Communicating with Aliens
Passive communication
The space probes Pioneer 10 (launched in 1972) and Pioneer 11 (launched 1973) visited Jupiter and Saturn, and subsequently became the first human-made objects to leave the Solar System. Routine radio contact with Pioneer 11 was terminated by NASA in 1995 due to power generation constraints on the spacecraft. A final weak radio transmission was received from Pioneer 10 in 2003 before it, too, fell silent.
Pioneer 10 is heading in the general direction of the star Aldebaran in the Taurus constellation and will take more than two million years to reach it. Pioneer 11 is headed toward the constellation of Aquila (The Eagle) and will pass near one of the stars in the constellation in about 4 million years.
Both space probes carry plaques inscribed with our attempt at interstellar communication, in case they are ever found by aliens.
Photograph of Pioneer 10's plaque, which features a design engraved into a gold-anodized aluminum plate, 152 by 229 millimeters (6 by 9 inches). The Pioneer 11 plaque is identical.
Credit (image and some text): designed by Carl Sagan and Frank Drake; artwork prepared by Linda Salzman Sagan; photograph by NASA Ames Research Center (NASA-ARC) [link]
The Pioneer 10 plaque is attached to the spacecraft's antenna support struts, facing inward, to help shield it from erosion by interstellar dust.
Credit (image and some text): NASA [link]The Pioneer plaque deciphered
Annotated illustration of the Pioneer plaque.
Credit: vectorized in CorelDRAW from NASA image GPN-2000-001623; vectors by Oona Räisänen (Mysid; Wikimedia Commons) [link]; designed by Carl Sagan & Frank Drake; artwork by Linda Salzman Sagan; annotated by D. W. Hoard (2019)“…a present from a small, distant world…”
The Voyager 1 and 2 space probes, which became the third and fourth human-made objects to leave our Solar System, each carry a more sophisticated attempt to communicate with any extraterrestrial intelligences that might find the spacecraft. As of 2019, both Voyager space craft are still in routine radio contact with Earth and returning scientific data from their instruments, probing the edge of interstellar space outside our Solar System. Both spacecraft can continue operating until about 2025, after which operations will cease due to loss of electrical power. It will be forty thousand years before they make a close (less than 2 light years) approach to any other star.
The Voyager Golden Record
The Voyager 1 and 2 "Golden Records" mounted on both spacecraft are encoded with 116 images, sounds of Earth, spoken greetings in 55 languages, and selections of music.
The title of this section (“…a present from a small, distant world…”) is part of the greeting spoken by U.S. President Jimmy Carter on the Golden Records.
Instructions for playing the records and decoding the images are given on the case.
See “How to decode the images on the Voyager Golden Record” [link]
The record is constructed of gold-plated copper and is 12 inches (30 cm) in diameter.
A phonograph needle stylus is included in the case.
Many people were instrumental in the design, development and manufacturing of the Golden Record.
Blank records were provided by the Pyral S.A. of Creteil, France.
CBS Records contracted the JVC Cutting Center in Boulder, Colorado, to cut the lacquer masters which were then sent to the James G. Lee Record Processing center in Gardena, California, to cut and gold plate eight Voyager records.
Gold plating took place on 23 August 1977; afterward, the records were mounted in aluminum containers and delivered to the Jet Propulsion Laboratory in Pasadena, California.
See "Making of the Golden Record" [link].
Outside case of the Voyager Golden Record. The diagrams at the bottom are the same hydrogen spin-flip calibration for physical units and pulsar map to our Solar System from the Pioneer plaques. The other diagrams contain basic instructions for decoding the sounds and images from the record.
Credit: NASA [link]
The contents of the golden records were selected for NASA by a committee chaired by Carl Sagan. Dr. Sagan and his associates assembled 115 images and a variety of natural sounds, such as those made by surf, wind and thunder, birds, whales, and other animals. To this they added musical selections from different cultures and eras, and spoken greetings from Earth-people in fifty-five languages, and printed messages from U.S. President Carter and U.N. Secretary General Waldheim.
Each record is encased in a protective aluminum jacket, together with a cartridge and a needle. Instructions, in symbolic language, explain the origin of the spacecraft and indicate how the record is to be played. The 115 images are encoded in analog form. The remainder of the record is in audio, designed to be played at 16-2/3 revolutions per minute. It contains the spoken greetings, beginning with Akkadian, which was spoken in Sumer about six thousand years ago, and ending with Wu, a modern Chinese dialect. Following the section on the sounds of Earth, there is an eclectic 90-minute selection of music, including both Eastern and Western classics and a variety of ethnic music.
For a listing of the full contents of the Voyage golden records, see “What are the contents of the Golden Record?” [link]
An image from the Voyager Golden Record is shown here. It is called "Demonstration of licking, eating, and drinking".
Credit: NASA [link]Active Communication
The Arecibo Message
Transmitted from the Arecibo Radio Telescope in Puerto Rico on 16 November 1974 for three minutes.
Aimed at globular cluster M31 (25,000 light years away).
The message contained 1679 binary digits (0, 1) arranged in 23 columns and 73 rows.
We assume that aliens know about prime numbers!
1679 is a "semi-prime number", the product of two prime numbers.
Thus, the only choices for arranging the bits in the message are 23x73 or 73x23.
Even if the aliens do it the wrong way the first time, the message still looks "orderly" enough that it should be clear that it contains information and is not a natural phenomenon. Hopefully they flip it around and try again!
This was more of a symbolic gesture than an actual attempt to contact alien civilizations.
For example, the message transmission will not reach M31 until 25,000 years into the future.
An explanation of each section of the Arecibo Message is given below.
The Arecibo message with color added to highlight its separate parts. The actual binary transmission carried no color information.
Credit: Arne Nordmann (norro; Wikimedia Commons), CC BY SA 3.0 [link]Numbers 1-10 in binary format.
"This is our system of mathematics."
Numbers 1, 6, 7, 8, 15 in binary.
These are the atomic numbers of hydrogen (H), carbon (C), nitrogen (N), oxygen (O), and phosphorus (P), which are the components of DNA.
"This is what we’re made of."
Formulas for DNA nucleotides using atomic numbers.
"This is the recipe for our building blocks."
The DNA double helix (blue) and the approximate number of nucleotides in binary (white).
"This is how to build us."
Shape of a human (red, middle).
"This is what we look like."
Height indicator (blue) with measurement (1.74 meters given as a binary number, 14, to be multiplied by the wavelength of the broadcast, 126 mm; left, blue & white)
"This is how big we are."
Population of Earth in 1974 (4.3 billion) as a binary number (right, white)
"This is how many of us there are."
Map of our Solar System.
Planet #3 is special.
This might be confusing, since Pluto is shown as a planet!
"This is where we come from and what our home looks like."
Diagram of the Arecibo Radio Telescope that was used to send this message; the “M” shows that it is a reflecting telescope (purple).
Size indicator for Arecibo (blue) with binary number 2430 (white), which gives the telescope diameter (306 meters) when multiplied by the broadcast wavelength (126 mm).
"This is the technology we have to receive a reply."
Since the Arecibo message was sent in 1974, about 30 radio messages have been transmitted into space in the directions of various stars.
Almost all messages of this sort are considered to be mainly publicity stunts.
Many scientists believe that attempts to actively communicate with alien civilizations should only follow a worldwide discussion and consensus among scientific, political, and humanitarian organizations.
But Earth has been "leaking" radio and television signals for almost a century by now…
The Altair Message
In 1983, a message similar to the Arecibo Message was transmitted toward the star Altair (alpha Aql), located at a distance of 17 light years.
The Altair message was conceived and transmitted by Japanese astronomers Hisashi Hirabayashi and Masaki Morimoto.
It contained 13 binary-encoded images (two examples shown here), 71 x 71 pixels each (prime numbers again!).
We could have received a reply as early as 2017.
But there has been no reply so far...
Pestering Polaris
In 2008, the song “Across the Universe” by the Beatles was transmitted by NASA toward the star Polaris.
It will arrive there in 2455.
This was an event to celebrate the 40th anniversary of the song's recording, the 45th anniversary of the Deep Space Network of radio telescopes, and the 50th anniversary of NASA.
In 2016, another message was beamed toward Polaris, consisting of thousands of text messages from humans answering the question “How will our present environmental interactions shape the future?”
Waiting for contact
We mostly listen for artificial radio transmissions from space.
Observations of this sort have been carried out somewhat haphazardly since the 1970s.
Somewhat more focused projects by the Search for Extraterrestrial Intelligence (SETI) Institute have been in place since the 1990s.
For example, using the SETI Institute’s Allen Telescope Array:
42 six-meter radio telescopes at Hat Creek Radio Observatory (290 miles NE of San Francisco).
Significant funding from the late Paul Allen (Microsoft co-founder).
So far, no signals have been received that can be conclusively identified as artificial.
Closeup front view of one antenna of the Allen Telescope Array, a radio telescope for combined radio astronomy and SETI (Search for Extraterrestrial Intelligence) research, located outside San Francisco. Note the person near the telescope base for scale.
Credit: Colby Gutierrez-Kraybill, CC BY 3.0 [link]