# Is a space probe communication relay possible?

Is it possible to maintain a connection with space probes that are far away by sending other space probes behind it. For example -

[earth] ------1---- 2-------3--------4------5....and so on if required (it depends on the distance from earth)

These 1,2,3,4, 5 are space probes and they are supposed to pass the command given to them to the next probe?

My question is that, is it possible to navigate or maintain a connection with the last probe in that way?

• Yes, theoretically. It hasn't been done yet, but Breakthrough Starshot is investigating it as a possible way to make lightyear-scale communication with nanosats easier. Commented Jun 5, 2017 at 18:25
• Regarding gravity, mass doesn't change trajectory - a smaller probe has a smaller force due to gravity, but also has less inertia and changes direction more easily, so it will have the same trajectory as a larger probe. However, it's lower inertia means that the same booster will accelerate it to a higher velocity, than although this improvement becomes less relevant as the mass of the probe compared to the booster becomes negligible. Commented Jun 5, 2017 at 18:28
• @Deimophobia... If a small probe is compared with a larger probe, (both having the same boosters) which would travel fast? Commented Jun 5, 2017 at 18:35
• the smaller would travel faster, but if both are very small compared to the booster then the difference would be very slight. Commented Jun 5, 2017 at 18:37
• Commented Jun 5, 2017 at 19:22

That wouldn't be a good approach, for a number of reasons, but I'll focus on power here.

The strength of a signal drops off proportionally to the square of the distance, so probe #1, at half the distance, would have to transmit 25% of the power as the Earth to have the two signals be the same strength at probe #2. It's a lot easier to put a big transmitter on Earth, where we can plug it in to mains power, than on a spacecraft where it has to be powered by the solar array or RTG.

Putting some numbers to this, the Deep Space Network can send 20 kW, compared with a few 10s of Watts on a spacecraft.1

(The DSN also has 70 m diameter dishes, which allow the power to be concentrated into a much narrower beam than a spacecraft antenna does, giving the ground station an even greater advantage.)

• Worth noting: this holds true for distances of scale of the solar system plus some. When it comes to distances of order of a light year, a series of relays may be better than a single, powerful source. (still, they'd need pretty strong power sources, and these, being heavy, are hard to accelerate to speeds that would cover light years in any reasonable time.)
– SF.
Commented Jun 6, 2017 at 22:39
• This answer only applies to communication from the Earth to the satellite, and as far as I know there are no probes that are seriously constrained by bandwidth in that direction. On the other hand, this answer does not apply to communication from the satellite to Earth, for which it is reasonable to consider communication relay sats. Commented Jun 5, 2020 at 19:40

A supplementary answer, in response to @JessRiedel's comment:

this answer does not apply to communication from the satellite to Earth, for which it is reasonable to consider communication relay sats.

There are two problems, broadly speaking: orbital dynamics and economics.

Orbital dynamics means that you can't keep a satellite permanently half way from Earth to Jupiter. After a few years, the various orbital motion under the effect of the Sun's gravity will have moved things so it's actually further from Jupiter than Earth is. So any such relay is either a one-shot for a particular mission for a relatively short period, or you need a whole lot of them spaced out around an orbit, which is getting crazy expensive.

The economic problem is essentially a variant of the uplink considerations in @djr's answer: it's almost always cheaper to upgrade facilities on Earth than to put something in space big enough to be useful. Let' try some numbers: Suppose we put a relay half way between the probe and Earth. On Earth we have a very high quality 70m dish and absolutely state of the art amplifiers, unlimited power to run those amplifiers, liquid helium to cool them, etc. If our relay is half way between Earth and the target, and somehow manages to have a 70m antenna of its own (massively bigger than anything ever flown) of the same quality and equally good amplifiers and signal processing it will quadruple the available data bandwidth. We could alternatively do that by building three more 70m antennae on Earth and linking them (a well-developed technology). Not only would that be massively cheaper, but they could be used for other purposes when not needed.

What is useful and has been done, is for a bunch of small probes in the same part of the solar system at the same time to relay their communications through one "mother ship". It means you only have to budget the mass for a 2 or 3 metre antenna and a relatively powerful transmitter, once, and you can put the relay in an orbit where it will usually be visible from Earth. This is happening now for Mars missions. If there were a flurry of missions to the Jupiter system, for example, the same approach would be worthwhile -- put a comms relay in high Jupiter orbit and have all the missions relay through it using less massive and power hungry systems.

• Thanks, this is definitely useful. Following your logic with respect to "mother ships": it seems like repeaters become worthwhile when the number of interplanetary probes operating in distant orbits (e.g., Kuiper belt) exceeds the number of repeater satellites you need dispersed in intermediate orbits (e.g., between Saturn and Uranus). Suppose you need 10 repeater satellites at the intermediate orbits to ensure one's always in position. If there are 100 probes dispersed in Kuiper belt, then you can make them all smaller in the presence of repeaters. Commented Jun 6, 2020 at 12:15
• Note also that there must be a limit to your argument regarding economics, i.e., there must be a parameter regime where it's more economical to improve the strength of the signal from the probe than to improve the sensitivity of the Earth receivers. (Otherwise probes would broadcast at arbitrarily low power levels and we'd build titanic receivers on Earth.) Presumably this is something like "you might as well increase the signal power emitted by the probe until it become comparable to the non-communication probe power requirements". Commented Jun 6, 2020 at 12:22
• @JessRiedel The Mother Ship is really only a win when it's MUCH closer to all the probes than they all are to Earth. I am not sure it's ever really worth having a relay half way. If you think about it, that is almost never done on Earth, except where line of sight is a concern. Commented Jun 6, 2020 at 16:18
• Why would that be? It's true that the optimal location for the mothership is to be close to all the probes when that's possible, but when it's not the (e.g., when the probes are dispersed) it's optimal for the repeater to be halfway. And it seems pretty clear that in the limit of extreme (e.g., interstellar) distances repeaters are better: total power string of fixed-separation repeaters scales linearly with distance to Earth, but power usage by un-repeated probe scales quadratically. So if not useful in solar system, it must be due to details. Commented Jun 6, 2020 at 21:36
• Consider also the extreme limit where there are, say, millions of microprobes in the outer solars system. Now consider whether to add a single repeater in orbit. That seems worthwhile even if for some reasons it were prohibited from repeating messages from probes in its immediate vicinity. Commented Jun 6, 2020 at 21:39