Calling all aliens: What's the best way to contact our galactic neighbors?

August 20, 2017 marks the 40th anniversary of the launch of the the first NASA Voyager mission, which is carrying a golden record filled with messages to potential civilizations beyond our solar system. This year is also the 20th anniversary of the sci-fi film Contact that dealt with receiving radio messages from extraterrestrials. Both the record and the film were brain children of the late Carl Sagan and raise an interesting question: which approach has the greater chance of success of making contact with aliens – sending radio messages or unmanned probes?

First contact with extraterrestrial civilizations has long fascinated scientists, philosophers, and writers. It's been the topic explored by serious scientific studies, crackpots, tabloids, science fiction epics, and international debates. The speculated results of the first meeting of man and alien run the entire gamut of imagination. Visits by aliens or receiving greetings from the stars has been seen as ranging from wonderfully transcendent, with the human race raised to the next step in evolutionary perfection, to us ending up as the main course on someone's dinner table.

Whether the outcome is the end of 2001: a Space Odyssey or To Serve Man, how will we establish contact with whoever or whatever lives beyond our solar system? Will our first contact be an alien spaceship carrying little green men? A probe operated by an artificial intelligence? A mysterious artifact buried on the Moon? A radio signal blaring out from the stars? Zaphod Beeblebrox crashing a party in Islington?

The problem with answering this question is that we know literally nothing about any other intelligent life forms. We don't even know if they exist or even how probable their existence is. As to how they think, their limitations, or what manner they might choose to make themselves known to us, these are questions that are so complex that it often wanders into the realm of metaphysics, if not theology.

A much easier way of reaching an answer is to ask not "How will they contact us?" but "How will we contact them?" If we can answer the latter, then we are a great deal closer to answering the former. Knowing how to send messages tells you how to receive them.

First attempts

The idea of trying to contact ET is the reverse of the conventional practice of the Search for ExtraTerrestrial Intelligence (SETI). Founded in 1984, SETI is a passive search for signs of other civilizations, usually in the form of radio signals showing definite signs of intelligence, though other evidence might be sought. Deliberate contact attempts are called Active SETI or METI (Messaging to ExtraTerrestrial Intelligence), a term coined by the Russian scientist Alexander Zaitsev to denote an aggressive program of composing and sending messages to the stars.

But this idea isn't particularly new. In fact, proposals for contacting other planetary bodies go back as far as around 1820 when the Joseph Johann von Littrow, director of the Vienna Observatory, suggested creating circles, squares and triangles 30 km on a side in the Sahara Desert by digging ditches one kilometer-wide and filling them with water topped with kerosene. This would be set alight at night in the hopes of contacting anyone on the Moon or Mars.

In 1868, inventor Charles Cros put forward a plan to the French Academy of Sciences to set up giant parabolic mirrors reflecting arc lamps with a focal length equal to the distance between Earth and Mars. Cros' plan was to concentrate the sun's rays on the Martian desert to carve geometric figures and numbers in molten glass on the surface. In another proposal, he suggested building an enormous checkerboard with shiny surfaces that could be uncovered to form shapes and patterns like a mechanical digital display that the Martians could view using telescopes.

In the 1890s, the Reverend W S Lach-Szyrma suggested lighting up the Riga, the Malvern Hills, or Lake Michigan with geometric patterns. Meanwhile, German mathematician Karl Friedrich Gauss recommended creating a giant right-angle triangle in Siberia using 15 km-wide strips of forest with fields of wheat as the background to form the famous geometric solution to the Pythagorean theorem. The whole thing would have been about the size of Ireland.

Not to be outdone, electrical pioneer Nikola Tesla in 1896 claimed that a more advanced version of his device for transmitting electrical power without wires could be used to contact Mars, and in 1899 he said that he'd detected signals from the Red Planet.

But perhaps the prize for the most ambitious early scheme should go to the popular science publisher and science fiction pioneer Hugo Gernsback, who in the February 1927 edition of his Radio News magazine put forward the idea of building a directional radio transmitter belting out 100,000 kilowatts in the two-meter band. True, it would have to have been a heavy bar of silver or copper glowing white hot to take all that power, but Gernsback claimed that it could not only communicate with Venus or Mars, but could also bounce radio signals off the Moon and back to Earth.

Is there anybody there?

But if we're going to talk to other civilizations, where do we start? We start with answering a few basic questions, like is there anyone to talk to? For our purposes, we don't need to go into all the complexities of astrobiology, planet formation or how to define the habitable zone. What we need is a rough idea of the probability of the present existence of intelligent life, how far away they are, and how advanced they are. This will tell us not only where to direct our efforts, but also when we can expect a reply, and whether they'll understand us.

These are questions that go back to the late 1950s when the SETI field was first pioneered by Cornell scientists Giuseppe Cocconi, Philip Morrison and Frank Drake at the Green Bank radio observatory. Back then, the field was marked by swift innovation and brilliant out-of-the-box thinking because these scientists didn't know what was possible or impossible. Many of our ideas about METI date back to this time.

One of the key tools for finding out if anyone is out there is the famous Drake Equation written in 1961, which is expressed as follows:

N = R* • f_p • n_e • f_l • f_i • f_c • L

• N is the number of technological civilizations in our galaxy
• R* is the average rate of star formation in our galaxy
• f_p is the fraction of stars that have planets
• n_e is the average number of planets that can support life
• f_l is the fraction of those planets that develop life
• f_i is the fraction of planets with intelligent life
• f_c is the fraction of planets with technological civilizations
• L is the lifespan of these technological civilizations

If you can find the numbers for each of these variables and plug them in, you should have a good idea of how many civilizations are out there for us to talk to. The problem is that even after almost 60 years of research, there are no reliable numbers for any of these variables. True, we know more about stellar evolution, we have discovered thousands of planets orbiting other stars and we do have a better idea of what type of planetary systems there are out there, but the specific numbers remain unknown.
The most important variable is L, which denotes the lifespan of a technological civilization. Even if all the other variables are nailed down, this one will determine the final answer. If such a civilization lasts only about a century, then we may be the only one. If they last for millions of years, there could be millions of civilizations out there. The irony is that we have no way to set L until we actually witness the rise and fall of other technological civilizations.

Our alternative is to take our only example of a planet with intelligent life (Earth) and look for somewhere that's a relatively close match. That means looking for a single G main sequence star not too close to the galactic center and not too far on the edge. It should have a rocky planet about the size of the Earth with a large moon and sit in its star's habitable zone.

Such a search would have been beyond our capabilities just a few decades ago, but modern exoplanet-hunting techniques have changed the game. True, nothing close to a near-Earth analog has been found (yet) and the nature of planet hunting tends toward extreme examples, but the ongoing planetary surveys have allowed us to eliminate many systems as candidates in the same way as the Mars and Venus probes put paid to any future projects to contact Venusians and Martians.

It would be nice if the Drake Equation was more tractable, because if we knew how probable another civilization was, we would know how likely it is that one was within a hundred light years of us. If it is very probable, then the probability of another civilization in our neighborhood increases. If it's improbable, then such a civilization could be thousands or even millions of light years away, if it's out there at all.

On the bright side, since we don't know, we have no reason not to send our message to nearby candidate stars unless our surveys show they have no Earth-like planets orbiting them. According to some estimates, there are 19 G-type stars within 10 parsecs (32 light years) of Earth, with the nearest only four light years away, so we have some to start with. And with hundreds of million more in the galaxy, we won't run out anytime soon.

Are aliens watching Hitler on the telly?

The next question is, how are we going to send our message? The obvious answer is radio. But it's not a matter of pressing the mic button and starting to talk. One common misconception fostered by a certain movie is that radio communication with the stars is so easy that we're doing it now without our knowledge. Are the inhabitants of some planet about 80 light years from Earth watching television broadcasts of Adolph Hitler opening the 1936 Berlin Olympics? Very probably not.

There are many different kinds of radio and most of them are unsuitable for communicating with the Moon, much less the stars. True, we can communicate with a deep space probe 11 billion mi (19 billion km) from Earth, but that's because we use very powerful transmitters on Earth focusing a very tight beam, while the receivers are giant dishes precisely aimed at the transmitting spacecraft.

Other forms of radio don't have a hope. AM transmissions simply can't cover much distance and shortwave broadcasts bounce off the Earth's ionosphere. As for television broadcasts, they can travel beyond our atmosphere and into deep space, as can very powerful military radars. For decades, these have been blasting out into space in a bubble that now has a radius of about a hundred light years.

At first, this makes the Earth seem like a bright radio beacon with aliens 50 light years away able to tune into Star Trek on a weekly basis. But the problem is two-fold. First, the television and similar transmissions are being broadcast in all directions, meaning that their strength is weakened by the factor of the radius squared. This means that the entire Earth from, for example, 1966 would have a brightness of 10-55 watts per square centimeter at a distance of 10 light years. That's ten million times too faint to be detected at all, much less not be lost in all the background static. To be picked up at 100 light years, a television broadcast would need 10²⁰ watts behind it.

It gets even worse. These broadcasts are coming from fixed spots on the Earth, which is rotating and revolving around the Sun. This makes the signals intermittent and subject to Doppler effects that distort them. Then there's the effect of distortion by the Earth's atmosphere, the Earth's magnetic field, the Sun's magnetic field, other stars, interstellar dust and gas, energetic objects, and the omnipresent cosmic microwave background radiation (CMBR) left over from the Big Bang. Add all that together and Hitler's cosmic broadcast petered out at less than two light years.

What we are talking about here are the ultimate limits of radio communications. Despite having been at this for a little over a century, we're already close to these limits and anyone else out there will likely be, too. This means that so long as we're sending out electromagnetic waves, we're likely on a level playing field no matter how advanced the other party is.

Designing the transmitter

To communicate with the stars, we need to consider three factors. First, the transmission, which must be in the form of a directed, monochromatic beam. Second, the power behind that beam must be high enough to carry information. And third, the frequency of the beam must be able to penetrate space for thousands of light years, yet have enough bandwidth to carry a message.

Ideally, the best system would be one where we design both the transmitter and receiver. Of course, we can't do this for the first message, but there's no reason why that message can't include instructions on how to build a compatible receiver.

The transmitter we'd use isn't too hard to figure out because we've already built several of them in the form of the radio telescopes at Arecibo in Puerto Rico, Jodrell Bank in England, and Pingtang in China, among others. To these giant dishes can be added arrays of multiple dishes, including the Very Large Array and the Allen Telescope Array.

One requirement for setting up a communication base is that it needs to be in a radio quiet zone where even mobile phone use is heavily restricted. The scene in Contact where Jodie Foster's character is excitedly shouting orders into a walkie talkie about the message from space she's discovered as she drives by the radio telescope dishes would have been more realistic if her colleagues at the other end angrily shouted back for her to shut up because she's drowning out the signal.

Choosing a frequency

The next step is choosing what frequency to transmit at, which is a mixture of technical details, economy, and second guessing whoever is listening.

One premise that SETI scientists work on is astronomer Frank Drake's Principle of Economy, which, to put it simply, is anyone we're likely to contact will be economical and minimize the personnel, materials, and energy to achieve their ends. In other words, their bureaucrats will be as penny pinching as ours because a species that is careful with its resources will have a better survival advantage.

This means that the aliens will also assume that the frequency we choose will be the one that conserves transmitter power and costs the least energy per bit to send. At the same time, the frequency needs to be easy to generate and detect, not susceptible to much deflection, interference, or absorption by interstellar dust and gas. One other factor that helps whittle down the options is that the signal has to go through our atmosphere and we assume that the receiver is inside a similar habitable atmosphere.

Leaving out the math, the most likely band to achieve all this is between 1,000 and 10,000 MHz. The problem is that there are nine billion frequencies in this range, so which to choose? The answer is to pick one that would be recognized anywhere in the universe. SETI researchers consider two in what is called the "watering hole" as the most likely. That is the frequency of neutral hydrogen at 1,420 MHz and the Hydroxyl (OH) radical frequency at 1,721 MHz. Since Hydrogen is the most common element in the universe, and combining H with OH produces H₂O, these are most likely spots on the spectrum that someone will be listening to.

"Nature has provided us with a rather narrow band in this best part of the spectrum that seems especially marked for interstellar contact," said leading SETI advocate Bernard Olive. "It lies between the spectral lines of hydrogen and the hydroxyl radical. Standing like the Om and the Um on either side of a gate, these two emissions of the disassociation products of water beckon all water-based life to search for its kind at the age old meeting place for all species: the water hole. Water-based life is almost certainly the most common form and well may be the only naturally occurring form.

"Romantic? Certainly. But is not romance itself a quality peculiar to intelligence? Should we not expect advanced beings elsewhere to show such perceptions? By the dead reckoning of physics we have narrowed all the decades of the electromagnetic spectrum down to a single octave where conditions are best for interstellar contact. There, right in the middle, stand two signposts that taken together symbolize the medium in which all life we know began. Is it sensible not to heed such signposts? To say, in effect: I do not trust your message, it is too good to be true."

Lasers, neutrinos and other exotics

But radio isn't our only option. What about lasers? We're already experimenting with them for communication with deep space probes, and SETI researchers are looking for signs of someone else using them, too.

Lasers have many advantages. They're tightly focused, highly directional, and monochromatic. Using an infrared beam focused by a large mirror or array, it could transmit messages at a much higher rate than a radio transmitter. Or it could be used to distort the solar spectrum by tuning the laser in to a stellar absorption band, which would look like an artificial spectral line to an alien astronomer. This would certainly gain attention and it could be made to wink on and off to send messages.

If we want to go further afield, we could use High Energy Particles (HEPs), including gamma rays, neutrinos, gravitons, and tachyons. Some are little more than theoretical, and others pose technological barriers that may never be surmounted, but they potentially have tremendous advantages.

Neutrinos, for example, would be an ideal communication medium. Sixty-five billion neutrinos emitted by the Sun pass through every square centimeter of the Earth every second and hardly any of them are stopped by the mass of the planet. If we could generate and detect them easily, they would provide us with a transmitter of unlimited range that would be almost impossible to block.

Going even farther afield, we could turn the Sun into a giant beacon by changing its spectrum directly by dumping about 400 tons of some man-made element into a heliocentric orbit. If someone light years away looked at the solar spectrum and saw something like technetium present (an element not found in nature), they'd certainly take notice. Alternatively, Philip Morrison once suggested placing opaque clouds in orbit around the Sun in a pattern that would make it seem to blink or even spell out short messages.

Oddly, one thing we don't have to worry about for the moment are inadvertent messages. While aliens aren't watching I Love Lucy, they could still be able to see the radio spectrum of our planet, and until recently it would have seemed very odd. Due to analog TV broadcasts and military radars, for much of the 20th century the Earth had a temperature in the radio band of the spectrum of 300 K (27° C, 80° F). In the absence of those artificial transmissions, for that to be true the Earth's black-body temperature would need to be 40 million degrees, which is about seven thousand times hotter than the surface of the Sun.

But during the past 20 years, television has switched over to digital, which requires much smaller bandwidths, and more efficient radars have been developed, so the Earth is currently dark. But don't get too comfortable. As we start moving out into the Solar System, there will be significant deep space traffic being tracked and tight communication beams transmitted, so things will get noisy again over the next generation.

How to write a message

So, we have our transmitter, but what do we say? To avoid the first conversation between worlds descending into awkward small talk about the weather, we need to come up with a message that's worth listening to. More importantly, it needs to be one the recipient will understand.

It also needs to be a message that will be recognized as a message. It has to be unambiguously artificial and distinctly different from natural sources. This isn't as easy as it sounds. A regular, repeating pattern in a radio signal may seem like an obvious beacon being transmitted by intelligent life, but radio astronomers keep being caught out by natural phenomena.

For example, on November 28, 1967, Jocelyn Bell Burnell and Antony Hewish at the Mullard Radio Astronomy Observatory in Cambridge, England observed a sequence of pulses coming at intervals of 1.33 seconds from the same point in the sky. Though a number of explanations were put forward to explain it, the idea that it might be artificial was reasonable enough for the signal to be nicknamed LGM-1, for Little Green Men.

LGM-1 turned out to be the first pulsar to be discovered, and it highlighted a problem with interstellar communications. Just because something is repeating, regular, or forms an obvious pattern, doesn't mean there's an intelligence behind it. Nature is filled with such things and, as the saying goes, although rare things occur rarely, it is also true that rare things occur rarely.

Imagine, as one philosopher put it, that you're on a train from London to Cardiff. As you look out the window, you see a scattering of white stones in a field on a hillside. Is this an intelligent message? No. Stones show up in places all the time. But what if the stones form a pattern, like a series of lines or a triangle? It might be a message, but there are any number of processes that might arrange stones in an orderly manner that don't require human intervention.

Now imagine that the stones spell out "Welcome to Wales." Is this a message? This is much more likely because stones don't generally form words or phrases, but it's not outside the realms of possibility that it's some remarkable coincidence or that we're imposing our assumptions on what we see.

But what is the stones spell out "Welcome to Wales" and the hillside is just inside the Welsh border? Now the stones aren't just forming a pattern, they're expressing an actual true fact that we're able to identify. It is a message and the probability of it being otherwise is infinitesimal.

So, if we are sending a message by radio, it has to be clearly artificial and it proves this by conveying verifiable facts that the recipient can recognize and understand. The problem is that this assumes some common frame of reference. If it hadn't been for the Rosetta Stone repeating the same message in both Greek and Egyptian, hieroglyphics would still be as impenetrable today as they were three hundred years ago.

We need a similar Rosetta Stone and since there aren't any monuments written in both English and Betelgeusian, we need something that is truly universal – science and mathematics.

Our cosmic message must be simple, but it must also show intelligence, so it can't be just a repeating series of radio pulses. Instead, these pulses can be used to form binary code to convey data. It might be a series of binary numbers equivalent to 1,2,3,5,7,11 for the first prime numbers, or 1,4,9,16,25 for the first squares, or 3,1,4,1,5,9 for pi, or any of a number of other things.

This would certainly tell ET that we're here and we're intelligent, but our message can't just be a string of simple numbers or it will go down as the most frustrating communication of all time. Our message has to be long. Much, much longer than these simple sequences meant as just a way to get attention. The first part might last only a couple of hours or days. The rest of the message would carry on for months.

Basically, what we're doing is an exercise in anti-cryptology. Where a cryptographer comes up with ways to make a message harder to read or even find, we're making one that's as easy as possible to read, yet will still hold the reader's attention by actually saying something worthwhile.

We could do this by splitting up the message into three types with each type alternating with the other two. To make sure we're making up for data lost through interference or to take into account those who started listening in the middle of the message, everything would be repeated several times and perhaps on several neighboring frequencies.

The first type is made up of numbers, physical constants, arithmetic, mathematical concepts, formulae, common scientific facts, and a vocabulary. The second type would be language lessons including syntax, grammar, ideas, logic, sentences, paragraphs, and abstract concepts. Of course, this wouldn't be in English, but more of a kind of binary Pidgin that would be intelligible to both parties.

The third type would be what we actually want to say and we'd only be limited by bandwidth and our own perseverance. We could send a very focused message, the complete sum of human knowledge in a giant encyclopedia, or we could, as the astronomer Fred Hoyle once suggested, send them instructions on how to build a computer and a copy of the software to program it with, creating a kind of electronic ambassador.

The first message

What we've discussed so far is what we could do if we wanted to get into some serious interstellar messaging, but it isn't theoretical or a someday thing. In fact, Earth started sending messages into space almost half a century ago.

The first radio message to be beamed at the stars went out on November 16, 1974 using the 1,000-ft (305-m) dish antenna at the Arecibo Observatory in Puerto Rico as part of a ceremony to inaugurate a major upgrade. That day, under the eye of then-director Frank Drake, at 17:00 GMT the great dish was aimed at Messier 13 (M13) in the constellation of Hercules.

M13 is a globular cluster made up of 300,000 densely packed stars about 25,000 light-years from Earth, but it's still within range of Arecibo, which is sensitive enough to detect a television station at a range of 1.8 light-years, BMEWS radar at 18 light-years, or its duplicate on the other side of the galaxy.

Set to 2,388 MHz, the signal shot out in a tight beam with 2 x 10¹³ watts behind it for two minutes and 49 seconds as 1,679 frequency pulses or bits modulated between two different frequencies to create binary code at 10 bits per second.

Unsurprisingly, 1,679 was not a number pulled out of a hat. It was very carefully chosen by Drake, who wrote the message itself before sending it to his colleague Carl Sagan to see if he could decipher it. One thousand six hundred and seventy-nine is the product of two prime numbers, 73 and 23. This is a vital clue to anyone or anything that intercepts the Arecibo Message, as it's now known, which we've reproduced here.

Being the product of two prime numbers tells the recipient to set the binary numbers into a square 73 bits on one side and 23 bits on the other. There are only two ways to do this. One produces nothing but gibberish. The other forms a very low-resolution image, which is very clear if the binary ones and zeros are replaced with dark and light squares.

The Arecibo message is short, but it includes a lot of information about humans and the Solar System. The top section (colored here in white for clarity, though there is no color in the message) are the numbers one to 10 in binary with a "least significant digit" marker to show where the number begins. Below this, in purple, are the atomic numbers for hydrogen, carbon, nitrogen, oxygen, and phosphorus, which are the basic constituents of DNA.

In the next section, in green, are the formulae for the sugars and bases that make the nucleotides of DNA. These are in the form of sequences of the five elements previously described. Below this are a pair of spirals, in blue, representing the structure of DNA and a center bar, in white, that is the number 4.3 billion in binary, which is the number of nucleotides thought to make up human DNA in the 1970s.

The next section is a bit more obvious, with a stick figure, in red, in the center. Next to it is a bar, in blue, with the height of the average man represented in binary as 14 times the wavelength of the message (126 mm times 14 equals 1.7 m (5.8 ft). On the other side is the size of the human population in 1974 (4.3 billion)

Next, in the yellow, is a chart of the Solar System with a rough representation of each planet's size. The symbol for Earth, which sits directly under the stick figure and is indented towards it to show a connection.

Finally, at the bottom of the image, is the outline of the Arecibo telescope and the binary representation of its diameter as a multiple of the message wavelength.

Because the Arecibo Message was really a stunt to show off what the telescope could do, it may have been much shorter than the ideal message we outlined above, but it still tells any recipients a lot about us. It shows a common numbering system and implies that we use a decimal system. It also tells them that we are carbon-based lifeforms, that our genetic structure is based on DNA, and something of our biochemistry. The message also shows that we're bipeds and our size tells them something about Earth's gravity. In addition, they know something about the structure of the Solar System and the nature of our technology.

Are we better to remain silent?

Since Arecibo, there have been about 12 other attempts to send messages to other civilizations, though none have been very long or repeated too many times. Part of the reason there have been so few and such modest attempts has been partly insufficient radio telescope time, but also the firm opposition of most of the astronomical community to sending such messages at all.

Though the idea of communicating with extraterrestrials has grown in popularity with the public, the SETI field has faced increasing difficulties in getting funding after nearly 60 years of failure, to the point where many researchers regard SETI, while laudable, as a pseudoscience without a subject and without a testable hypothesis.

According to astronomer and science fiction author David Brin, the strenuous efforts of some SETI researchers to keep the organization from being identified with UFOs and little green men has "pushed away a field that was very kind to them — bona fide science fiction" and walled in their community, isolating them from mainstream science. This has made some SETI proponents very sensitive and frustrated, leading some to advocate going straight from listening to shouting out the existence of mankind to the Cosmos in hopes of spurring a reply.

The problem is that one never knows who is going to get the message. They could be one of Sir Arthur C Clarke's godlike, totally altruistic beings; friendly, logical Vulcans; H G Wells' ravening Martian hordes bent on conquest; or C S Lewis's demonic creatures motivated by pure evil. It's this uncertainty that makes most astronomers prefer that sending any message should wait until the matter has been thoroughly discussed at the very least.

The Physicist Stephen Hawking, in an interview with the Sunday Times, said that if the human race is anything to go by, it would be better to remain silent.

"We only have to look at ourselves to see how intelligent life might develop into something we wouldn't want to meet," says Hawking. "I imagine they might exist in massive ships, having used up all the resources from their home planet. Such advanced aliens would perhaps become nomads, looking to conquer and colonize whatever planets they can reach."

Even early studies of the 1960s and '70s said that one of the top three criteria for a civilization becoming an active transmitting one was having the technology and resources to fend off an alien invasion force or other military threats.

Probe ambassadors

So does this mean that caution dictates that we never try to communicate with another civilization? Not necessarily. There is an alternative – one that we've already used to send messages to the stars. It's slower than radio, but potentially much safer. This surprising competitor was actually the very first to carry a message addressed to some unknown extraterrestrial civilization years before Arecibo. The first will take tens of thousands of years to reach even the distance of the nearest star, if they ever do. Yet in this cosmic tortoise vs. hare race, it has some surprising advantages.

On March 3, 1972, Pioneer 10 lifted off from Cape Canaveral, Florida. Along with Pioneer 11, launched 11 months later, these unmanned deep space probes were tasked with making flybys of Jupiter and Saturn, setting them on a hyperbolic slingshot trajectory that made them the first spacecraft to ever set out from the Solar System, never to return.

Not wanting to pass up the opportunity to create the most audacious messages in a bottle ever tossed into infinity, NASA turned the Pioneer probes into Earth's first cosmic emissaries by tacking a gold-anodized aluminum plaque to each one. Measuring 9 x 6 in (229 by 152 mm), these plaques were engraved with a pictogram that may one day become the most important postcard in history, if by some miracle it's ever found.

First suggested by journalist Eric Burgess and designed and constructed in three weeks by Carl Sagan, his then-wife Linda Salzman Sagan, and Frank Drake, the plaque is the icing on the cake for Pioneer. Since the aliens will already have the inert probe to study at their leisure, the plaque's job is to provide a bit of context as to where this mysterious spacecraft came from and who sent it.

The plaque shows the outline of the Pioneer probe, in front of which stand the nude figures of a man and a woman. The man's hand is raised in a gesture of greeting, while the woman has one foot set slightly forward to give some idea of how humans move. Below them is a representation of the Solar System with an arrowed line showing that the Pioneer came from the third planet from the Sun. To one side is a strange diagram of spreading lines with binary symbols next to each one, while above this is a figure of two circles separated by a line.

The two humans are probably the most difficult figures to decipher, since we have no idea how much of how we see two dimensional representations is universal and how much is peculiar to us. But the rest of the plaque should give the finders the ability to deduce some basic information about us.

The two circles at the top are a schematic of the "hyperfine transition" of neutral atomic hydrogen. That is, when the spin of the electron and the proton in a hydrogen atom align shift to when they are opposed, which is when the atom emits radio waves at 1,420 MHz. Since hydrogen is universal, this fact should also be universally known.

The clever bit is that 1,420 Mhz is a wavelength of 21 cm and that gives us and the aliens a common yardstick. Underneath the line connecting the two circles is the number one in binary code. Next to the two humans is the number eight in binary, which tells the aliens that humans are 8 x 21 cm tall – a figure that is confirmed by comparing them to the height of the spacecraft drawn behind them. Since the aliens should have the probe as well as the plaque, this allows them to double check their deductions.

The Solar System diagram also includes binary numbers under each planet showing their distance from the Sun.

As to the enigmatic spider next to the humans, this is a map showing the relationship between 14 pulsars identified by binary representations of their periods, with a 15th line showing the distance between Earth and the center of the galaxy. Since the periods of pulsars are precisely measured and their rate of slowdown is also known, the aliens should be able to pinpoint the date of the probe's launch to within 100 to 1,000 years and our position to within 60 light years. Not exactly a GPS fix, but at least it would get them close enough to find a gas station and ask for directions.

"The Pioneer plaques are destined to be the longest-lived works of mankind," said Sagan and Drake in 1975. "They will survive virtually unchanged for hundreds of millions, perhaps billions, of years in space. When plate tectonics has completely rearranged the continents, when all the present landforms on the earth have been ground down, when civilization has been profoundly transformed and when human beings may have evolved into some other type of organism, these plaques will still exist. They will show that in the year we called 1973 there were organisms, portrayed on the plaques, that cared enough about their place in the hierarchy of all intelligent beings to share knowledge about themselves with others."

The Pioneer message is in some ways as simple as the later Arecibo Message, but in other ways is more complex. It has only a few sections intended to convey a few pieces of information, but the resolution is higher than what could be included in a radio message and the pulsar map goes a step further by telling the recipients where and when the probe came from.

Voyager

In August 1977, the next level of SETI messaging went into space with the first of the two Voyager probes. Voyager 1 and 2 were larger and more versatile spacecraft tasked with a more ambitious mission. Their trajectory not only sent them to Jupiter and Saturn, but also to Uranus and Neptune, then set them on a velocity that is sending them out of the Solar System before their predecessors.

Even 40 years later, the two spacecraft are still partially functional and will continue to operate until the nuclear power system runs out sometime between 2025 and 2030. But even after their electronics go cold, the Voyagers will still have a job as carriers of the most ambitious space message sent so far.

Mounted on the fuselage of each Voyager's main section is the Golden Record. It's actually a 12-in (30-cm) copper gramophone record plated in gold and sealed in a gold-electroplated aluminum cover. The latter includes an ultra-pure sample of radioactive Uranium 238, which has a half life of 4.468 billion years and provides the finder with an accurate way to calculate how much time has elapsed since Voyager left Earth. To back this up, the cover is etched with the same pulsar map found on Pioneer. Like any user-friendly product, the cover includes an operating manual showing how to use the record as well as a phonograph needle to play it.

Where Pioneer was a simple message, the Voyager record is a flat-out information dump selected for NASA by a committee led by Carl Sagan. Along with lessons in number system, units of measurements, and biochemistry, the record contains 115 images, greetings in various languages, sounds of everyday Earth life, and 90 minutes of music from around the world.

The images are stored using a simple analog technique developed in the 1920s as a way of recording television on audio records. Since television is a made up of a series of still images, this turned out to be an excellent way to store relatively high definition images. The first image is a simple circle that acts as a calibration aid for the finder.

This is followed by a solar location map; mathematical and physical unit definitions; a tutorial in human biochemistry, anatomy, and reproduction; information about the planet Earth and its structure; images of terrestrial geology, climatic regions, animal life, and plant life. There's also a large compendium on human life, activities, architecture, eating, technology, and music, as well as printed messages from US President James Carter and UN Secretary General Kurt Waldheim.

It will be a good 40,000 years before any of these probes comes within two light years of any other star systems and odds are that none will be found for millions or even billions of years.

AI ambassadors

But what does this slowpoke approach for sending messages have over light-speed radio signals? Not much at the moment, but while we're at the physical limits of what radio can do, interstellar travel still has a long way to go. At the moment, we're limited to using primitive chemical rockets or ion drives that aren't really suited to the task of jumping between the stars, but that could well change one day, if we're patient enough.

In 1964, Soviet astronomer Nikolai Kardashev came up with a way of classifying civilizations based on how much energy they are able to harness. A Type I civilization is limited to the power available on a single planet – about 4 X 10¹² joules. A Type II would be able to use the output of an entire star, which comes out to 4 X 10²⁶ joules. Meanwhile, a Type III civilization would have the output of a galaxy at 4 X 10³⁷ joules.

The more power a civilization has at its disposal, the more efficient it becomes. One interesting point that SETI scientists have found is that when a culture reaches the point beyond a Type I civilization, the difference in efficiency between sending radio messages and sending unmanned probes becomes negligible.

In 1960, Ronald N Bracewell of Stanford University put forward a proposal for using robotic probes rather than sending radio messages as our way of opening contact with other beings. Let's look an updated version of his idea.

Imagine it's a few centuries from now when the energy problems of today seem as quaint as a flint or deer antler shortage in Neolithic Britain. Humanity now has so much surplus energy at its command that sending an interstellar probe seems no more farfetched than sending a probe to Pluto does in our day.

But these are far more advanced spacecraft than any we have today. They are larger and more powerful. They are self-refueling, self-repairing, and can even duplicate themselves as required using advanced 3D printing techniques. They are also fully autonomous with computers that have an almost organic level of artificial intelligence.

These probes aren't very fast, reaching only 10 percent of the speed of light, but they don't have to be. With no passengers or crew, they can afford to spend a few decades or even centuries getting to their destination. As they approach the candidate star selected by mission control, each probe has the ability to study the system in detail, identify the planets most likely to possess intelligent life, and make an assessment of whether to proceed or carry on to a more promising system.

If a planet does turn out to have a civilization advanced enough to make contact with, the probe would be programmed to discretely stand off and listen to radio, television, and data transmissions. Unlike trying to pick up signals from light years away, the probe could do so from only millions of miles or might even send in scouts to orbit the planet for a closer look and listen.

Already the advantages of sending out such a system become obvious. The Bracewell probe would be intelligent enough and programmed with enough precautionary algorithms to determine if the civilization in question is safe to contact or whether it could be more in Earth's interests to stay silent. It could even remain on station for decades or even centuries as it sends back reports to Earth.

The same watching brief might even apply if it finds a civilization that hasn't reached a high enough level of technology to communicate with. It could patiently wait and watch as it evolves, then decide whether to communicate as soon as it starts receiving radio transmissions. Or it could leave behind an artifact, as in 2001: a Space Odyssey, that would inform Earth if it was ever disturbed, while the probe itself moves on to more productive targets. Or it might duplicate itself and send the new one on.

If the probe did decide to make contact, it would be in a far better position than someone trying to start a conversation across 100 light years of space. For one thing, the probe would have no trouble making its presence known by blasting a powerful signal at the planet, perhaps re-broadcasting television programs with a six hour delay on the same frequency as the original broadcasts to make clear that this isn't some kind of an echo.

When contact is established, communications would be clear with a minimum of interference and responses would be received in real time. In addition, a probe orbiting the planet would have huge bandwidth at its disposal to send and receive a very large amount of data – much of it in the form of video.

It would also be a simple task for the probe to instruct the natives on how to build compatible transceivers for the most efficient exchanges or to speak directly to Earth. On the other hand, the probe could act as a gatekeeper by relaying messages to Earth and censoring information, like our location, if the indigenes prove untrustworthy.

Because the probe has artificial intelligence, it can adapt its communications to suit the recipient. It could indulge in true conversations with the natives, asking questions and being asked questions in turn as an exercise in both teaching and learning. It could even provide language lessons with suitable feedback. It might even be able to connect directly to the planet's version of the internet and use deep learning to better understand the culture or even to communicate directly with individuals.

Indeed, these exchanges could make such a probe, in the broad sense, profitable. Instead of blasting energy into space from Earth with no known return on the investment, the probe could send its findings to Earth much more economically. In fact, unlike radio signals, an autonomous probe program would continue operate long after it had been abandoned back on Earth.

It might even act as an insurance policy for our civilization. If Earth is destroyed, then at least our culture might live on – if only as a record, albeit an intelligent record. Or the probe could be programmed with a wide sampling of the human genome and supplied with information on how to construct a biological printer that would allow it to build human cells and clone them. Scientists are already doing this with simple viruses, so it may one day be possible with Homo sapiens. It may even be possible to bioengineer the ova at code level to adapt the colonists to their new environment.

Taking things a step further, there's no reason why the probe must be mechanical. Today, scientists are able to encode images and even videos on bacterial DNA. Perhaps, in time, some sort of microorganism could be developed with complex messages encoded on it as a form of self-replicating courier placed in small probes or turned into spores and carried on the solar winds into the galaxy. Maybe one day our first message won't be heard over the radio, but seen through a microscope.

Today, we have a lot more experience beaming messages into deep space as our unmanned missions probe the edges of the Solar System and beyond. We know more about how to send and receive data with a minimum of wattage, how to use tight-beam radio, and how to carry out precise tracking of space objects. We're even experimenting with laser communications in deep space and looking at new ways to send spacecraft to the nearest stars.

But whatever methods we may adopt in the future, our first messages are already on their way and we're waiting for the reply. The odds are long, however, and we won't hear much for about 50,000 years, so there's time to put the kettle on.