We have sent four maps, each plotting a course to Earth, out into the deep dark void. You can find them on Pioneer 10 and 11 – launched in 1972 and 1973, respectively, to study the gas giants and the asteroid belt – and Voyager 1 and 2 – launched in 1977 to study the outer Solar System.
The maps, which were part of a range of anthropogenic designs, information or schematics sent into space, are specifically there for the purpose of finding the pale blue dot we call home. The rather romantic idea is that if intelligent life were to ever find them, they’d know how to come over and say hi, or at least beam a transmission our way.
Frank Drake, an American astronomer who pioneered the first modern search for extraterrestrial intelligence (SETI) back in the 1960s – as well as a controversial equation that predicts the likelihood of alien life existing elsewhere in the universe – designed these maps.
Speaking to his daughter for National Geographic for the 40th anniversary of the Voyager probe launches, he points out that a few scientists out there are little concerned about the consequences of these cosmological cartographic creations.
“In those days, all the people I dealt with were optimists, and they thought the ETs would be friendly,” Drake says. “Nobody thought, even for a few seconds, about whether this might be a dangerous thing to do.”
The idea is that these maps could one day be intercepted by a hostile alien force, which could then come and destroy us, or harvest us, or engender some sort of apocalyptic event. According to some planetary researchers, we should just be listening, not sending out messages into the unknown.
So what are the odds of aliens actually finding these four spacecraft and using them to plot a course to Planet Earth? Let’s take a look.
First, we have to assume these aliens are pretty good at reading maps. As you might imagine, what looks like a map to us won’t look like a map to an octopus – so an alien life form will probably not use coordinates and directions in the same way we do.
They certainly can’t use constellations, as they’ll look different from every single angle – and, to be honest, they’re so arbitrary that what we sketch out as a stellar cat could easily be rearranged to be a space dog, or a Milky Way monkey or something. So what else is there?
Carl Sagan and Drake had a chat about this when they were designing the plaques for the Pioneer probes. They reasoned that they could use pulsars to mark the position of Earth, and indeed, that’s what they plotted out on the maps.
Pulsars are the highly magnetized, rapidly spinning remnants of either a neutron star or a white dwarf. First discovered in 1967 by Jocelyn Bell Burnell and Antony Hewish, they were found to rotate and send out beams of intense radiation into the cosmos at very predictable rates for many millions of years. You can literally set your watch by them, and in a way, they are the lighthouses of the shadowy star ocean.
Assuming that any spacefaring aliens can see them, they would recognize them in the night sky. Along with the binary code attached to each pulsar – indicating how far or close it was to us – aliens could definitely use these to work out how to get to the Sun, and then find us.
So the maps check out. All these hypothetical space squidlings need to do is catch one of these four probes – so how likely is this?
Well, to quote the late, great Douglas Adams, “Space is big. Really big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist, but that's just peanuts to space.”
It’s arguably infinite in size, and the space between planets and stars is only increasing by the day. So in order to make it likely that a passing alien starfighter or freighter will spot one of these probes, there needs to be a hell of a lot of them out there.
A recent study used the aforementioned Drake Equation to calculate how likely it was that an advanced alien civilization existed on any such planet. Using the most cutting-edge data, the researchers took a look at a variety of variables, including the rate of formation of stars suitable for the development of intelligent life, and the fraction of life bearing planets on which intelligent life emerges.
They estimated that the chance of humanity being alone in the Milky Way is less than one-in-a-quintillion, or one in a billion billion. So it’s very unlikely we are alone in our galaxy, but let’s zoom in a little.
Even at the ridiculous speeds that these probes are moving at – Voyager 1, for example, is moving at speeds of 17 kilometers (about 11 miles) per second – they haven’t managed to get very far. This spacecraft is the furthest man-made object in the universe right now, having breached the heliosphere back in 2012, the outermost boundary of the Sun’s solar winds and the Solar System as a whole.
Right now, it’s about 21 billion kilometers (13 billion miles) from the Sun. That, pathetically, means it’s still in our galactic neighborhood. At this rate, it will take 40,000 years to reach the nearest star along its journey.
So what are the odds there are aliens in our galactic neighborhood? Well, the team from earlier worked this out too. Even at the most optimistic odds – that advanced alien life has a one-in-10,000 chance of existing – we’re still likely the first galactic civilization in our region of the galaxy.
So when you combine that with the fact that aliens would have to spot this probe moving at a breakneck speed through the darkness merely by chance, the odds are vanishingly small – essentially impossible. It’d be difficult to catch anyway, and it’s more likely to eventually slam into something and shatter into a trillion pieces.
When you extend this to the entire Milky Way, with the same odds, we are one of roughly 6 million civilizations out there. That sounds like, perhaps eventually, Voyager 1 will come across some benevolent or malevolent aliens.
Let’s assume that there are 6 million civilizations out there, living on their Earth-sized worlds, and let’s assume Voyager would have to crash-land on the planet for the map to ultimately be seen – and that Voyager 1 is passing through the galaxy rather than escaping it.
Earth has a volume of around 1.1 trillion cubic kilometers (0.26 trillion cubic miles). If there are 6 million Earth-sized civilizations out there, then that equals a total volume of 6.6 quintillion cubic kilometers (1.6 quintillion cubic miles.)
The volume of the Milky Way, based on its average thickness at the center and its length, is around 6.7 x 1051 cubic kilometers (1.6 x 1051 cubic miles) in volume, a staggeringly huge number.
Ultimately, this means that these 6 million alien civilizations make up just 0.000000000000000000000000000000001 percent of the entire space of the Milky Way. There’s basically zero chance that Voyager 1 will ever see anything other than emptiness – so in this regard, Earth will never be found.