I'm working on an alternate-history story in which peaceful alien satellites are surveying the solar system and passively observing human activity. Many of the deep-space satellites would have been in long orbits, communicating with one another mainly through radio signals.

(Edit: They are mostly doing a mineral survey of the solar system at large, and studying the course of humanity's technological development. They intend to make contact with humans at some later point, but have not yet decided when to do so. There are a few long-orbit satellites studying the outer planets, but the greatest concentration is from the asteroid belt inward, as well as communications relays in Earth orbit.)

In real history, pioneering radio astronomers in this era detected radio signals from distant celestial objects such as Cygnus A, and were able to map many stars in the Milky Way.

  1. If there had been alien satellites in the solar system at that time, would radio astronomers have been able to intercept their communications?
  2. Would it have been possible for mid-20th-century terrestrial radio technology to send signals back to space, and establish two-way communication with these devices?
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    $\begingroup$ The radio signals detected from Cygnus A compared to the signals generated by, e.g., deep space satellites like the Voyagers is something like comparing attending a heavy metal concert to the sound created by a butterfly's wing in flight. To better answer your question, exactly where are the alien satellites in question? Detecting orbits around Pluto (in any era) would be considerably more difficult than those in orbit around Mars. Also, what's the maximum distance between any two Alien satellites we can expect (that gives us a relative signal strength). $\endgroup$ Dec 30, 2020 at 7:20
  • $\begingroup$ I haven't pinned down numbers yet - that point had been flexible in my notes! There would be a modest fleet of them, trying to do a mineral survey of the whole solar system but willing to take their time about it. (Decades, certainly; perhaps centuries.) A few might head out as far as Pluto to do a flyby and then swing around and come back. But the greatest concentration would be from the asteroid belt inward. They're interested in predicting the course of human development, and want to know what resources will be available to us if and when we develop space travel. $\endgroup$
    – Crystal E
    Dec 30, 2020 at 15:07
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    $\begingroup$ @JBH: Really an inadequate analogy. More like a spark generated by stroking your cat vs a multi-megaton nuclear explosion. $\endgroup$
    – jamesqf
    Dec 30, 2020 at 17:33
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    $\begingroup$ @CrystalEidson Thanks! Please remember to edit your question with the clarifications. As a rule, never trust that people will read through all the comments. $\endgroup$ Dec 30, 2020 at 18:15
  • $\begingroup$ If the alien probes don't want to be heard, they probably won't be. There are directional forms of communications. Also, could they change their encodings such that we would most likely think they're just noise? $\endgroup$
    – NomadMaker
    Dec 31, 2020 at 17:17

10 Answers 10


Assuming they communicate in (almost any) radio frequency
They are communicating directly at Earth.
We are actively looking for something, or their probe accidentally happens to be in the exact same direction as one of ours. (accurate to within a few thousandths of a degree)

If the aliens device communicated via laser, or tachion beam, or polarized gravitometric pulses, or hyperwave, or by rapid engublification of it handwavium vanes, then obviously there is no ways we can detect it, even if it is sitting in the vacant lot next door.

As for them needing to be talking directly at us... Consider the case of a Voyager Probe.
When it is talking directly at us, the signal is so strong that you only need an antenna the size of 4 football fields, pointed exactly at it, and listening for a signal on its known frequency.
If the signal is pointed far away from Earth, say more than 1/2 of a degree off target(thus out of the peripheral lobes of the x-band antenna), then that receiving antenna would need to be several millions of times bigger.
And if the Voyager is not actively transmitting, then the detecting antenna would need to be.... many times bigger than the Planet Earth itself.

We can safely assume that alien probes have better antennas with better focus.

So, realistically? Yes but only if the aliens WANT us to hear them.

As for communication: If we can actually receive (and send) signals with the alien probe, then of course we will be able to communicate.
Because, as shown above, this will only happen if the aliens want to communicate.
Being much much much more advanced than us, they will of course have listened in on our own radio & tv broadcasts, figured out suitable communications protocols, and will be transmitting it to suitable receivers at sufficient amplitude in familiar encoding formats.

  • $\begingroup$ Ah, this is directly relevant! For the purpose of my story, I want human astronomers to detect an alien satellite sometime in the 40s or 50s - so I need to work backwards and figure out how to make this possible. I had already planned that there will be some larger communications relay modules in lunar or earth orbit. So perhaps - if the timing was right - could a signal sent from a satellite towards the lunar array get picked up by, say, the Mount Pleasant Observatory in Tasmania? My aliens DO want to communicate with us, eventually. But they hadn't planned to do so just yet. $\endgroup$
    – Crystal E
    Dec 30, 2020 at 15:20
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    $\begingroup$ @CrystalEidson If the aliens are talking to their own relays on/around Earth, and doing so from a long ways off, then yes the signal could very well be intercepted. We would only hear the incoming signal, not the replies, of course. And making sense of the transmission... that would be a monumental task. Depending of the aliens but the transmission is likely to be digital, compressed and in an unknown protocol. Think something like the computing team trying to figure out the German Enigma Codes in ww2. $\endgroup$
    – PcMan
    Dec 30, 2020 at 16:01
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    $\begingroup$ You might not even need a relay around earth. That looks like a bad position for a relay, in fact. Too much interference. Instead, say the relay is halfway between Mars and Earth. The probes studying Earth are nearby, however. As a result, the signal from the relay to the probe would be reliably detectable $\endgroup$
    – MSalters
    Dec 31, 2020 at 12:16
  • $\begingroup$ @PcMan, decoding the radio signal to binary is already difficult if they use some more complex modulation like QAM. And that's still assuming they use a fixed carrier. Some kind of spread spectrum would be impossible to even detect, because the transmission power is spread over large spectral width and without knowing the sequence, it may not be much stronger than background noise at any frequency we'd try to amplify. So the aliens can hide what they don't want us to find even over radio (IIRC stealth fighters use this to share sensor data without giving themselves up). $\endgroup$
    – Jan Hudec
    Jan 1, 2021 at 10:53
  • $\begingroup$ @JanHudec Which come back to my point in my answer: "So, realistically? Yes but only if the aliens WANT us to hear them." $\endgroup$
    – PcMan
    Jan 1, 2021 at 13:04

Yes, probably, in a fashion.

Remember, we're talking about the decade of Sputnik and Explorer, and Mars programs failed in 1960. Building the ground-based portion of such a program just a few years ahead of the historical schedule should be possible, given proper motivation.

Sending signals to them is not the same as establishing two-way communications. Unless the alien sats have been programmed to aid human communications attempts, mankind would face quite a challenge to get into their communications protocols.

Do you know what an email header is? Could you write a plausible set of headers down from memory? And that's with the fields being in English, now imagine a space-saving format of bits and bytes. (How many bits to the alien byte? You can see the problem, I guess ...)

That challenge would be taken by plenty of very smart people, using the best computers available. I'd compare it to breaking codes in WWII. The codebreakers have messages without context and look for patterns.

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    $\begingroup$ +1 for noting that the problem is much more complicated than just detecting/sending signals. What if the aliens don't use binary? Or they do, but rather than a hexadecimal coding system, they use elevanary or base-23? What if they're using terahertz frequencies (which the 50s might have detected, but done nothing about)? And then you get into the actual application protocols (e.g., email headers) and that's assuming they're not encrypted because the aliens are smart enough to know the humans could detect the signals. What if they're using lasers like we do today? Lotsa ugly, here. Cool! $\endgroup$ Dec 30, 2020 at 8:20
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    $\begingroup$ "Email headers" is a very useful analogy for me, thank you! Most of the information the satellites are exchanging is astronomical survey data - the locations, orbits and composition of various objects each module has observed. I'd been thinking that might be a kind of Rosetta stone for humans seeking to decode them. For unrelated plot reasons, the aliens DO use binary as well as base 10. But now that you mention it, the fact that their system of numbers is more easily comprehensible to humans is surprising, and is something my characters should remark upon. $\endgroup$
    – Crystal E
    Dec 30, 2020 at 15:10
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    $\begingroup$ @CrystalEidson Binary is not strange, but decimal is. $\endgroup$
    – user253751
    Dec 30, 2020 at 15:28
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    $\begingroup$ @CrystalEidson To add on a bit to what user253751 is saying, binary is about as simple and reliable as digital transmission can get (it's either a high or a low, with a substantial dead zone in the middle, easy to define and detect), and it's capable of representing any other data. Decimal on the other hand is a quirk of having ten fingers, and absent that compelling bit of evolutionary history there is no particular reason to use it. It would be very odd for any alien species that doesn't have ten fingers/tentacles/whatever to regularly use it $\endgroup$
    – BThompson
    Dec 30, 2020 at 16:27
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    $\begingroup$ @AustinHemmelgarn brings up yet another complication (and the OP another, though she might not realize it). Austin's point is like comparing CISC CPUs to RISC CPUs. CISCs are powerful, RISCs are efficient. There's often not a "best choice." However, one look at a Chinese keyboard will demonstrate that culture often overwhelms engineering efficiency. Crystal's point is that the other culture may not use our points of reference at all. The math is the same, but the constants are all different. So might the map references. Figuring out where the sats think they are might be a challenge. $\endgroup$ Dec 30, 2020 at 18:23

Don't confuse detection with communication.

Detection is just the reception of the signal and the determination of it being of artificial origin. That's highly plausible or at least not out of the realm of possibilities.

Communication means delivering signals in a way that can be read and understood by the receiver.

For the reception part from our side, that would require being able to determine how to read the data and to break their eventual coding protocol (to stay simple, think of some aliens capturing the sentence "the Eagle has landed" from Apollo program, what could they infer from it? If you want to be more complex, think of how data from space probes are compressed.).

Then for the sending part, it would require knowing how and if the receiver is programmed. I would say this is extremely unlikely.

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    $\begingroup$ It's actually the other way around. Detection in the sense of interception would be hard - spread spectrum modulations are practically invisible to 1950's tech even if someone is looking in the right time/direction/band: eg even contemporary GPS satellites would probably be missed and they'd be found only by passively reflecting solar or radar illumination. But communication would be easy if the satellites were designed to make it so, by using easy frequencies, simple modulations, and self-explaining information coding of the sorts we've used in our own subsequent attempts. $\endgroup$ Dec 30, 2020 at 23:03

Most systems are encoded on several layers.

First encoding: the channel

There is a channel encoding, on the bottom-most level, which deals with the specific stuff needed for the analogue world. Normally, if it gets sophisticated and high speed com, it resembles white noise and so is difficult to decode. Especially if you have much slower signal processing than the sender assumes, you just can't get the content out in time to react.

In the analogue world, those normally are a combination of too-high or too-low frequencies in several steps, phase shift of the waves as often as every quarter wave (the wave comes a bit too early, on time or too late); amplitude changes (louder or more silent), all of this in arbitrary combinations and arbitrary fine steps. 8 bit is a digital native, the analogue world can have chunks which are smaller or bigger or oddly numbered, depending only on what is practical with this transmission technique. Imagine you have 3 phase steps, 3 amplitude steps and 3 frequency steps, then every quarter wave will carry 27 bits. Some of it will be used for error correction and has to be removed in the net transmission cpacity, but still, these can be quite unusual numbers.

There are two upsides: satellite communication is often very slow for long ranges, in order to avoid transmission errors, and can be repeated several times. So you have a chance. And all those changes move around a central frequency, the carrier, which is mostly detectable.

second encoding: the network protocol

Then you normally have some kind of network protocol, the best known of which here on earth is tcp/ip. Those protocols have the task to determine sender and receiver and routing, provide some extra data and additional error correction. The point is, any message can be cut up in many small chunks and every chunk will contain all the protocol header. This is a repeated part of the message, a gold nugget for the deciphering cryptography people and probably the part which enables the cracking of the channel protocol above.


This is the hardest. Compression algorithms can be incredibly sophisticated. You won't understand jpeg or mpeg by just looking at the data. Zip compression yes, if you're lucky. But then, who says that the aliens are not using something even more crazy? Even we have jp2 since 20 years as replacement for jpg (even if it is somehow not used), they may have something way over our heads.

email header and text

Someone else here mentioned the email header. This is a protocol overhead for the payload. If you are here, you have won! After all, you'll get the payload now.


I think you can detect it. I seriously doubt you can decipher what it says to another satellite with fifties tech. I guess you can send something to the satellite with a big enough dish. And then, if the satellite is programmed to react on this crude analogue signal, it could answer using the earthling's protocol. That would be communication, but the satellite must be specifically programmed to do this.


Anywhere from ~1940 to not yet.

The first purpose-built radio telescope was a 9-meter parabolic dish constructed by radio amateur Grote Reber in his back yard in Wheaton, Illinois in 1937.

The cosmic background radiation (CMB) was measured by Andrew McKellar in 1941 at an effective temperature of 2.3 K using CN stellar absorption lines observed by W. S. Adams. Theoretical work around 1950 showed the need for a CMB for consistency with the simplest relativistic universe models. In 1964, US physicist Arno Penzias and radio-astronomer Robert Woodrow Wilson rediscovered the CMB, estimating its temperature as 3.5 K, as they experimented with the Holmdel Horn Antenna.

Discovered in 1941, the CMB took until 1950 to surmise what it was, and till 1964 to prove it.

It took two years to find the first two pulsars (1967-68) to discount the 'Little Green Men' hypothesis of having found the first one. However, pulsars were first posited in 1934.

So let's say by 1940 you have the tools to do it, and an inkling of what it might be (or more importantly, what all these stray radio signals from space are and are not), but proof is still a few decades off. That is unless you figure out some kind of signal to send to one of the satellites that triggers a response.

The ability to send the signal isn't the problem, it's what to 'say'.

If it's a 'first contact' system they would've made it easy for us. If it's encrypted you're going to need computers. Or if you're lucky they didn't even bother because they didn't plan on us being here.

  • $\begingroup$ It doesn't even need to be encrypted. If they're using a Forward Error Correction Code you're straight out of luck. Turbo codes were only discovered in 1990, but were directly obvious for use in space communications. Even Reed-Solomon codes date to the 1960's, one decade after the time frame of the question. $\endgroup$
    – MSalters
    Dec 31, 2020 at 12:29

For detection, no, assuming that the aliens don't actually want their satellite communications to be detected, and are competent electrical engineers. They would have used beamed radio communications, or possibly lasers &c, and would have taken care not to point the beams at Earth. For comparison, consider satellite TV antenna dishes, which have to be pointed pretty much directly at at a particular satellite (a mere 22K miles/35K km away in geosynchronous orbit),and which is trying to have its signals received.

Communication, maybe, if the satellites have been detected and frequencies known. After all, radar beams were bounced off Venus, and the return signal detected, starting in 1961.

The problem, though, is discovering the communications protocol to be used. After all, if you don't use the correct protocol for your cell phone, bluetooth device, WiFi router, or whatever, it's just noise, not communication.

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    $\begingroup$ If you wanted to contact a primitively electronic society, you'd use frequencies that are physically easy and modulation schemes that can be noticed as interesting by biological signal detectors to get them intrigued enough to hook up a chart recorder and pass the plot off to the math / codes biologicals. In English: you beep prime numbers. $\endgroup$ Dec 30, 2020 at 22:57
  • $\begingroup$ @Chris Stratton: Yes, IF the aliens wanted to use the satellites to communicate. I'm assuming that they don't, and that the Earthlings have detected them due to e.g. sloppy alien engineering, and want to eavedrop on the communications and eventually send their own messages. $\endgroup$
    – jamesqf
    Dec 31, 2020 at 18:24

If there had been alien satellites in the solar system at that time, would radio astronomers have been able to intercept their communications?

Only if the observers were lucky and the satellites had no desire to hide their transmissions.

First, you have to be looking in the right direction, at the right time, at the right frequency.

But then you have to recognize that the incoming energy is a signal - which means that it has to be coded in a way that you can detect, and expect. 1950's technology still used fairly simply effectively analog modulations - signals of interest had lots of energy concentrated in narrow bandwidths. But many modern schemes are instead spread-spectrum and can only be detected by a receiver seeking a comparable pattern. Even something not trying to hide - say our own contemporary GPS satellites which transmit different coding sequences all in the same frequencies - would be difficult to detect as intentional radiators with 1950's gear, unless someone knew exactly what to build gear to look for. Something so close might be seen by reflecting sunlight or on an early active radar search, but that's distinct from intercepting its signal.

Would it have been possible for mid-20th-century terrestrial radio technology to send signals back to space, and establish two-way communication with these devices?

If the satellites were design to make that possible, then yes, most definitely.

If you wanted to communicate with an early electronic society, you'd go to efforts to make things easy. You might try some very longwave frequencies for ultra-primitive situations, and your classic physics-inspired SETI ones, but if you're orbiting someone's planet you'd mostly you'd pick the low VHF range where you easily get energy through an ionosphere, while remaining in the realm of what can be done with vacum tubes.

Then you'd use (at least at the outer attention level) simple modulations, for example toggling between two discrete nearby frequencies in such a way that you elicit a response from all of your basic detector types (BFO, AM, FM). And you'd use it in a way that creates a "curious" pattern such that a lifeform manually noticing the signal finds it interesting enough to bring it to the attention of someone who can hook up more instrumentation to look at it. Finally, you encode your meaning in terms of basic mathematics - prime numbers and all that, and in a self-teaching way. Our own science fiction authors and actual real world outgoing contact attempts have examined the possibilities there.

To some extent though, to have a meaningful "conversation" you have to pack a fair amount of AI into that satellite. And if you're going to do that for the meaning, you might as well do it for the communication, too. In other words, you monitor your target civilization's own communications, and you respond in kind. If they call each other with a particular pattern, you use the same pattern. If their broadcast stations turn off a certain period after the local star rotates past the horizon, you jump in an instant later with your own "greetings planetenials" message - or simply replay their own traffic of the previous platentary revolution. In short, you get their curious individuals curious, and you make the task of responding within reach


Many of the deep-space satellites would have been in long orbits, communicating with one another mainly through radio signals.

The alien satellites would know where each other is, and would transmit using dish antennae aimed at each other just like we do, to concentrate the signal; otherwise, 99.999% (probably much more) of the energy would be wasted going in directions which serve no purpose.

Therefore, no we would not detect those satellites in the 1950s or even now.

Here's a picture of the antenna on the Voyager spacecraft. Without the dish aimed directly at Earth, we couldn't detect it's faint signal. Alien craft would have a similar problem (though presumably with a higher energy budget).

enter image description here

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    $\begingroup$ Such a dish isn't so precise that a satellite in the outer solar system transmitting to one near the earth wouldn't also be illuminating the earth; but anyone wanting to hide would use challenging frequencies and spread spectrum coding schemes that would make it challenging to even someone who knew where to look. $\endgroup$ Dec 30, 2020 at 22:56
  • $\begingroup$ @ChrisStratton "Such a dish isn't so precise" that depends on whether or not OP wants the Earthlings to detect the satellites... :) $\endgroup$
    – RonJohn
    Dec 31, 2020 at 13:37
  • $\begingroup$ no, it is simple physics that a dish isn't that precise $\endgroup$ Dec 31, 2020 at 16:29
  • $\begingroup$ @ChrisStratton interstellar space-travelling aliens know more than we do. $\endgroup$
    – RonJohn
    Dec 31, 2020 at 22:46
  • $\begingroup$ Like many current earth humans, they'd know the limitations of dish directionality, and that this is a matter of basic physics, not technical sophistication. The question isn't seeking ideas from the imaginery realm of magic such as you are trying to provide. $\endgroup$ Dec 31, 2020 at 23:01

In addition to all the other answers indicating that we would only be able to intercept radio communications from the alien sattelites if they were transmitting directly in the direction of earth, I'd like to point out another way to observe them:

Satellites in low earth orbit are visible with the naked eye, because they reflect sunlight. Just like the moon. If you go outside on a clear night you can see them, dots moving along the sky. If the aliens have satellites in earth orbit they will be visible, unless they take steps to hide them. I'm not sure if satellites in medium or high orbits will be as visible, but at least they would also be visible with simple telescopes that were available long before the 1950's. My guess is that satellites in earth orbit would be discovered before the 1950's unless they had measures to hide themselves in place. Hiding a satellite is probably as simple as putting a big mirror on earth-facing side, and making sure the mirror is oriented so that it reflects sunlight to anywhere but earth.

Without such hiding measures, satellites would need to stay out of earth orbit. Maybe a very distant orbit would work, I'm not sure where the limit is. At least beyond the distance of the moon, I'd guess. Or they would need to use orbits at the lagrange point behind the moon or the lagrange point between earth and the sun (you can't point your telescope at the sun).

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    $\begingroup$ But the question says that these are deep space satellites, not ones in Earth orbit. Given the difficulties in detecting e.g. football field sized asteroids making fairly close passes to Earth (when we know such exist, and are deliberately looking for them), the alien satellites would need to be pretty large to be seen. (And you certainly can point telescopes at the sun, though they have rather different design criteria than ordinary astronomical telescopes: en.wikipedia.org/wiki/Solar_telescope ) $\endgroup$
    – jamesqf
    Dec 31, 2020 at 18:30
  • $\begingroup$ @jamesqf The OP indicated in a comment to another answer that they also planned to have satellites in earth and lunar orbits. Regarding sun telescopes: In that case I guess a satellite should orbit close to the sun without crossing the sun's disk. The glare of the sun should make it invisible. $\endgroup$
    – JanKanis
    Dec 31, 2020 at 23:09

There are two ways an alien probe might indicate its presence: optically or via radio waves. Technically radio waves are incorporated into optics, but by optics I'm referring to the visible part of the electromagnetic spectrum (red to violet light: 380 to 750 nm wavelengths).

Detection via Visible Light Waves

Visible light astronomy, could detect a moving object within the solar system if telescopes where pointing in the right direction. The movement of the object would be noticed against the background of distant stars that from our perspective appear stationary.

By observing the moving object over a period of time a course can be determined. If the course followed by the object orbits the Sun it would be assumed the object could be an asteroid or a comet. If the object followed a "weird" path that did not orbit the Sun, but moved from planet to planet, or something else, it would be noted as something peculiar.

Consider the consternation caused by Oumuamua in 2017 when it came very close to Earth. It was the first object from outside the solar system to enter the system that was noticed. There was speculation by some people that instead of being just a rock from somewhere else it might have been an alien probe with the appearance of a flat rock.

Now if an object was detected optically, in addition to its path being weird if it flashed in an unnatural way, something akin to a type of alien Morse code, or a repetitive sequence of prime numbers then someone might postulate the object might be of extraterrestrial origin.

The technology and means for optical detection existed in the 1800s.

Detection via Radio Waves

Regarding radio waves the availability of technology and people needs to be considered. World War 2, which occurred during the first half of the 1940s affected both.

Radio Astromony

The first detection of radio waves from an astronomical object was in 1932, by Karl Jansky who was with Bell Telephone Laboratories when he was investigating static that interfered with transatlantic shortwave voice communications.

Inspired by Jansky's work Grote Reber built a 9 m parabolic radio telescope in his back yard, in Wheaton Illinois, in 1937. As others have stated in their answers, to be able to detect anything one has to be using the correct radio frequencies. Reber's first two attempts to detect anything from the cosmos failed because he used frequencies of 3300 MHz and 900 MHz. He only started getting results when he used 160 MHz. He ended up completing a radio frequency map of the skies in 1941, which was extended in 1943. Until the end of World War 2, Reber was the only radio astronomer in the world.

People started to enter the field of radio astronomy after World War 2. Confirming radio emissions from the Sun was some of the first work undertaken, usually using repurposed military radar equipment from World War 2. Ruby Payne-Scott was the first female radio astronomer from 1946 to 1951.

Many of the world's iconic radio telescopes where not constructed until the late 1950s, or even later:

So if you want to set your story in the 1950s, then the signal could have been received in either: the Netherlands (1956), the United Kingdom (1957), the Soviet Union (1959), USA at Goldstone (1958) or possibly the University of Illinois (1957).

The other thing is, just because a radio telescope exists doesn't mean it can receive all signals. Size matters, the bigger the better, particularly for very weak signals, as NASA discovered during the Apollo 13 emergency when its 22 m diameter dish at Honeysuckle Creek, near Canberra, Australia could not clearly and reliably receive communications from the stricken Apollo 13 craft. The Parkes radio telescope had to be quickly converted from a dish that just received radio signals to one that could send and receive.

A serendipitous signal like the Wow! signal, received in 1977, could be a good analogy for what might have happened.


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