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How far away from Earth could we reliably communicate using radio?

I'm thinking that a primary factor would be how much RF power could be kept in a "tight beam" and accurately pointed.

Assumotions:

  1. Users do not care about lag time between sending and receiving.
  2. Using present technology for sending and receiving.
  3. "Reasonable" power is available for use. No harnessed stars or fusion generators.
  4. Large antennas of present construction capability could be placed well away from the noise of Earth.
  5. Do not care about data rate.
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    $\begingroup$ I really wonder at what distance two fully functional Arecibos could talk to each other... $\endgroup$
    – SF.
    Feb 15, 2023 at 12:01
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    $\begingroup$ @SF. I had done that calculation once, years ago, and it was 10,000s of light years if I remember correctly, all comes down to what an acceptable data rate is.... $\endgroup$ Feb 17, 2023 at 11:04
  • $\begingroup$ Why RF? Optical works better because apertures are far smaller for the same gain, and depending on band, noise and foregrounds can be far better. Search term: optical SETI. $\endgroup$
    – user71659
    Feb 19, 2023 at 23:34

3 Answers 3

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Frame challenge, regarding point 5, do not care about data rate: That is problematic. At extremely low rates (e.g., a bit per hour) communication is pretty much useless, and in many cases, it is impossible. No vehicle that I know of is capable of slowing its transmission rate down to a bit per second, let alone a bit per hour.

For example, the Voyager satellites will be dead in terms of communication well before they have to throttle their transmission rate down to one bit per second. They are not capable of transmitting at such extremely low rates. In addition, the ground systems are not capable of receiving such extremely low transmission rates signals.

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    $\begingroup$ yeah... I knew before I added #5 that it would be an issue but I wanted to leave that door open. CW transmission (continuous wave) morse code via RF has a very low data rate but can be effective over great distance at low power. I guess the question needs definition of "effective" $\endgroup$
    – BradV
    Feb 15, 2023 at 14:33
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    $\begingroup$ It is reasonable for the designers to have chosen to make communication at low bit rates impossible, indeed because they thought, like you, that it would be pointless. But that is a design choice not an inherent impossibility and it would be interesting to hear what “the limits of the possible” are here. $\endgroup$ Feb 15, 2023 at 14:33
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    $\begingroup$ After all, the receipt of a CW pilot tone is a respectable method of getting information about a spacecraft’s state when it is (eg) being woken from hibernation. $\endgroup$ Feb 15, 2023 at 14:36
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    $\begingroup$ The lowest data rate in use I know of is the 50 bit/s of GPS. $\endgroup$
    – asdfex
    Feb 15, 2023 at 16:14
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    $\begingroup$ The Juno mission to Jupiter was capable of data rates as low as ten bits per second, but I don't know how low they actually went in operation. $\endgroup$
    – Ryan C
    Feb 15, 2023 at 21:16
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“It depends” doesn’t answer the question of “what distance”, but max distance depends on engineering more than limitations imposed by physics.

1) What is the signal/noise ratio you require? The background noise will depend on the frequency you chose and the direction the spacecraft is communicating from.

2) What is the effective area, aperture efficiency and gain of the receiving antennae?

An example given in https://en.wikipedia.org/wiki/Parabolic_antenna is a 25m satellite ground antenna at 21cm which gives a gain of 52 dBi. The Deep Space Network 70 meter dishes have gain of 74dBi

For 2.5cm-12m waves, the atmosphere is transparent so the antennae can be ground-based. Placing the receiving antenna outside the atmosphere will allow reception of shorter wavelengths and therefore increase the gain of a given size antenna. Gain is proportional to (diameter/wavelength)^2. The mass of a given size space antenna could be less than that of a ground based antenna of the same size but there are obviously challenges designing, launching and assembling a precision structure of this scale.

3) Similar issues of size apply to the spacecraft's transmission antenna. An inflatable antenna may prove to be the optimum design. There may be significant distortion due to acceleration. You are interested in maximum distance, so presumably there is been a long thrust phase.

An alternative is to deploy and tune the antennae only after the start of the coast phase of the mission. An intriguing option is to use the antennae as a parabolic solar collector to drive a solar thermal rocket while inside the solar system, and then convert it to a high gain communication antenna during coast phase.

4) How much power is available for transmission? RTGs are limited by the lifespan of the isotope: 89 years for 238Pu, 432years for 241Am (with a half life of , but only a fifth the power density of 238Pu). Mass of isotope and shielding becomes a significant factor if you are going for maximum distance.

You could consider a nuclear reactor, but the lifespan of fuel rods, even the highly enriched rods used in submarines, is only about 50 years.

Available power will decrease as the mission progresses, but required power for successful communication will increase.

5) How intermittent is the signal transmission? Storing power for a short burst could produce a much stronger signal than continuous transmission. But power storage in cold interstellar space is a challenge. Batteries and supercapacitors don’t like the cold. Insulation and waste heat from RTG could keep them warm.

Answer: it depends.

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  • $\begingroup$ Nature was much better, the en.wikipedia.org/wiki/Natural_nuclear_fission_reactor continued fission for a few hundred thousand years. So design of a reactor for much more than 50 years should be possible. $\endgroup$
    – Uwe
    Feb 16, 2023 at 15:37
  • $\begingroup$ @Uwe .. That one was a bit too heavy to launch. A 'slow-poke' reactor which ticked over producing isotopes and subsequent heat for RTG type generation would be a good cross-over technology. Know of any such design? $\endgroup$
    – Woody
    Feb 16, 2023 at 15:55
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tl;dr: Interplanetary and interstellar scintillation!

At what distance does radio communication become unusable

Assuming:

  1. users do not care about lag time between sending and receiving.
  2. using present technology for sending and receiving.
  3. "reasonable" power is available for use. No harnessed stars or fusion generators.
  4. large antennas of present construction capability could be placed well away from the noise of Earth.
  5. do not care about data rate.

Interesting question! And deserving of an answer.

There are some unsatisfying responses:

Unlike conventional deep-space optical communications technologies, for low data rates, signal strength at the destination will drop as roughly $1/r^2$ out to intergalactic distances. There will be a small amount of attenuation in some directions due to scattering and absorption, but mostly dust gas and even the charged particles in interstellar and intergalactic space isn't enough to seriously attenuate radio, as long as you choose your frequency carefully. Otherwise radio astronomers would have a less interesting job.

However, it's another matter for SNR. Interplanetary and interstellar scintillation!

While the integrated electron density along these very long interstellar (and intergalactic) paths is still to low to attenuate or reflect radio signals, they are high enough to refract them slightly!

What that means is that due to density inhomogeneities, and turbulent fluctuations over the timescale of hours to days to years, the radio sky shimmers like "heat waves" over a hot road at sunset will cause mirages that change with time.

This causes the apparent source size and shape to change and become larger, and imposes a random intensity modulation that could mix with real modulation in nonlinear ways in order to depress the SNR well below what you'd expect from pure $1/r^2$.

Even if you build a ginormously large aperture to collect signals from far away, you have this big, shifting phase noise across your aperture due to fluctuations in integrated electron density along the path to each part of your aperture.

Interstellar scintillation is certainly a known thing, but people are just starting to learn about it in detail and how it varies over time and in different directions. It's an ongoing field of research.


1Of course you can sent one bit of real information per minute even if your data stream contains 100 bits per second. Take for example LORA:

where it's constantly sending streams of repeating patterns at a high rate in a very high noise environment while the bit rate of communicated information could be much lower.

In a deep space context, this might be like sending one long command for a "1" and a different long command for "0" and choosing the details inside those two commands to be such that their bit patterns are orthogonal and even in a noisy environment they look maximally different.

That may let you get around "modern spacecraft can't do low data rates" as long as you can update the spacecraft's software to handle this mode. Modern deep space spacecraft often use software defined radios so I think this is sufficiently plausible that your question should not be dismisses so easily.

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