Is it viable to broadcast a radio/EM distress signal from a space vessel that can be detected within a star system?

  1. Think of an unexpected distress signal i.e. the receiving apparatus shouldn't necessarily be pointed at that origin of the signal to receive it;

  2. The signal should be able to carry a small message (for instance, the relative location and motion vector). It is ok the signal is required to be a burst of repetitions and to have a minimum duration to increase the chance of it being detected;

  3. It should be detected within a star-system, so I expect it to take hours to propagate.

  4. Is should be detected within a star-system, so I suppose it must be potent enough to be detected yet not so potent it'll harm the vessel's occupants. Maybe this toll should go on the receiver (i.e. be extra sensitive and be computationally capable of filtering it from other sources);

  5. Ideally, occupants should be able to send more bursts if they decide to;

  6. Receiver design must consider signal location ain't expected, so it must be able to detect and receive a signal from any direction;

  7. Triangulation should not be a problem: the signal would pack location and current speed vectors.

Is it feasible? What should be the minimum duration? What is the power involved and relevant technical details? Would it require any unavailable (as of today) tech?

  • 1
    $\begingroup$ While the idea is feasible, the response depends on a huge ton of factors, including, but not limited to available delta-V, the number of ships in that star system, the origin of the ship in distress, actual tech level of either ship or base(s) and more. My answer is based on current tech, as well as others' answers as I can see, but specifying the base tech level for the story would help, at least to yourself to decide whether the ship's crew expect the response at all, or they just are to send a SOS and perish. $\endgroup$
    – Vesper
    Commented May 9 at 8:39
  • $\begingroup$ we comunicate with spaceships on the edge of our star system using radio, so yes. see voyager 2 $\endgroup$
    – John
    Commented May 12 at 12:46

3 Answers 3



First, you can arrange the frequency range to be used solely by distress signals, and arrange a set of omnidirectional listeners to detect the signal in that frequency range, to minimize unwanted interference from common broadcast. Next, while the signal emitter would also be an omnidirectional, you might feasibly arrange the situation on a distressed ship to allow a transmitting antenna to focus the signal at least into a single hemisphere, towards the star, both lowering the required power to be heard, and protecting the ship from the signal itself. In fact, ships on Earth have buoys designed to emit SOS and remain afloat without relying on the ship, your spaceships should be equipped with similar devices powered by say an RTG, so that they would have power to detach from a distressed ship, compensate its rotation, position itself between the ship and the star, align speeds with the ship, aim towards the star (then aim at a rescue base, if desired) and then transmit SOS with a small amount of data received from the ship (current course, local time, expected destination, flight number, problem code, etc - this can be compacted into 1 kB together with ECC) until deactivated.

Now, for the required signal power. Voyager 1's transmitter has "mere" 23W of signal power, and is heard from more than 150 AU away. Given that your "star system" range does not exceed 100AU (and usually the planets of interest are relatively lose to the star, give or take several AU), you don't need to broadcast your signals too far. With Voyager's antenna saying to have a 48 dBi gain, which translates into 10^4.8 or 63000x power (with 1/63000 area of full sphere), your omnidirectional or one-sided transmitter with 23W power would be heard from a 250 times less distance, or 0.8 AU, at the same power level as Voyager's, which is "barely hearable". If you employ a simpler antenna that can be directed and has a gain of ~20 dBi and aim it to the location of the nearest known (or just "known") SOS receiver, you can extend that distance tenfold, which would be 8 AU and quite viable to both travel in the first place, and be heard in the second place. Then, emitting a four times stronger signal would allow its detection from double the distance, although shortening the sending time by a factor of 4. An example proposed emergency buoy should be hearable in an omnidirectional mode from ~4 AU, expecting its RTG power of 4 kW, and ~40 AU if it succeeds to aim properly and use the directional antenna to send SOS.

Now for the reaction and signal time. With a payload of 1 kB, and current downlink speed of Voyager 1 of 160 bps, the signal's length should be longer than 50 seconds - it's a lot for an emergency, but since we're in SPAAACE, we don't care about length but care about being heard. I'd design several standard baud rates for SOS beacons, based on distance from the star measured by luminosity, or rather by the ship route's largest distance from rescue stations, and ensure that the ship captains would program their SOS beasons prior to leaving the docks according to their declared route and destination. This way the receivers could be more precisely filtering out random noise and better detect distress calls, if any,effectively making a SOS be heard from farther away. So, the minimum signal length is effectively dictated by whatever SOS protocols are running in your star system, and the distance between transmitter and receiver, the farther, the longer.

In total: Yes, the idea itself is feasible; your ship that falls into distress can send a SOS so that it'll be heard at the "base", that SOS can also have a payload, the minimum length of the SOS is determined by payload, distance and other predefined constants, but will be longer than 0.1s per AU^2, give or take an OOM, with currently used receiver of Voyager 1's signal. Using a worse receiver would proportionately lower the maximum detection range, but if there would be a space base, they will likely have a receiver of similar capability because of enough space around themselves, and low amount of unexpected emission.


Communication with Voyager continues even though it's left our system so it's definitely practical to communicate complex information over such distances with acceptable loss.

Voyager continues to communicate because it can focus its signal in one very specific direction, requiring exponentially less power than an omni-directional broadcast. So your transmitter needs to have enough power to broadcast in all directions. Or it could save a bit on power by sweeping a beam periodically.

The other element is the listener - something needs to be able to receive the signal, and someone needs to be monitoring that. Ideally multiple things are receiving, so the origin of the signal can be triangulated - that tends to be much harder to do with just one receiver, unless it can receive the signal enough times that its own movement through space can afford the triangulation.

Presumably this signal uses a standardized protocol - specific frequencies and patterns. If so, then it should be fairly robust even at the minimum broadcast power needed to cross the system. Assuming the standard was designed with the system's noise sources in mind, it should require little or no filtering to receive clearly.

Line of sight is also a consideration - a vessel on the opposite side of Jupiter is unlikely to get any signal to Earth on its own. A satellite network deployed on planets throughout the system could compensate for this, relaying emergency signals between planets using much more efficient directional communication. This probably also solves the triangulation problem, and reduces the broadcast power as well because you only need to reach satellites at the nearest planet.


Of course it's viable. The problem is reception

Check out this wonderful image showing the approximate distance human RF transmissions have traveled in the Milky Way.

enter image description here
Please click on this image to see it full-size. Courtesy Popular Mechanics (of all places).

By the way, what that image is showing you is the approximately 200 light year bubble around the Earth that encompasses the extent of the broadcasts. It's not the square box you're looking at... it's the blue dot in the inset. That's the bubble. "Space is big," intoned Douglas Adams. "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's, but that's just peanuts to space."

It's important to understand that RF transmissions are just another kind of photon, just like visible light and microwaves. They don't diminish over distance (but they can be absorbed... hence sunburns). In space, distance isn't the problem.

However, as they travel, there are fewer and fewer "per square meter" to be received. This is also true for light. It takes a LOT of photons to be easily "seen" or detected at a great distance. So the further one is from the source, the more sensitive the receiver must be to clearly detect the signal.

There's also a "window of opportunity." If the distress signal is broadcast for one hour, then there is a one-hour window of opportunity at any point in space to receive it. Assuming detection isn't a problem, that signal would be "heard" on the other side of the galaxy. Yeah, 106,000 years later, but if you were listening at just the right hour, you'd hear the signal.

So there are the limits of your effort to receive the distress signal. It can be received anywhere in the solar system so long as...

  1. The receiver can detect the weak signal (weakened by distance).
  2. Whomever is listening is, indeed, listening at the right time to hear the signal. This is more important than you might think. A signal broadcast from Earth can be received on Mars 4–22 minutes later depending on the position of the two planets in their orbits (and assuming that position doesn't put the sun between them... absorption...). So even if the distress signal is broadcast for days, there's a minimum amount of time before it arrives at the listening post before it can be heard. And after those days and that delay, it can't be heard. Hopefully the broadcast is long enough to do a little triangulation, which might require the receiving ship to move... which takes time.
  • 1
    $\begingroup$ Normally SOS receivers are on constantly, and in space I expect them being more than that, say passively scanning the entire sky sphere once in several minutes. Thus if the signal is longer than that period, which is known for every ship that's using the SOS system (or even globally as a system-wide or empire-wide standard), it will be listened for. Actual hearing would require receiver capabilities and specific signal characteristics to filter it from the space noise, this is possible to negotiate prior to launch tho. $\endgroup$
    – Vesper
    Commented May 9 at 6:34
  • $\begingroup$ @Vesper That may be true here on Earth where transmission distances are incredibly limited, and it certainly makes sense that some authoritative agency would specify a broadcast on frequency X, modulation Y and encoding Z to always be an SOS, but it doesn't change the fact of anything I've said. A continuously broadcast SOS is easy to find. A brief SOS is not. And having a 360x360 antenna array on every ship capable of listening across the entire heliosphere (OP didn't specify spatial limits, so I set the worst case) is impractical. Antennae in space are not the same as antennae on Earth. $\endgroup$
    – JBH
    Commented May 9 at 6:52
  • $\begingroup$ I'm speaking about having such an antenna at every base, not ship, although ships can also have antennae dedicated to listening for SOS signals. Say, if there are 10000 ships in flight in a star system, each equipped with one, the average distance of a SOS to reach the nearest ship would be about 0.5 AU, which is even easier to hear and react than expected. If there are just a few like here, it'll be easier to plain track each of them from the base, knowing their relative coordinates if they don't deviate from the route, eliminating the need to scan all the sky sphere. $\endgroup$
    – Vesper
    Commented May 9 at 8:32

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