The Voyager Golden Record

The Voyager Golden Record carries a small sample of uranium on its cover, with a built-in clock for a message engineered to last around a billion years        

Each of the two Voyager spacecraft, launched in 1977, carries a phonograph record. The records hold sounds and images chosen to represent Earth, assembled by a committee led by Carl Sagan. The records themselves are the part most people know about. The cover is the part worth a closer look. Electroplated onto the aluminium cover of each record is an ultra-pure sample of uranium-238. It sits in a small area about two centimetres across, and it is there to serve a single function. It is a clock. The Golden Record Pioneers 10 and 11, which preceded Voyager, both carried small metal plaques identifying their time and place of origin for the benefit of any other spacefarers which might find them in the distant future. NASA placed a more ambitious message aboard Voyager 1 and 2, a kind of time capsule, intended to communicate a story of our world to extraterrestrials. The Voyager message is carried by a phonograph record, a 12-inch gold-plated copper disk containing sounds and images selected to portray the diversity of life and culture on Earth. The Golden Record Cover is an easily recognized drawing of the phonograph record and the stylus carried with it. The stylus is in the correct position to play the record from the beginning. Written around it in binary arithmetic is the correct time of one rotation of the record, 3.6 seconds, expressed in time units of 0,70 billionths of a second, the time period associated with a fundamental transition of the hydrogen atom. The drawing indicates that the record should be played from the outside in. 

Below this drawing is a side view of the record and stylus, with a binary number giving the time to play one side of the record. The information in the upper right-hand portion of the cover is designed to show how pictures are to be constructed from the recorded signals. The top drawing shows the typical signal which occurs at the start of a picture. The picture is made from this signal, which traces the picture as a series of vertical lines, similar to ordinary television. Picture lines 1, 2 and 3 are noted in binary numbers, and the duration of one of the "picture lines," about 8 milliseconds, is noted. The drawing immediately below shows how these lines are to be drawn vertically, with staggered "interlace" to give the correct picture rendition. Immediately below this is a drawing of an entire picture raster, showing that there are 512 vertical lines in a complete picture. Immediately below this is a replica of the first picture on the record to permit the recipients to verify that they are decoding the signals correctly. A circle was used in this picture to ensure that the recipients use the correct ratio of horizontal to vertical height in picture reconstruction. The drawing in the lower left-hand corner of the cover is the pulsar map previously sent as part of the plaques on Pioneers 10 and 11. It shows the location of the solar system with respect to 14 pulsars, whose precise periods are given. 

The drawing containing two circles in the lower right-hand corner is a drawing of the hydrogen atom in its two lowest states, with a connecting line and digit 1 to indicate that the time interval associated with the transition from one state to the other is to be used as the fundamental time scale, both for the time given on the cover and in the decoded pictures. The reasoning is straightforward. Uranium-238 decays into a chain of daughter products at a fixed, known rate. A finder who measures how much uranium-238 remains, against how much of the daughter material has accumulated, can calculate how long the decay has been running. The figure is the time elapsed since the sample was placed on the spacecraft. It tells the finder how old the record is. The uranium is not the only timekeeping device on the record cover. It is one of two, and the two are meant to be checked against each other. The cover also carries a pulsar map, the same basic diagram used earlier on the Pioneer 10 and 11 plaques. It shows the position of the Sun relative to 14 pulsars, with each pulsar’s rotation period written in binary. Pulsars spin down slowly and predictably over time. A finder who knows how fast those 14 pulsars are spinning when the record is found, and compares that to the periods recorded on the cover, can also work out roughly how much time has passed. So the cover gives a finder two independent ways to date the record. One is the decay of the uranium. The other is the slowing of the pulsars. If both methods point to a similar launch epoch, a finder can have more confidence in the answer. The redundancy is deliberate.

Electroplated onto the record's cover is an ultra-pure source of uranium-238 with a radioactivity of about 0.00026 microcuries. The steady decay of the uranium source into its daughter isotopes makes it a kind of radioactive clock. Half of the uranium-238 will decay in 4.51 billion years. Thus, by examining this two-centimeter diameter area on the record plate and measuring the amount of daughter elements to the remaining uranium-238, an extraterrestrial recipient of the Voyager spacecraft could calculate the time elapsed since a spot of uranium was placed aboard the spacecraft. This should be a check on the epoch of launch, which is also described by the pulsar map on the record cover. The  Voyager Golden Records  are two  phonograph records  which were included aboard both  Voyager spacecraft. The records contain sounds and images selected to portray the diversity of life and culture on Earth, and are intended for any intelligent  extraterrestrial life  form who may find them. The records are a sort of  time capsule . Although neither Voyager spacecraft is heading toward any particular star,  Voyager 1  will pass within 1.6  light-years ' distance of the star  Gliese 445 , currently in the constellation  Camelopardalis , in about  40,000 years . The spacecraft will be encountered and the record played only if there are advanced space-faring  civilizations  in  interstellar  space, but the launching of this  'bottle' into the cosmic 'ocean'  says something very hopeful about life on this planet. The choice of isotope is the whole point. Uranium-238 has a half-life of about 4.5 billion years, meaning that after that span roughly half of any given quantity has decayed. It's is a slow clock, and a slow clock is what a message of this kind needs.

A fast-decaying isotope would be useless here. If the half-life were measured in years or centuries, the sample would have decayed away to almost nothing long before any plausible finder encountered it, leaving nothing to measure. A half-life of billions of years means the clock stays readable across the kind of timescales the Voyager spacecraft will actually be drifting. The Voyager record is often described as a message built to last around a billion years, and the figure is worth handling carefully. It is an estimate of the physical survival of the record itself, not a guarantee and not a precise prediction. The record is gold-plated copper in an aluminium cover, mounted on the spacecraft body. In interstellar space it faces erosion mainly from micrometeoroid impacts and cosmic rays, both of which act slowly. Estimates of how long the record’s grooves would remain physically readable run to a timescale of that order, hundreds of millions to a billion-plus years. These are estimates of material durability, and they carry wide uncertainty. The uranium clock is well matched to that lifespan. With a half-life of about 4.5 billion years, the uranium-238 sample will still be measurable, still a working clock, long after a billion years have passed. The clock is designed to outlast the message it dates. 

The selection of content for the record took almost a year. Sagan and his associates assembled 115 images and a variety of natural sounds, such as those made by surf, wind, thunder and animals (including the songs of  birds  and  whales ). To this they added musical selections from different cultures and eras, spoken greetings in 55 ancient and modern languages, other human sounds, like footsteps and laughter (Sagan's), and printed messages from US president  Jimmy Carter  and  UN   Secretary-General   Kurt Waldheim. The Golden Record also carries an hour-long recording of the brainwaves of  Ann Druyan . During the recording of the brainwaves, Druyan thought of many topics, including Earth's history, civilizations and the problems they face, and what it was like to fall in love. The  pulsar  map and hydrogen molecule diagram are shared in common with the  Pioneer plaque. The 115 images are encoded in analogue form and composed of 512 vertical lines. The remainder of the record is audio, designed to be played at 16⅔ revolutions per minute. The drawing indicates that the record should be played from the outside in. A circle was used in this picture to ensure that the recipients use the correct ratio of horizontal to vertical height in picture reconstruction. Color images were represented by three images in sequence, one each for red, green, and blue components of the image. A color image of the spectrum of the sun was included for calibration purposes. 

The record also includes the inspirational message  Per aspera ad astra  in  Morse code. The collection of images includes many photographs and diagrams both in black and white, and color. The first images are of scientific interest, showing mathematical and physical quantities, the  Solar System  and its planets,  DNA , and human  anatomy  and  reproduction . Care was taken to include not only pictures of humanity, but also some of animals, insects, plants and landscapes. Images of humanity depict a broad range of cultures. These images show food, architecture, and humans in portraits as well as going about their day-to-day lives. Many pictures are annotated with one or more indications of scales of time, size, or mass. Some images contain indications of  chemical composition . All measures used on the pictures are defined in the first few images using physical references that are likely to be consistent anywhere in the  universe. The Voyager spacecraft were not aimed at any recipient, and the distances between stars are such that neither craft will pass close to another star for tens of thousands of years at least. The record cover is built the way it is not because a finder is expected, but so that the information is complete and self-checking if, against very long odds, one ever exists. This is the quiet logic of the uranium spot. It is two centimetres of metal added to a cover, on the assumption that the most useful thing you can give a hypothetical finder, along with the message, is an honest way to know how old the message is.

Voyager 1 was launched in 1977, passed the orbit of  Pluto  in 1990, and left the  Solar System  (in the sense of passing the  termination shock) in November 2004. It is now in the  Kuiper belt . In about 40,000 years, it and  Voyager 2  will each come to within about 1.8  light-years  of two separate stars:  Voyager 1  will have approached star  Gliese 445 , located in the constellation  Camelopardalis ; and  Voyager 2  will have approached star  Ross 248 , located in the constellation of  Andromeda . In March 2012,  Voyager 1  was over 17.9 billion km from the Sun and traveling at a speed of 3.6  AU  per year (approximately 61,000 km/h (38,000 mph)), while  Voyager 2  was over 14.7 billion km away and moving at about 3.3 AU per year (approximately 56,000 km/h (35,000 mph)). Voyager 1  has entered the  heliosheath , the region beyond the termination shock. The termination shock is where the solar wind, a thin stream of electrically charged gas blowing continuously outward from the Sun, is slowed by pressure from gas between the stars. At the termination shock, the solar wind slows abruptly from its average speed of 300–700 km/s (670,000–1,570,000 mph) and becomes denser and hotter. On 12 September, 2013, NASA announced that  Voyager 1 had left the heliosheath and entered  interstellar space, although it still remains within the Sun's gravitational sphere of influence.