November 16 – Home Phone E.T.

Today’s factismal: The first interstellar message was sent on November 16, 1974. It will arrive in 25,000 years.

Quick! What’s big enough to hold 10,000 gallons of guacamole, deep enough to put a submarine in, precise enough to see supernovae 44 million light years away, and sent the first message to the stars? It is the Arecibo National Astronomy and Ionosphere Center (the Arecibo Observatory, or just Arecibo, for short).

Words (My camera)

The Arecibo “dish”, big enough for a submarine to hide in (My camera)

Back in the late 1950s, scientists were just learning about the ionosphere and wanted to develop a tool that would allow them to probe its secrets. And other scientists were learning about radio emissions from planets and stars, and wanted a tool to learn about those. And when the first group of wonks met the second group of wonks, a new telescope was born.

The idea was simple: because the same energy (radio waves) that is used to probe the ionosphere is also used to learn more about distant planets and stars, instead of building two small instruments, why not build one huge one? They would get better resolution (thanks to the size of the reflecting dish), more power (thanks to the size of the transmitter/receiver), and more funding (thanks to the size of the project). And so they started looking for a place to build what would be the world’s biggest single aperture telescope, a title it would hold until 2016 when the Chinese opened their Five-hundred-meter Aperture Spherical Telescope.

The remains of the very first supernova ever recorded (Image courtesy NASA)

The remains of the very first supernova ever recorded
(Image courtesy NASA)

They had quite a few requirements on the location. It had to be in the US (thanks to the Cold War). It had to be near the equator (so it could see the planets). It had to be in an area with eroded limestone features called karst (so that it would be easy to build). And the spot that best fit was a little place called Arecibo on the island of Puerto Rico. So that’s where they built it and, on November 1, 1963, they started getting signals.

An image of the Crab Nebula at radio frequencies (Image courtesy NASA)

An image of the Crab Nebula at radio frequencies (Image courtesy NASA)

And what amazing things they saw! At the end of six months, they had discovered that Mercury wasn’t tidally-locked to the Sun like the Moon is to Earth; instead, it had a funny 3:2 rotation so that the day on Mercury appears to take two years! Soon they proved the existence of neutron stars, and mapped asteroids, and found complex molecules in outer space. But they weren’t limited to discovering things; they could also help things discover us. On November 16, 1974, Carl Sagan and friends took over Arecibo and used it to send a message to globular cluster M13, letting ET know where to phone.

Globular cluster M13, target of our first message (Image courtesy NASA)

Globular cluster M13, target of our first message
(Image courtesy NASA)

Unfortunately, Sagan was a better showman than he was an astronomer. In sending the message, he forgot about “proper motion” and sent the message to where the cluster appears to be in the sky. Because M13 is some 25,000 light years away, where it appears to be tonight is actually not where it is now – or where it will be when the message arrives! Imagine that you are walking along and tossing out pebbles every so often. The pebbles take a second to so to hit the ground so that you are in a different place when they land than you were when they were thrown. The same thing is happening here; the light from M13 left 25,000 years ago when the globular cluster was in a different place and it will have moved yet more in the 25,000 years it will take for the message to arrive.  As a result, our message will miss M13 almost entirely and instead head out into deep space.


Arecibo continues its mission of discovery today. One of its most important missions today is the search for black holes – and they need your help! Just go to Radio Galaxy Zoo and help match radio telescope pictures to infrared telescope images. Fun, easy, and really, really cool!

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