What would you say to an alien civilisation on an Earth-like planet far, far away?

"Greetings from a girl on Earth who, every so often, looks up at the night sky and waves hello in the hope that someone on another planet is doing the same."

This message from Sophie of Longmont, Colorado, in the United States, is just one of almost 26,000 sent from Australia to an Earth-like planet 20 light-years away.

It's been a decade since NASA transmitted these goodwill messages, and this week the transmission passed the halfway mark on its long, lonely journey through the silent cosmos.

The project, called Hello from Earth, began as a science communication campaign to get people excited about Australia's National Science Week.

Those of us running the annual 10-day event were looking for an idea that would create a buzz on social media.

We decided on a kind of "Twitter to the stars". We would collect short messages from the public and transmit them to the nearest habitable planet beyond our solar system.

Each message would be short, later packaged into a single transmission and sent using one of NASA's facilities.

Our target was Gliese 581d, a "super-Earth" orbiting the habitable zone of its parent star.

First detected in 2007, studies in 2009 suggested it could have large oceans.

And since it was 20.4 light-years away, it would help give people a real appreciation of just how big the universe is.

"If you come to Earth, look into: music, the beach, ice cream, hugs, family, love, dancing, cheese, trampolines, friendship, books and dreams. Just for a start." — Tamasin, Richmond, Australia

'It might trigger an invasion'

When I suggested the idea, the bureaucrats involved with National Science Week were intrigued, if a little sceptical, but asked me to explore it.

In the months that followed, I had conversations with sometimes quizzical senior CSIRO staff, leading astronomers and US government officials, negotiating terms and agreeing to specifications.

We hoped sending messages through the cosmos would get people excited about the International Year of Astronomy. ( Supplied: Canberra Deep Space Communication Complex )

Surprisingly, we didn't need approval to transmit an interstellar message — but we would have if we wanted to respond to an extraterrestrial signal.

You can understand why: if an extraterrestrial signal is received, you can't have everyone with a high-gain antenna answering back.

So who speaks for Earth? That turned out to be the SETI Post-Detection Subcommittee, which at the time was chaired by astronomer Paul Davies of Arizona State University, an old friend and former colleague.

"What do you think?" I asked in an overnight phone call after explaining Hello from Earth.

"Will we breach any unwritten rules in the scientific community?"

"Well, there's no statute covering interstellar messages, and no-one has jurisdiction over transmissions," Davies said from his home in Tempe, Arizona.

"But it will upset some people."

"Some scientists believe that sending such messages alerts potential extraterrestrial civilisations of our existence, which they consider unwise and potentially catastrophic."

"Seriously? What, it might trigger an invasion?" I chuckled, if a little awkwardly.

"But we've been broadcasting our existence since the 1930s, haven't we? Television transmissions from every city go into space every day, blanketing the whole sky as the Earth turns. Military radar is even more powerful, and probably detectable for, what, 100 light-years?"

"That's largely true," he said. "Nevertheless, there's a debate. It's a minority of the scientific community, but it exists."

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Despite the debate, Davies said the academy had no official opinion on interstellar messages. And personally, he didn't object.

"There's a 'radio bubble' emanating from Earth in all directions, and it's almost 100 light-years in diameter. Extraterrestrials with technology advanced enough to threaten us would also have much better antennas and would know we're here."

So, no jurisdiction but no objection. Just the infinitesimal chance that Hello from Earth might prompt an invasion fleet from Gliese 581d.

"Hi there. Sorry about the Outer Limits; hope you enjoyed I Love Lucy. Have you got all our missing socks? Love, Earth." — Fred Mason, Roberts Creek, Australia

A history of interstellar transmission

It was the Arecibo Radio Telescope in Puerto Rico that inspired me to propose Hello from Earth.

That telescope sent the first intentional interstellar message in 1974 — a three-minute broadcast aimed at the M13 globular star some 25,000 light years away.

But it turned out that I wasn't the only one it inspired: I was working on the 20th interstellar transmission, and there have been another 11 since.

Some have been serious attempts to hail alien civilisations, but most have been science communication exercises aimed at fostering public engagement.

One was sent in 2008 by a large 70-metre dish outside Madrid to commemorate the 50th anniversary of NASA.

It also happened to be the 40th anniversary of the recording of the Beatles song, "Across the Universe", hence it was selected for transmission — with approval from Paul McCartney, Yoko Ono, and Apple Records.

The song was transmitted to Polaris, which is 431 light years distant and unsuited for life, as it is a triple-star system anchored by a yellow supergiant.

"If someone is reading this, I hope that our children will someday have the privilege of meeting one another." — Tegan Larsen, San Antonio, United States

Collecting messages from Earth

NASA Administrator Charles Bolden Jr eventually approved the Hello from Earth proposal, just eight days before the start of National Science Week.

In those eight days, I hired a company to fast-build a website that allowed people to register via email and upload a message that would appear on Hello from Earth, as well as be collected for transmission.

We leased a dedicated server at a large-scale data centre in Sydney and built a website, which included information about the Gliese 581 system, the 350 planets beyond our solar system we knew about a the time (we now know of 4,000), and background on the scientific thinking on extraterrestrial life.

We also assembled a team of 10 moderators around the world so that the messages uploaded could be read 24/7 and quickly accepted or declined.

Former science minister Kim Carr was a big supporter of the project and sent the first message. ( Supplied: Questacon )

The then-science minister Kim Carr launched the project at Questacon in Canberra on August 12, submitting the first message:

"Hello from Australia on the planet we call Earth. These messages express our people's dreams for the future. We want to share those dreams with you."

The reaction from news outlets around the world ranged from jovial to breathless.

"Dear alien: Can I add you to Facebook?" was the headline in The Sydney Morning Herald, while The Daily Mail cried, "Is there anybody out there? Australian minister leads nation in contacting planet Gliese 581d".

In the days that followed, the website was bombarded with visitors from all over the world and we had back-to-back calls for radio interviews from everywhere.

The messages that came in were an insight into people more than anything: there was a lot of humour, sure, but also heartfelt yearning:

"What do you see when you look up into the sky? Do you feel small and lonely, just like us? From now on, I can assure you one thing: you are not alone. Be happy." – Sergio Camalich, Hermosillo, Mexico

"Hello Baba, if you are out there I love you and hope you are watching me. I wonder if when you died you went to this planet." — Liam Oliver, Coogee, Australia

"All our petty disputes, disagreements and wars fade into insignificance when we consider our tiny world's place in the cosmos." — Silvio Zarb, Melbourne, Australia

There were rules about what messages could contain, mostly common sense: use ASCII plain text, and list no email addresses, URLs or HTML tags (the Internet may be everywhere, but it is limited to this planet).

We also stipulated that messages would be moderated and rejected if deemed inappropriate — that is, if they contained profanity, racism, derogatory comments or personal attacks.

This was also a more innocent time, and only 1 per cent of messages violated these rules.

NASA; however, insisted on a very high level of decorum: nothing remotely suggestive (including the word 'breast'!), no risque humour or anything aggressive.

To be fair, NASA had reason to be cautious. In 1973, they added a gold-anodized aluminium plaque to the Pioneer 10 and Pioneer 11 space probes — the first to leave the solar system — with an illustrated diagram in case either was found by extraterrestrials.

It depicted two hydrogen atoms, an array of lines and dashes — a kind cosmic address for our Sun on the radio spectrum of the galaxy — as well as an illustration of a naked man and woman, showing what our species looks like.

NASA received complaints from members of US Congress, and newspapers ran letters objecting to NASA "exporting pornography to the stars".

NASA was cautious after this gold plaque from Pioneer space probe drew complaints. ( Supplied: NASA )

By the end of National Science Week, the website stopped accepting new submissions, and we finished up with 25,880 messages.

We merged them into a single document, and it ran to 1,003 pages and 524,704 words.

"There is only one thing bigger than this vast universe, the desire to discover. I hope I discovered you." — T.S.M., Skopje, Macedonia

Figuring out where to point the dish

All these messages then went to NASA's Jet Propulsion Laboratory in California, where they were encoded into a binary signal for transmission. They also worked to establish where to point the radio dish.

This was no mean feat: engineers needed to not only point the dish exactly where Gliese 581d would be on its orbit around its parent star when the message arrived, but figure out where the whole solar system would be 20 years and 145.7 days in the future.

The powerful transmission is almost half-way to Earth-like exoplanet Gliese 581d. ( Supplied: European Southern Observatory )

That's because our galaxy is rotating, and all its stars move around the centre.

Our Sun — which is about 24,000 light-years from the core of the Milky Way — orbits at about 220km per second, taking 230 million years to make one revolution.

To cap it off, each star tends to bob and ebb in the cosmic ocean, often displaying its own idiosyncratic motion.

Hence, extremely careful calculations had to be made about where the Gliese 581 star system would be, as well as establishing where along its orbit the fourth planet would be located when the signal arrived.

When you think about it, it's astonishing that we can do this. Adding to the fact that it's already mind-boggling we were sending goodwill messages from a random selection of humans to a potentially habitable planet that might have a technical civilisation. Granted, that chance is highly unlikely, but it's not zero.

"MY AIM OF CONTACTING YOU IS TO SEEK YOUR ASSISTANCE IN TRANSFERRING THE SUM OF THIRTY-FIVE MILLION US DOLLARS OUT OF NIGERIA AND INTO YOUR TRUSTED BANK ABROAD." — Hapatikiatwengo, Australia

Message in a bottle

The message was scheduled for transmission from the Canberra Deep Space Communication Complex (CDSCC) on August 28, 2009 at 11.30am.

By chance, Indigenous students from Karalundi College in Meekatharra, 821km north-west of Perth, were visiting that morning.

They watched as the 70-metre dish weighing 3,000 tonnes gently slew toward Gliese 581d.

The Canberra Deep Space Communication Complex, which transmitted the Hello from Earth messages. ( Supplied: Canberra Deep Space Communication Complex )

The DSS-43 is the largest steerable parabolic antenna in the southern hemisphere and it's part of part of NASA's Deep Space Network.

There are two other facilities, one outside Madrid, Spain and another in Barstow, California. Together they are the largest and most sensitive scientific telecommunications system in the world.

The packaged messages were transmitted at a frequency of 7.145 gigahertz and a power of 18 kilowatts, and repeated twice over two hours "at a power level and frequency that will be obvious to anyone who might be listening," the then-director of the CDSCC, Miriam Baltuck, told the assembled crowd near the foot of the dish.

"That's equivalent to using the combined power of over 300 billion mobile phones at once."

It seemed fitting that the first interstellar message from Australia was being witnessed by Indigenous kids, representatives of a culture twice as old as the time it takes light to travel to Earth from the centre of the Milky Way.

Hello from Earth was probably just a 'message in a bottle' tossed into a silent interstellar ocean, a cosmic "cooee!" shouted into a long and empty night.

But, if a reply does eventually come, it will arrive decades from now.

Maybe these students — representatives of the oldest continuous culture on Earth — will be around to see it.