The man's hand is raised as if to say 'hello', although this would probably not be understood by aliens, it does illustrate our opposable thumbs[8c].
These images drew a number of complaints. Some argued that the figures were not racially ambiguous enough. Some did not think people should be depicted naked, but others did not think the images were explicit enough, showing male but not female genitals. It's rumoured that the image was censored for NASA's approval but Sagan stated that this was a stylistic decision, and that the people are modelled on Greek sculptures.
The shape on the left shows fifteen lines originating from the same place, which represents the Sun. The line to the right, which extends behind the human figures, represents the Sun's distance to the centre of the Galaxy, and the fourteen other lines represent pulsars, which are used as reference markers. The lines represent the positions of the pulsars relative to the Sun, and the lengths of the lines represent their distance[8d].
The marks on the lines give the periods of the pulsars in binary code, using the frequency of the hydrogen spin change as a reference unit. These periods change at a predictable rate over time, and so this information could be used to determine where, and when, the probe was launched[8e].
The bottom of the image shows the trajectory of the Pioneer 10 spacecraft through the Solar System. The distance of the planets from the Sun are given in binary next to each planet, in multiples of Mercury's orbital distance[8f]. Pluto is depicted as it was considered a planet until 2006.
2. Voyager 1 and Voyager 2 ↑
The Voyager 1 and Voyager 2 probes were launched in 1977 in order to study Jupiter and Saturn, although Voyager 2 also travelled past Uranus in 1986, and Neptune in 1989. The famous 'pale blue dot' image was taken by Voyager 1 in 1990. This shows the Earth from beyond the orbit of Neptune, at about 40 AU.
'Pale blue dot', image taken by Voyager 1 at over 40 AU. Earth is the dot in the middle of the orange streak (rollover to view). Image credit: NASA/JPL/Public domain.
Both probes are expected to keep transmitting signals back to Earth until at least 2025. Voyager 1 is currently the furthest human-made object from Earth, having reached the edge of the solar wind, and therefore entered interstellar space, in 2013. Voyager 1 is currently about 134 AU away, and Voyager 2 is currently about 111 AU away[2c]. The Voyager probes are both travelling in the general direction of Pioneer 11.
2.1 The Voyager Golden Record ↑
Each of the Voyager probes contains a gold-plated copper record. The cover contains an isotope of uranium, which could be used to work out when the probe was sent, and it's etched with the image of the hydrogen atom and pulsar map used in the Pioneer probes[15a].
The top left of the cover shows the image of a record player with the speed to turn it given in binary, using the hydrogen atom for reference. The top right of the cover shows how to view the video portion of the record[15b].
The Voyager Golden Record cover. Image credit: NASA/JPL/Public domain.
The Voyager Golden Record. Image credit: NASA/JPL/Public domain.
The record itself contains sounds and images representing the diversity of life and culture on Earth. The contents were selected by a NASA committee chaired by Sagan. Sounds from the Voyager record can be heard here, and images can be found here.