Arecibo Update

Since the publication of this post on 2 December 2020 it appears that Arecibo observatory may be rebuilt. Although the $8 million pledged by the Puerto Rico Rico government is clearly not enough

I was saddened to see that the Arecibo Observatory in Puerto Rico has closed following catastrophic damage to its cables and the platform’s ultimate collapse on 1 December 2020. 

The 305 metre diameter bowl was for over fifty years the world largest single dish radio radio-telescope and made some major contributions to science during its lifetime. I’ve listed a few of them below:


  • Radio echoes determined the rational period of Mercury as 59 days (not 88 days which has been assumed previously)
  • Pulsars are rapidly rotating neutron stars. The first binary pulsar was discovered at Arecibo in 1974. The first ‘millisecond pulsar’ PSR +21, which performs 642 revolutions per second, was discovered in 1982. The first planets around a pulsar were discovered in 1989.
  • The large collecting area enabled very sensitive observations to be taken in the 21cm hydrogen line (and indeed other spectral lines as well). The enabled the distribution of hydrogen gas in our Milky Way galaxy. Its great sensitivity allowed the detection of galaxies which would be too faint to see with a less sensitive instrument. (Indeed, the author took some observations at Arecibo for his PhD thesis in the mid-1980s 😉)
  • Using radar, maps could be produced of Venus.


Outside the scientific community perhaps the thing the Arecibo observatory was most famous for is the Arecibo message. This was first attempt to send a message from Earth intended for an alien civilisation. I  have therefore decide decided re-blog part of a post I wrote previously on this topic.

The key challenge is writing  a message so it could be [potentially] understood by an alien civilisation that happened to receive it. The recipients would have no knowledge of Earth or of any of its culture and would in all likelihood look and act very differently from human beings.

The difficulties faced when trying to decipher a message in an unknown language are illustrated by the hundreds of stone tablets on the Greek island of Crete written in a language called  Linear A. All these tablets were produced around 3500 to 4500 years ago but nobody has managed to decipher them. Most of the letters in the Linear A alphabet appear in no other known language. Huge efforts have been made over the years to decipher the Linear A, but to no avail.  What is needed is a clue like the Rosetta Stone, which contains the same text in three different languages and its discovery provided the key to finally understanding Egyptian hieroglyphics.

A Stone tablet with an inscription written in Linear A -Image credit Wikimedia Commons



The first and the most famous message sent from Earth intended for alien civilisations was the Arecibo message. It was constructed by Frank Drake who decided to convey basic information about humanity by sending a simple picture which an alien intelligence receiving the message could hopefully  construct. The message was transmitted from the Arecibo telescope at a wavelength of 12.6 cm and consisted of a sequence of 1679 zeros and ones. The number 1679 was chosen because its only factors (numbers which divide into it without giving a remainder) are 23 and 73. Drake felt that when the message was interpreted by an alien civilisation they would naturally write it out in a 23 by 73 or in a 73 by 23 grid.

Understanding the message

The message consists of seven separate components.

The Arecibo Message  – Image credit Wikimedia Commons. 


The message is written out in a grid of 73 rows and 23 columns, which is the way it was intended to be read. All the 0’s in the message are coded black. The 1’s in the message are colour coded to show which of the seven components they belong to.  A civilisation receiving this message would have to work out the components themselves.



Component 1 – Numbers from one to ten

The blocks at the top show the numbers from one to ten.

The first three rows show the numbers in binary format. The fourth row marks the beginning of each number and is to be ignored.  The first, second and third rows give the value of the number as shown in the table below.

Component 2- Chemical Elements in DNA

The purple blocks give the numbers 1, 6, 7, 8 and 15 in binary.

These are the atomic numbers of the elements hydrogen(H) , carbon(C), nitrogen (N), oxygen(O), and phosphorus(P), which make up DNA. DNA holds the genetic information of all living organisms found on Earth except viruses. If the civilisation reading the message had their genetic information held in a similar way, then this part of the message should make sense to them.


Component 3 – DNA Building Blocks Formulae

The green blocks give the chemical formulae, i.e. the number of atoms of hydrogen, carbon, nitrogen, oxygen and phosphorus of each of the basic building blocks of DNA.

The basic building blocks are described as sequences of the five atoms that appear in component three. Each sequence represents its molecular formula.


For example, the first block is the molecular formula of deoxyribose. This has seven atoms of hydrogen, five atoms of carbon, one atom of oxygen and no atoms of nitrogen or phosphorous. It is represented as

Component 4 – The Double Helix

The blocks in the middle show the double helix which is the basic structure of DNA.

Superimposed is a massive number – approximately 4.3 billion.

In 1974 this value was thought to be the number of basic building blocks in human DNA. The actual number is around 3.2 billion.

Component 5 – Humans

The central figure in red  shows a graphic of a human.


To its left is a number which gives the height of the average human. Clearly this could not be expressed in any Earth-based units such as metres or feet and inches, because these would not make sense to an alien civilisation. Drake thought the only natural unit to use was multiples of the wavelength the Arecibo message was transmitted on (12.6 cm).  So, the value given is 14 because 12.6 cm multiplied by 14 gives 176.4 cm which is the average height of a human.


To the right of the figure, is a very large number, just over 4 billion, which was the human population of Earth in 1974. Today it is approximately double that value!






Component 6 – the Solar System, the Sun and the Earth

The blocks near the bottom show a plan of the Sun and the nine planets – the Solar System. The third block is raised to indicate which of the planets the message is coming from. Interestingly, this is now rather out of date as there are now considered to be only eight planets. Since 2006 Pluto has no longer been classified as a planet because of its very small size.


Component 7 – The Telescope

The bottom of the message shows a picture of the Arecibo telescope and a number giving its diameter in multiples of 12.6 cm.


The message was transmitted on 16 November 1974 and was aimed at an object called M13 about 25 000 light years away. This is a globular cluster, a dense collection of stars. It was sent a total of three times and will take about 25 000 years to reach the position where M13 was in 1974, by which time it will have moved, although, in theory, it could be intercepted by any alien civilisation along the way.

In 1999 and 2003, the Arecibo message was re-sent to eight nearby stars, which were known to have planets

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