How We Came to Know the Cosmos: Space & Time

Discover How We Came to Know the Cosmos

Chapter 27. The Pioneer and Voyager Missions

27.1 Pioneer 10 and Pioneer 11 probes

NASA launched the Pioneer 10 and Pioneer 11 spacecraft in 1972 and 1973 to study solar winds and cosmic rays as they travelled through the Solar System.[1] Cosmic rays are extremely high-energy radiation that comes from space. They were discovered in the 1900s and 1910s.[2]

Pioneer 10 became the first spacecraft to travel through the asteroid belt in 1972 and the first to observe Jupiter in 1973.[1] Pioneer 10 is currently about 125 AU away[3] (where 1 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.[4]

A photograph of Pioneer 10, showing the Pioneer plaque.

Figure 27.1
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Pioneer 10.

Pioneer 11 reached Jupiter in 1974 and became the first probe to reach Saturn five years later.[5] Pioneer 11 is currently about 101 AU away, also beyond the Kuiper Belt.[3] 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.[5]

27.1.1 The Pioneer plaque

The British journalist Eric Burgess was the first to suggest that the Pioneer probes should contain a message from Earth, and he approached the American astronomer Carl Sagan with the idea.[6] Sagan was already involved in the SETI (Search for Extra Terrestrial Intelligence) program, which actively searches for signs of extraterrestrial intelligence[7].

Sagan liked Burgess’ idea and took it to NASA, who gave him three weeks to design the message. Sagan enlisted the help of fellow SETI member Frank Drake. Drake had previously developed the Drake equation, which can be used to estimate the number of extraterrestrial civilizations in our galaxy that we may be able to communicate with (discussed in Chapter 29). Sagan, Drake, and the artist Linda Salzman Sagan designed a picture to be engraved on two 23 cm wide aluminium plaques[8] (shown in Figure 27.2).

Depiction of the Pioneer plaque.

Figure 27.2
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The Pioneer plaque.

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.[8]

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 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.[8]

These images drew 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.[9]

The shape on the left side of the plaque shows 15 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 14 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.[8]

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.[8] 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.[8] Pluto is depicted as it was considered a planet until 2006.[10]

A diagram of the Solar system, showing the location of the Pioneer and Voyager probes and New Horizons.

Figure 27.3
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Location and trajectories of spacecraft as of 2011.

27.2 Voyager 1 and Voyager 2 probes

The Voyager 1 and Voyager 2 probes were launched in 1977 to study Jupiter and Saturn,[11] although Voyager 2 also travelled past Uranus in 1986, and Neptune in 1989.[12] 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.[13]

A photograph of Earth, where Earth is only a pixel wide.

Figure 27.4
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‘Pale blue dot’, an image taken by Voyager 1 at over 40 AU. Earth is the blue dot in the middle of the orange streak of light.

Voyager 1 is currently about 147 AU away, and Voyager 2 is currently about 121 AU away.[3] The Voyager probes are both travelling in the general direction of Pioneer 11. 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. Both probes are expected to keep transmitting signals back to Earth until at least 2025.[14]

27.2.1 The Voyager Golden Record

Each of the Voyager probes contains a gold-plated copper record. The cover (shown in Figure 27.5) 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.[15]

The top left of the cover shows the image of a record player with the speed that it should be turned 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.[15] 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.[16]

A photograph of the Voyager Golden Record cover.

Figure 27.5
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The Voyager Golden Record cover.

A photograph of the Voyager Golden Record.

Figure 27.6
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The Voyager Golden Record.

Carl Sagan’s influence on space exploration

Carl Sagan began writing to scientific journals in his late teens, and his suggestion that organisms could exist on Mars was mentioned in an article in the New York Times in 1956, while he was still an undergraduate.[17] Sagan completed his PhD at the University of Chicago in 1960 and became a Fellow at the University of California, Berkeley. While at Berkeley, Sagan was involved with the Mariner 2 project, which sent a space probe to Venus in 1962. During this time, Sagan wrote scientific articles, gave speeches, and began to be quoted by the media.[17]

Sagan’s first success with a mainstream scientific journal came with ‘The Planet Venus’, published in Science in 1961.[18] That same year, Sagan’s greenhouse model of Venus, along with the suggestion of ‘terraforming’ it, a term he borrowed from Jack Williamson’s science fiction of the 1940s, made it into the New York Times and Newsweek.[17] Sagan’s views earned him a place at the 1961 Green Bank convention, which became known as the first SETI (Search for Extra Terrestrial Intelligence) meeting.[19]

In 1965, when Mariner 4 sent back photos showing the ‘dead’ surface of Mars, Sagan published an article in Icarus juxtaposing the images of Mars with photographs showing the Earth from the same distance, making it clear that evidence of life might not be observed.[20]

Sagan’s first mainstream book Planets was first published in 1966. In Planets, Sagan presented the planets as physical places to be explored rather than points of light in the sky. He talked of going to new places in the spirit of adventure, discovering new lands and perhaps even new wildlife.[17] Sagan repeated the Russian astronomer Iosif Shklovsky’s suggestion that there could be “libraries and museums” on Mars’ moon Phobos.[21]

In 1972, Sagan became involved with composing data to be sent out of the Solar System with the Pioneer probes. Sagan’s The Cosmic Connection: An Extraterrestrial Perspective was published in 1973 and he made many television appearances to promote the already very successful book, including an appearance on Johnny Carson’s The Tonight Show. Sagan went on to become the show’s ‘house astronomer’ and hence became one of the most famous American scientists of the time.

Sagan had a direct effect on NASA’s expenditure by bringing publicity, and therefore funding, to the projects he was interested in. When NASA was refused funding for a crewed mission to Mars, Sagan suggested launching robotic probes to land on the surface instead and made it his mission to win over the public. Carson helped endorse the view, and the success of this project led him to be profiled by Rolling Stone magazine.[17]

Sagan’s influence meant that the Viking missions went ahead and contained cameras capable of spotting moving objects. Sagan made what were seen as outrageous claims about the possibility of “polar bear-sized” animals on the surface[17]. The first view of the Martian sky came from Viking 1 in 1976, and there was no sign of life. In 1977, Sagan became involved with composing data to be sent out of the Solar System with the Voyager probes.

The 13-part television series Cosmos aired in 1980. Cosmos won many awards and the accompanying book became a New York Times bestseller for over 70 weeks. By then, the SETI program was severely underfunded, and so Sagan cofounded the Planetary Society to fund it privately. His 1985 science fiction novel Contact[22] was, among other things, an assault on critics of the SETI program.

After his death in 1996, Nature described Sagan as,

“...the greatest populariser of the 20th century”.[23]

27.3 References

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