Game of Probes: The First Probe Sent to Another Civilization Won’t Be the First to Arrive

If we ever detect an Extraterrestrial Civilization (ETC) and start communicating with them, the messages could take years, decades, or even centuries to travel back and forth. We face a challenging 49-minute long delay just communicating with the Juno spacecraft orbiting Jupiter, and that’s well within our Solar System. Communicating with an ETC that’s hundreds of light-years away or even further is a daunting task.

It’s even worse if we’re sending probes.

Imagine if a robotic probe arrived in our Solar System, sent by an ETC. They detected us and sent their probe to introduce themselves and learn more about us. We would be shocked, and the event would change civilization’s trajectory forever. Imagine we waited for another one, and imagine that we had to wait centuries. Generations of humans would live and die, we would’ve learned all we could from the probe, and it would sit in a museum somewhere.

As humanity waited for the next probe, the scientific community would publish a flood of papers on the probe and what might happen next. A whole new field of study would be born. Politicians would stake entire careers on the issue, and opinion-piece writers, artists, and musicians would have a field day. Scams would appear, and cults might spring up.

Then imagine our sensors detect another incoming probe from the same place as the previous one. Imagine our surprise when we receive it, retrieve it, and begin to study it, only to find that it’s not as advanced as the previous one and contains older information and messages than the first one. Its technology would be less advanced, and it would be more primitive.

That’s exactly what might happen, according to Graeme Smith. Smith is a professor and an astronomer at the Department of Astronomy and Astrophysics at the University of California. He published a paper in the International Journal of Astrobiology titled “On the first probe to transit between two interstellar civilizations.

“In other words, which probes have the potential for instigating a first-contact event?”

Graeme Smith, Dept. of Astronomy and Astrophysics, U of C.

Smith points out that sending probes on journeys of such extreme distances means the first one received wouldn’t be the first one that was sent. As technology advances, we can expect probes to become faster. Eventually, a probe launched later than its predecessor would overtake its predecessor and reach the destination first.

“If a space-faring civilization embarks on a program to send probes to interstellar destinations, the first probe to arrive at such a destination is not likely to be one of the earliest probes, but one of much more advanced capability,” Smith writes. “This conclusion is based on a scenario in which an extraterrestrial civilization (ETC) embarks upon an interstellar program during which it launches increasingly sophisticated probes whose departure speed increases as a function of time throughout the program.”

The technological gap between probes could depend on different things, but it would be worse the greater the distance between us and the ETC. “The more distant the site from which an ETC is launching probes, the greater will be the technology gap between a first-encounter probe and terrestrial technology.”

A lot of work has gone into calculating how widespread ETCs might be and how far they could spread after developing interstellar travel. There are all sorts of different conclusions based on all kinds of calculations and premises. One paper examined the idea that an ETC might be able to spread through the Milky Way with self-replicating probes and that rather than using SETI to search for radio signals, we should be searching for probes.

Smith’s paper touches on similar themes. He focuses on a hypothetical ETC in the early stages of a probe-sending program with lots of technological advancement in its future. “In this paper, we focus upon a hypothetical earlier phase of such an ETC in which it first embarks upon sending probes of increasing sophistication into interstellar space,” Smith writes.

The first probe might not even be interstellar, at least not on purpose. Our own Voyager probes can rightly be called interstellar probes, though that wasn’t their intent. The same could be true for an ETC just embarking on a program of sending probes to other star systems. The first one could’ve been meant to explore their own solar system or the nearest star, then eventually arrived at another civilization’s home. From a zoomed-out perspective, that’s kind of what humanity’s doing right now.

This artist's illustration shows Pioneer 11 on its way to interstellar space. NASA lost contact with the probe in 1995, but they expect it's still travelling at 40,260 km/h (25,010 mph.) It won't come close to another star for more than 900,000 years. Image Credit: By NASA/Don Davis -, Public Domain,
This artist’s illustration shows Pioneer 11 on its way to interstellar space. NASA lost contact with the probe in 1995, but they expect it’s still travelling at 40,260 km/h (25,010 mph.) It won’t come close to another star for more than 900,000 years. Image Credit: By NASA/Don Davis –, Public Domain,

This could be how first contact happens. Rather than an overt message from sender to recipient, the recipient receives a technological artifact to ponder. In Voyager’s case, both crafts carry an artifact designed for alien eyes, just in case.

Smith isn’t the first one to point out a technology gap in this situation. In 2006, Andrew Kennedy published a paper in the Journal of the British Interplanetary Society titled “Interstellar Travel – The Wait Calculation and the Incentive Trap of Progress.” He pointed out that civilizations know their technology will progress, so they may wait before sending any probes, knowing that their first efforts will likely be surpassed by their subsequent, more technologically advanced efforts. Is there a point where it won’t make sense to wait anymore?

In this paper, Smith assumes an ETC won’t want to wait. There could be myriad reasons why, and they’re beyond this paper’s scope. Instead, Smith assumes an ETC launches a program without waiting until technology reaches some critical stage. He tackles a different question: “What probes will be the first to arrive at a disparate planetary system within which there is a civilization capable of retrieving the vehicle?”

“In other words,” Smith writes, “which probes have the potential for instigating a first-contact event?”

To examine that question, he hypothesizes two different ETCs around different stars. One has developed the capability to send probes on interstellar journeys, and Smith calls it the active ETC. The other one hasn’t developed interstellar probe technology yet but does send probes out to explore its own solar system. Smith calls this one the passive ETC. Smith examines first encounter events, where the passive ETC receives the probe from the active ETC.

In Smith’s scenarios, the active ETC would continue to develop its probe technology, and the probes would become faster and faster. They would keep advancing their technology because they perceive some benefit to themselves from shorter interstellar transit times. That’s how we think, so it’s not unreasonable to think another technological civilization would think the same.

Smith looks at two different cases of technological advancement in his paper. In the first, the speed of the probes scales linearly with the launch date. In the second, the speed scales exponentially. For each case, he looks at two examples. One for a probe sent from Earth, a passive ETC, to an active ETC. The second is for a probe sent from an active ETC to a passive ETC, Earth.

In the first case where we send probes out into interstellar space, the Voyager probes could very well be the first ones. Smith encourages enormous caution when considering this idea, but he includes it for completeness. “The first spacecraft have begun to leave the Solar System within 100 yr of the launch
of the first liquid-fuel rocket by Robert Goddard,” Smith writes, and that forms the backdrop for this scenario.

As a loose, illustrative case, Smith calculates that a next-generation probe would launch every 100 years as Earth transitioned from a passive to an active ETC. 100 years is the same span of time between Goddard’s rocket and the Voyager probes. Using the Voyager 2 probe as humanity’s generation zero probe, Smith calculates it would reach an interstellar target in our stellar neighbourhood in about 80,000 years. Every 100 years, humanity would launch another probe to the same destination. In 2700 years, we’d launch our 27th-generation probe. That would probe would be far more advanced and travel at a much higher speed. It would reach the same destination as the zero-generation probe in only 5,560 years, 74,000 years before Voyager 2 would if it were a purposeful probe.

But that example has Earth sending probes to a relatively nearby star in our own stellar neighbourhood. What happens if we expand our reach for a greater likelihood of reaching an ETC and send probes to a star 31 parsecs (100 light-years) away?

In that case, our 140th-generation probe would be the first one to reach the destination. It would be launched in the fourteen-thousandth year of our interstellar probe program and would be much faster than our zero-generation probe (Voyager 2.) It would reach the destination 28,200 years after the beginning of the program. And the first probe sent in this scenario wouldn’t reach the same destination until a whopping 1,972,000 yrs later.

If this is the case, and if Voyager 2 or maybe Pioneer 11 ever made it to an ETC, they would be irrelevant. The inspiring golden record aboard Voyager 2, designed to introduce ourselves and put humanity’s best foot forward, would be an anachronistic oddity. The ETC would already know more about humanity than could be crammed into a whole spacecraft jammed full of golden records. It would be like finding a child’s piece of art and showing it to them when they were 80.

Earth's Greatest Hits: the Golden Record attached to the Voyager 1 and 2 spacecraft. Credit: NASA/JPL.
Earth’s Greatest Hits: the Golden Record attached to the Voyager 1 and 2 spacecraft. Credit: NASA/JPL.

The numbers for an active ETC sending probes to Earth in the same linear acceleration scenario are similar. But while our initial probes are extremely slow, a more advanced ETC would have faster probes. Smith calculates that their probes would have a speed of 1/10 light speed 200,000 years into their interstellar program. There wouldn’t be an almost 2,000,000-year gap between the arrival of their generation-zero probe and their 1/10th light-speed probe. But the first-to-arrive more advanced probe would certainly be extremely high-tech by our standards, and it’s doubtful we could learn much from it. We wouldn’t be able to reverse-engineer it. Indeed, we might struggle to understand its implications.

These are the linear expansion cases, where the speed of probes increases linearly. But Smith doesn’t end there. He also looks at the same two cases with exponential velocity increases.

In this case, the active ETC’s 6th generation probe would be the first to arrive at the destination. It would be launched within 1200 years from the beginning of their probe program and would reach the destination in only 200 years. So it would arrive there thousands of years before the zero-generation probe.

Whatever the actual numbers could turn out to be if there were active and passive ETCs sending probes to distant star systems, a number of things are likely to be true, according to Smith.

Smith agrees with other thinkers and researchers who point out that it’s highly unlikely that first contact would be between two equal civs. There’s likely to be a technological disparity between them, and it could be extraordinary. Some wonder if Unidentified Aerial Phenomena (UAP) could be probes. There’s really no way to know, but the idea’s worth thinking about. “Are flight characteristics of any UAP singular enough as to be consistent with an origin from a distant ETC?” Smith asks.

Proxima Centauri is our nearest neighbour and is 4.2 light-years away. It would take Voyager 1 a long time to reach Proxima Centauri, over 73,000  years. Credit: ESA/Hubble & NASA
Proxima Centauri is our nearest neighbour and is 4.2 light-years away. It would take Voyager 1 a long time to reach Proxima Centauri, over 73,000 years. Credit: ESA/Hubble & NASA

We assume that humanity would be the junior partner in any first encounter. Our level of technology doesn’t allow us to send probes to other stars except the nearest. If Voyager 1 was heading for our nearest neighbour, Proxima Centauri, it would take over 73,000 years to get there. (Breakthrough Starshot has a concept for a fleet of lightsail spacecraft that could reach Proxima Centauri in as few as 20 years, but it’s only a concept.)

We’re in the predicament Kennedy outlined in his 2006 paper. It’s pointless for us to try and send probes to even the closest star. We might as well wait until our technology develops. 73,000 years is an absurd amount of time. What will humanity look like by then? What will our civilization be like? Will humanity even exist?

It’s far more likely that we’re the passive ETC, on the receiving end of another civilization’s interstellar probe program. In that case, first contact might be with an active ETC’s tenth-generation probe or one even more advanced than that. What would we make of it? Would we even recognize it?

This is an artist's illustration of the SPEAR concept, or Swarm-Probe Enabling ATEG Reactor. The concept envisions a new type of NEP (Nuclear Electric Propulsion) system. What type of propulsion systems are needed for probes that could take centuries to reach their targets? Image Credit: Troy Howe
This is an artist’s illustration of the SPEAR concept, or Swarm-Probe Enabling ATEG Reactor. The concept envisions a new type of NEP (Nuclear Electric Propulsion) system. What type of propulsion systems are needed for probes that could take centuries to reach their targets? Image Credit: Troy Howe

If it was designed to enter orbit around Earth or the Sun, we certainly would recognize it as an artificial object. Then what?

We’d point as many telescopes at it as we could, and then we’d launch our own probe to observe the object and learn all we could. Nations might vie for access. Things might not go smoothly. There could be arguments, and conflict could erupt as nations realize what a technological treasure trove it might be and how it might benefit them. Many nations aren’t space-faring. What would all those people think?

This is all speculation, but that’s part of the value of work like this. It forces us to confront these issues, even though contact might not occur for thousands of years and may never occur.

But thanks to Smith and thinkers like him, first contact by probe might play out differently. We’d be better prepared. If a probe from another ETC arrived and we retrieved it and studied it, we’d know what we might expect next. We’d know that it may not have been the first one sent, and we’d reasonably expect that the next ones to arrive might be more primitive.

In the end, it’s a mind-bending concept, a kind of ‘communication-by-probe relativity.’ The intent behind the first probe to reach us might be wildly different from the second one, which was actually sent earlier by a civilization whose values, government, and entire understanding had changed radically in the intervening centuries.

Science fiction often conceptualizes other ETCs as stolid, as if they’ve withstood centuries or millennia of change and development and are more stable than us. We often imagine that they’re unified. Meanwhile, our own world changes radically generation by generation as technology advances and our moral thinking advances along with it. We’re anything but unified. But predicting the course of events for planetary civilizations over thousands of years is impossible. We have so little to go on.

In this game of probes, our own intent would change over the centuries. We might be naive right now, imagining peaceful and fruitful first contact with another civilization, assuming they’d conquered their own dark impulses and they’d be benevolently disposed toward us. Like a big brother. But over time we might become more cautious, guarded, and even paranoid. Who knows?

Stephen Hawking famously warned us about advertising our presence in the galaxy too soon. “As I grow older, I am more convinced than ever that we are not alone,” Hawking said in the film Stephen Hawking’s Favorite Places. “One day, we might receive a signal from a planet like Gliese 832c, but we should be wary of answering back.”

Artist's impression of rocky exoplanets orbiting Gliese 832, a red dwarf star just 16 light-years from Earth. Credit: ESO/M. Kornmesser/N. Risinger (
Artist’s impression of rocky exoplanets orbiting Gliese 832, a red dwarf star just 16 light-years from Earth. Credit: ESO/M. Kornmesser/N. Risinger (

Gliese 832 is a red dwarf about 16 light-years away. That is an extraordinary distance, and if a probe as fast as Voyager 2 were heading there, it would take over 250,000 years to reach it. Gliese 832c is a rocky planet in the star’s habitable zone, and it’s the fifth-closest potentially habitable planet to Earth that we know of. If an ETC was based there, and if they sent a probe to us, they’re likely to struggle with the issue Smith outlines in his paper.

Something like this might happen one day. We don’t know what’s out there, or how widespread life might be. But the idea that our old Voyager probes would be the first to reach another ETC, or that an ETC’s first probe might be the one to reach us, is unlikely according to Smith.

“If an object sent by an interstellar ETC were to enter the Solar System, chances are it would not be a relic artefact that is perhaps analogous to a timeworn Voyager-like probe, but rather one might expect a vehicle of much greater sophistication, even if it were no longer functioning,” Smith writes.

“This suggests a question: might a hypothetical first-encounter vehicle from an ETC be so sophisticated as to still be functioning upon arrival at the Solar System?” Smith writes. “As Hawking (2010) and others have drawn attention to, some potential consequences of such a first-contact event might be of serious concern for humanity.”

Science fiction writers have explored nearly every permutation and combination of these ideas, and that’s as it should be according to Smith. “This is a topic that has again found fruitful soil within the realms of
science fiction, but is beyond the context within which the present paper has been framed,” he concludes.