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The power that deep space probes have allows an RF communication capacity of a few kbps from a range like Pluto-Earth. If we go into optics, the range will be better. But, of course, we want to send scouts (small probes) to far away galaxies. This would potentially require having a chain of communication relays (or repeaters, let's say).

How can we send these repeaters, and at what periods, so that they always keep moving like a train, and so that we keep communications with the probe?

(irrelevant, but, the repeaters would probably be smaller, but they should be able to travel the same speeds as the probe, and should have one beam towards the probe (or the forward Repeater), and another beam towards the behind repeater)

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  • $\begingroup$ "of course, we want to send scouts (small probes) to far away galaxies." Has there been any serious suggestions to do anything even remotely like that? I'm not nitpicking; if such a proposal exists, I'd be very interested in reading about it, so please do provide some sort of citation for that statement. The only missions I've ever heard of have been within our solar system; the Voyagers have left the solar system, but are still a long, long, long way away from even the nearest star, let alone other galaxies. An intergalatic mission would be a gigantic undertaking no matter how you slice it. $\endgroup$
    – user
    Jul 17, 2015 at 12:54
  • $\begingroup$ @MichaelKjörling, I'm not in the field of space, but the curiosity of mankind is un-stoppable from thinking what's next (achievable or not). I don't know if there's any real studies regarding jumping to other galaxies, to be honest. The question might be asked to focus on solar to nearest star travels, but I guess it's too late to re-phrase it now. $\endgroup$
    – Gürkan Çetin
    Jul 19, 2015 at 16:59

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If you want to send a probe to another star with the kind of technology we can think about (if not design and build) today, you're talking about a mission lasting hundreds of years; the nearest stars are more than 8000 times farther away than Pluto.

We don't have the slightest idea how to send a probe to the next galaxy over, let alone "far away galaxies."

But in the general case, by simply sending relays on a following course at fixed intervals behind the prime probe, I believe you can turn the inverse square law problem into an approximately inverse linear one -- 100 relays each with transmission power x can transmit a signal 10 times as far as a single probe of power 100 x. For an interplanetary mission conducted in a reasonable amount of time, the delta-v provided by gravity assist from a Jupiter flyby is a relatively small component; perhaps the primary probe with a heavy scientific payload could use Jupiter to match the speed of the lighter relays which would be launched following it.

On an interstellar mission, solar power would be out of the question (unless you consider an absurdly vast number of really low power relay stations), so lifetime of the relays' power sources would be a huge constraint. RTGs are the power supply of choice for our outer system probes, and their performance degrades over a few decades.

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    $\begingroup$ It seem very hypothetical, but would a single failure of one of these probes render the entire system useless? $\endgroup$
    – duzzy
    Jul 16, 2015 at 20:54
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    $\begingroup$ Assuming the probes were close enough together and powerful enough to achieve high data rates normally, and weren't able to maneuver significantly to close the gaps between them, a single knockout would most likely force the two neighboring relays to shift to a lower data rate between them, which would bottleneck the whole pipeline, as if you'd only had half as many relay stations to start with. It could be designed to degrade "gracefully", but it would definitely degrade. $\endgroup$
    – Russell Borogove
    Jul 16, 2015 at 20:58
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    $\begingroup$ Within the solar system, what might be practical over the next hundred years or so is a whole bunch of relays in circular solar orbits along the ecliptic, for example at 3, 6, 9, etc. AUs from the sun, with multiple satellites spaced roughly 3AU apart along each orbit. That way there's nowhere near the ecliptic that's more than 3AU from some node on the network, and probes going to any of the planets can use it. (You'd have to schedule visits to Pluto and other high-inclination targets for periods when they were close to the ecliptic.) $\endgroup$
    – Russell Borogove
    Jul 16, 2015 at 21:24
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    $\begingroup$ The exact configuration of the network would be constantly shifting because the orbital periods are all different, but it would have the advantage that losing any one node would hardly degrade it at all; you could always route around it until a replacement was put in position. The speed-of-light latency would increase with the reroute, but the throughput wouldn't be reduced. $\endgroup$
    – Russell Borogove
    Jul 16, 2015 at 21:28
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    $\begingroup$ @RussellBorogove, the comments here deserve to be in the answer, they are quite helpful. especially that gravity-assist would not be required for an interstellar mission. $\endgroup$
    – Gürkan Çetin
    Jul 17, 2015 at 6:47
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Claudio Maccone has been pushing for cleverly exploiting physics of radiowave and light propagation, namely, gravitational lensing.

The main idea (attributed to Von Eshleman from Stanford - 1979) is to send a probe (or several) to the nearest star AND to send a large space radiotelescope in an opposite direction relatively to the Sun, 550 to 740 AU away. The Sun would focus the transmission from the probe, thus gaining around 57.5 dB at the 21 cm wavelength - this gain cannot be obtained by DSN dishes on the Earth. Certainly, economy-wise it is better to send one radiotelescope and one probe instead of a thousand or a hundred probes spaced in time (for the same order of expected scientific results).

How lensing works

FOCAL mission drawing 1

How to organize the interstellar comms network

Not to scale

Other technical tricks and tips to chirp (no pun intended) away at the problem

  • Raise EIRP. Instead of an RTG with decaying fuel, install a nuclear reactor with (say) sodium coolant, keep it turned off until flyby at the target star's planetary system, turn on and transmit all you want to transmit.

  • Make the communications asymmetrical (a laser downlink and microwave uplink), see the paper by Boone et al. (2002).

  • Another exotic line of thought relies on neutrinos. Sending a reactor will kinda help in generating some neutrinos, but we have no way to generate enough neutrinos to cover the horrendously large distances, and no way to focus them. Sci-fi at its best, and the "sci" part isn't realistic at all. Of course, you can provide antimatter propulsion, and communicate via modulated thrust - but please bear in mind antimatter is prohibitively expensive.

  • Please also consider the amount of data you want to transmit. At interstellar distances your probe is essentially travelling in a straight line, and there's little if any opportunity to deflect the trajectory to aim precisely at the other star or any of its planets, if any unexpected anomaly pops up. Thus the flyby of the other star's system will be quite unlike the New Horizons - you'll be sending back mostly multispectral data from far away - a few dots here, a few dots there, spectral analysis of the atmosphere, summary of intercepted radio transmissions :) That's more or less it. No full color spy sat pictures, no live feeds. Thus, the need in high-speed comms is not as acute as you'd thought.

References

  • Deep Space Flight and Communications: Exploiting the Sun as a Gravitational Lens Claudio Maccone. Springer, 2009.
  • Von R. Eshleman. Gravitational Lens of the Sun: Its Potential for Observations and Communications over Interstellar Distances. Science, 205 (1979). Pp.1133-1135. DOI https://dx.doi.org/10.1126/science.205.4411.1133
  • Bradley G. Boone; Robert S. Bokulic; G. B. Andrews; Ralph L. McNutt, Jr. and Nicholas G. Dagalakis. Optical and microwave communications system conceptual design for a realistic interstellar probe, Proc. SPIE 4821, Free-Space Laser Communication and Laser Imaging II, 225 (December 1, 2002) DOI https://dx.doi.org/10.1117/12.451807
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  • $\begingroup$ Worth mentioning that the nearest stars are 265,000+ AU away; your gravity telescope's primary is therefore some 400 times closer to Earth than an Alpha Centauri probe. High speed communication at 600-700 AU is a problem left as an exercise for the student? ;) $\endgroup$
    – Russell Borogove
    Jul 16, 2015 at 22:22
  • $\begingroup$ @RussellBorogove - you know the routine - the lower the total loss, the higher the bit rate. Raise the EIRP as well. $\endgroup$
    – Deer Hunter
    Jul 16, 2015 at 22:28
  • $\begingroup$ In a few words, what is the interest of sending the waves past the Sun and (much) farther than the Earth? $\endgroup$
    – Nicolas Barbulesco
    Jul 17, 2015 at 5:18
  • $\begingroup$ @NicolasBarbulesco - the Sun acts as a lens, focusing radiowaves and improving signal to noise ratio. $\endgroup$
    – Deer Hunter
    Jul 17, 2015 at 5:19
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    $\begingroup$ One might want to send some relays after the FOCAL spacecraft too since it will continue on and on and already 550 AU is a challenge. Btw I note that Eshleman came up with the idea same year that the first gravity lens was observed, the Twin-QSO $\endgroup$
    – LocalFluff
    Jul 19, 2015 at 18:05

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