What happens in outer space is a never-ending source of mystery and wonder. Black holes, pulsars, anyone? Cepheid variables, globular clusters, supernovas? But despite the awe-inspiring physics behind some of those, despite what they have taught us about the universe, despite the visual spectacles some offer—arguably the most beguiling question of all is an ancient one indeed: “Are we alone?"
Variations: Is there life somewhere out there? Another civilization? Technology comparable to ours?
Decades ago, we humans started sending spacecraft soaring through space that, among other things or maybe even peripherally, sought to answer such questions. I don’t mean the Sputniks and Apollos and space shuttles, valuable as all those were in their own ways. The missions I am referring to are intended to soar indefinitely, never to return to Earth. They go by the names Voyager, Pioneer, and New Horizons.
Here’s a quick rundown of some of these. Pioneer 10 was launched in March 1972. In late 1973, it began sending to Earth images of Jupiter and eventually got to within about 130,000 km of that giant planet. Over the next decade, it flew through the outer reaches of the solar system. It sent back ever-weaker signals till 2003, when it was about 12 billion km from Earth and the signals died out. Pioneer 11 followed about a year after Pioneer 10, also flying by Jupiter and to within 21,000 km of Saturn’s surface. Its last signal reached Earth in 1995, from a distance of about 6 billion km. Voyager 1 and 2 were launched in 1977, to study the large outer planets of the solar system. Both have sent us beautiful images of those planets and their moons.
By now, all these spacecraft have left the heliosphere—essentially, the outer edge of the Sun’s “atmosphere", about 18 billion km from us. They are now in what’s called “interstellar" space. There’s no realistic prospect that they will encounter and photograph objects like Jupiter and Saturn in our lifetimes. But we still get valuable scientific data. For example, in 2020, both Voyagers detected and reported to Earth a definite increase in density of matter around them. That confirmed a conjecture about the particular region in space they were traversing. We should keep getting their livestreamed bits for several more years.
But we were discussing life out there. Remember, these missions have been transmitting data back to Earth, encoded of course as streams of 1s and 0s. We pick these streams out of a blur of background “noise" from everywhere else in space. That is, the data from these missions has definite meaning and is identifiably not just more cosmic noise. But “identifiably" to whom? Certainly to us on Earth, but then we designed the format that data appears to us in and can recognize it. Who else? In particular, what if a sufficiently advanced civilization detected the Voyager signals? Would it know, like us, that this wasn’t just more noise? Would it know that it had to come from another source of life and technology?
How would it respond? When would it respond? And if it does respond, how will we receive and understand whatever it chooses to send our way?
These are not just idle questions. In effect, they have intrigued scientists ever since we first started transmitting radio and television signals around the world. It’s the nature of such signals that you can’t confine them to our planet. Just by being broadcast, they also travel out into space. Admittedly, they are pretty weak as they travel the universe, and the idea that an extra-terrestrial civilization might detect such a signal is somewhat fanciful. Yet the astronomer Carl Sagan thought it was plausible enough to make it an integral part of his novel Contact. The opening ceremony of the 1936 Berlin Olympics was an early global TV transmission, and in Sagan’s story, aliens intercept it and reflect it back at us. Thus our first contact with another civilization comprises, several decades after he died, images of Hitler. Easily understood here on Earth, if probably startling.
But we were discussing Voyager and its cousins. Signals from Earth to them and vice versa are necessarily stronger than a TV broadcast is. They have been zipping through space for decades now. There are some natural questions, then: What are the stars these signals could have reached? And if they do encounter some intelligent life that can respond, when’s the earliest we can expect such a response?
Well, last year a Nasa study tried to answer just those questions. It uses the so-called “Gaia Catalog of Nearby Stars" (GCNS). This is a list of over 300,000 stars in our “solar neighbourhood"—within 326 light years from us. To be sure, that’s a vast, vast neighbourhood. Yet on the scale of the universe, it is vanishingly small: our Milky Way galaxy alone is about 100,000 light years across, and the farthest-known object in the universe is a galaxy that’s over 13 billion light years away.
Still, the Nasa study considers “the first stars [in the GCNS] that will be reached by the transmissions of each spacecraft, including some stars that have already been encountered" (The Breakthrough Listen Search for Intelligent Life: Nearby Stars’ Close Encounters with the Brightest Earth Transmissions, Reilly Derrick and Howard Isaacson, 18 April 2023). The calculations are intricate, involving the trajectories of the spacecraft as well as the stars (the “ephemerides" of these objects).
In this way, the study found that Pioneer 11 will “encounter" 411 stars in the GCNS, Voyager 2, 325; Voyager 1, 289; Pioneer 10, 241; and New Horizons, 142. Most of these encounters are in the future, though. Voyager 1 will contact its first star only in 2044, New Horizons in 2119. On the other hand, transmissions to Voyager 2 and the Pioneers have already encountered a total of four stars. Pioneer 10 was the first, in 2002. Voyager 2 had two encounters in 2007. Pioneer 11 had one in 2018.
And if by some chance there’s something out there listening and reflecting these transmissions, it’s the Pioneer 10 signal we can expect a response to soonest, in 2029. After that, from the two Voyager encounters, in 2031 and 2033. Two more this century, but decades later. Of course, as the years go by, these probes’ signals will encounter more and more stars, as also any planets that orbit them and might harbour life.
Still, the chances of a response are low. But even so, it’s getting to be time that we start listening. Tell your children and grandchildren, and to tell their children and grandchildren.
Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. His Twitter handle is @DeathEndsFun.
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