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Let's suppose we have found a new human colony today, somehow populating Pluto. They have good technology, much like ours and we don't want to slave them. We want to establish a communication link between us and them. How could we do it using our technology?

Ok, let's assume some things first.

  • Wormhole, superdrive, novadrive, etc. was not invented yet.
  • We are at 2015.
  • Pluto is at 4,6 light hours from Earth.
  • We have a big budget to spend.
  • Our goal is to establish a 1Mb/s link (2,25 GB/5h).

With this, I think we could have some rudimentary communication established. We may send and receive videos, documentation and knowledge with a good speed. Within this scenario, we will never have a real time communication, since we can't break speed of light.

So, how do we address this issue?

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    $\begingroup$ Relevant: space.stackexchange.com/questions/6207/… $\endgroup$ – Rohcana Sep 22 '15 at 13:37
  • $\begingroup$ @Anachor, that was a lot of info. Thanks! $\endgroup$ – Rodmentou Sep 22 '15 at 14:41
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    $\begingroup$ Does it matter how long it takes to get the information between the two planets? $\endgroup$ – ASCIIThenANSI Sep 22 '15 at 15:45
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    $\begingroup$ "we don't want to slave them" - you don't know much about human exploration, do you? $\endgroup$ – corsiKa Sep 22 '15 at 18:39
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    $\begingroup$ Related; possible duplicate: Information Exchange In Space $\endgroup$ – a CVn Sep 22 '15 at 20:34
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Your question is misleading. Depending on the bandwidth allocated in the radio spectrum to such communication, we can very well have gigabit links to pluto and back. Major problem in deep space communication is latency, not bandwidth.

The limiting factor for digital bandwidth is signal to noise ratio (so, power alone wont cut) and bandwidth (this is the [in]famous shannon-hartley-nyquist theorem afterall), bandwidth here is the size, "width" of the radio spectrum allocated to transmissions. When you say "my FM transmitter uses the 100mhz frequency" you are not saying it all, because no transmission occurs in a single frequency, but on a continuous spectrum of frequencies called 'a channel', the size of such channel in MHz is the aforementioned bandwidth. Digital bandwidth is then a consequence of this channel size and the signal to noise level. Usually, the ammount of information you can inprint on a signal is half as much as the size of the channel in hertz (so 0.5 bits/hertz of bandwidth). For example, FM radio uses a ~200khz channel (this would mean that the 100mhz station mentioned earlier occupies from 99.900mhz to 100.100mhz contiguous spectrum). This size holds the guard bands between channels, two audio channels and a pilot tone that allows decoding the stereo arrangement. This same 200khz band would mean a ~100kbit/sec digital bandwidth using a first-order modulation scheme like PSK, by virtue of the nyquist limit. This cannot, however prevent second-order or higher-order modulation schemes (like QAM etc). Using higher order modulation schemes the same 200khz band can carr more information per second (called symbols per second) than the normal first-order modulation. But, as the symbol density of the modulated signal increases, the signal loses the capability to survive noise. This is where the signal to noise ratio comes into play. (The shannon part).

Another factor to be considered is the Friis equation, that allows calculating the signal power at the receiving end of a transmission/receiving system. Following this equation, a system with a low gain antenna and a very powerfull transmitter, will usually have a poorer performance than a system with low power but a very high gain antenna. Both characteristics (transmitter power and antenna gain) are expressed as decibels. The path loss (free space loss) is calculated as the sum of the gas loses (the energy that gases in the path convert to thermal energy) and the geometric loss (the decoherence of the beam, inverse square law). In space you should care only with the geometric loss. At such distances, the geometric loss is huge, so, a low directivity antenna pair will have bad performances, even if operating with very high power. The way to go is to have very very directional antennas. The Friis equation (also called radar equation) sums all gains and all loses on a system, like Rpower = AntennaGain1 + AntennaGain2 + Power1 - Pathloss. In decibels (a logarithmic scale) the difference between a 1w signal and a 1000w (1kw) signal is circa 36db(w). A parabolic antenna can usually have 36db gain over the isotropic transmitter. This means that a system transmitting at 1000w of power but using an isotropic antenna (theoretical) will reach the receiver with the same power as a transmitter with mere 1w of power but a 36db parabolic antenna, and cheaper too. Higher power transmitters tend to emit a lot more noise than lower power ones, so, in the end of the day, lower power is usually better than too much power (Your priority goes from finding the bigger gain antenna possible, to the lower noise equipment to the largest power, in that order).

Summarizing both poins above, the bandwidth is limited by the capability of the system to send a large bandwidth signal across space while maintaining a good signal to noise ration, and using very directional antennas. But, you asked about bandwidth, and there we have no big problems. Like I said earlier, major problem for space communications is latency, the time that a wave takes to go from pluto to earth will have a major impact on protocols like TCP/IP. We will need better protocols. If error rate is too high (your latency will prevent simple acknowledge protocols to work satisfactorily) we do have some forward error correction protocols which may prove good enough. TCP/IP does not use forward error correction protocols due to the best effort nature of our networks and the low latency usually involved. But, nothing prevents someone from using a error correction protocol at the physical layer of the network.

Forward error correction protocols go a step further from error detecting protocols, in that they allows not only to detect errors, but to use aditional information in the packet itself to correct a certain maximum ammount of errors induced by the propagation media. This why you decrease the need for acknowledge packets (wich are limited by the very high latency of the network). Its right at the aknowledge system where latency might become a bandwidth limitation, becase to transmit the next packet the network might need to wait for the aknowledgement of the previous packet sent. TCP/IP uses a sliding window protocol to prevent this, but, the size of the windows involved in such a case are too small to be used in huge latency situations like those in the question. So, you might use bigger, FEC-protected packets and go away with acks altogether.

tl;dr

Bandwidth is not a problem. Latency is the real problem and we dont have a way to break the speed of light limitations but we can mitigate that problem with forward-error-correction protocols.

(This question is right into my professional area (networks).)

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  • $\begingroup$ That was a good, detailed answer. Thanks, Jorge. $\endgroup$ – Rodmentou Sep 22 '15 at 21:53
  • $\begingroup$ There are some typos in the text that i might correct later... $\endgroup$ – Jorge Aldo Sep 22 '15 at 21:57
  • $\begingroup$ Just to make something clearer. What would happen if we had a Deep Space Network on Pluto? $\endgroup$ – Rodmentou Sep 22 '15 at 22:01
  • $\begingroup$ For a sense of scale, Voyager, using 1970's technology, a rather small power supply, and , currently transmits at 1kbit/s, one thousandth the goal rate you would like to see. There's room for smarter compression, higher powers, larger antennas, and better crafted directional antennas to improve on this if the financial incentives are there. $\endgroup$ – Cort Ammon - Reinstate Monica Sep 22 '15 at 22:17
  • $\begingroup$ To put things into perspective, if you study astronomical objects using radio waves, you will know that they usually have a ultra large power output (think about a quasar) much bigger than anything men can generate on earth, yet, you need antennas like the one in Arecibo Observatory to receive nanowatts of power here on earth. Thats why i say that power alone wont cut. $\endgroup$ – Jorge Aldo Sep 22 '15 at 22:25
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Get them a powerful directional transmitter and you shouldn't have any problems. New Horizons is transmitting at 20 Watts in our direction and we can receive data just fine. Increasing the power output of the Pluto transmitter along with a larger transmitter should radically improve the signal to noise ratio thus granting higher bandwidth.

If the Pluto antenna doesn't point towards us we won't receive much, if anything at all. If the Pluto signal is too weak, we will have difficulty picking up the signal from the cosmic background noise. If we pick a too-long carrier frequency then there's bandwidth we miss out on that could be had with a shorter wavelength.

Terrestrial TCP/IP protocols are poorly adapted to the unique constraints of deep space data transmission. Proposals have been made for TCP protocols, one called TCP-Planet that should prove interesting.

Any kind of interaction that requires rapid interactions, such as AJAX and Web 2.0 will obviously fail.

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  • $\begingroup$ Interesting. So, if I build a Deep Space Network 2.0 on Pluto, I would achieve my goal? What about using re-transmitters along the way? $\endgroup$ – Rodmentou Sep 22 '15 at 14:43
  • $\begingroup$ You would want to include a few hops in the form of retransmitters/routers (out there it will be the same hardware) between Pluto and Earth but yes, you could do it. $\endgroup$ – Green Sep 22 '15 at 14:46
  • $\begingroup$ I don't even think "giant" is necessary. With good directionality and a few hundred watts it shouldn't be hard to hit Megabit speeds. Assuming you are willing to set up reactors on Pluto a hundred watts is cake. $\endgroup$ – Sam Sep 22 '15 at 17:50
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    $\begingroup$ @JorgeAldo less detailed perhaps but not wrong. Further, there isn't anything in my answer incompatible with the Space SE answer cited in the question comments. $\endgroup$ – Green Sep 22 '15 at 21:40
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    $\begingroup$ @JorgeAldo Transmitter power is one side of the equation, antenna directivity (or "antenna gain") is the other. Both can be used to achieve a good enough link (and on the receive side, antenna directivity is the only way to increase the link budget) if you can maintain sufficiently accurate pointing of your antennas. Compare What is a link budget, and how do I make one?; note that you can directly replace power output with antenna gain (on the transmit and/or receive side). The hardest part with high-gain antennas is accurate enough pointing. $\endgroup$ – a CVn Sep 23 '15 at 7:27
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Related: https://www.nasa.gov/content/disruption-tolerant-networking/

As @Green said, getting a giant transmitter will ensure that the message gets across more often than not.

The other problem is the protocol. Again, as @Green mentioned, TCP/IP is rubbish in this instance. As an alternative, NASA are working on and testing sometime called "Disruption Tolerant Networking". This new protocol recognises that messages transmitted across space will contain errors, and that it won't arrive immediately. Here's a quote form their page:

Information processing nodes, satellites or ground stations, need to be able to store the data that they receive until they are able to safely send it to the next node in the network.

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Easy: use x-rays!

Radio and even visible-light lasers diffract too much to get good efficiency. At the frequencies that New Horizons uses (about 7 GHz (pdf)), even a huge antenna like Goldstone's 70-meter dish are diffraction-limited to a beamwidth of about:

$$ \theta \approx 1.2 \frac \lambda D = \frac{1.2c}{fD}=\frac{1.2c}{7~\text{GHz}\times70~\text{m}}\approx0.04^\circ\approx 2.5' $$

This may not seem very big, but at Pluto this translates to a spot about:

$$ w\approx \theta d = 2.5'\times 33~\text{AU}\approx 3.6\cdot 10^{6}~\text{km} $$

in diameter! On the other hand, if we use soft x-rays with an energy of about $1~\text{keV}$ and a much smaller "antenna" (say, one meter):

$$ \theta \approx \frac{1.2hc}{ED}=\frac{1.2hc}{1~\text{keV}\times 1~\text{m}}\approx 0.3~\text{mas} $$

That's in milliarcseconds, or one one-thousandth of one thirty-six-hundredth of one degree. The spot is now only:

$$ w \approx 0.3~\text{mas}\times 33~\text{AU}\approx 7.3~\text{km} $$

across! Of course, the small wavelength of x-rays mean that x-ray optics are in practice not diffraction limited. However, as long as we can achieve sufficient pointing accuracy we can pretty much make the beam as small as we want, achieving amazing link efficiency.

I'll assume that we can achieve a pointing accuracy of $0.1''$. This gives us a spot size of about $2400~\text{km}$. With a receiver diameter of just $2~\text{m}$, we get a total loss of about $120~\text{dB}$. Assuming that our detector is fairly sensitive and needs only about 100 x-ray photons per bit, then the transmitter power must be about:

$$ \begin{align} P&=E_\text{per bit, transmitted}B \\ &=E_\text{per bit, received}L_\text{free space}B \\ &= 100\times 1~\text{keV}\times 10^{-120~\text{dB}}\times B\\ &\approx 16~\text{mJ}\cdot\text{bit}^{-1}\times B \approx 16~\text{kW}\cdot\text{Mbps}^{-1}\times B \end{align} $$

In practice our pointing accuracy can get much better, and we have detectors capable of sensing individual photons, so the power usage could go even lower.


The only trouble is that soft x-rays don't penetrate through the atmosphere at all. This is not so much a problem at Pluto, but here at Earth we'd need put our x-ray transceiver in space. However, communicating at high data rates with spacecraft near Earth is not a problem, plus a spaceborne transmitter affords us the pointing accuracy that we need.

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    $\begingroup$ Interesting approach. I look forward to your explanation of how you convince the general public that this is safe. :-) $\endgroup$ – a CVn Sep 22 '15 at 20:39
  • $\begingroup$ @Michael As I mention, soft x-rays don't penetrate through the atmosphere at all. I actually work for someone who's working on x-ray comm, and the lab demo we have is totally blocked by a piece of paper or just a couple feet of air. Its really no more dangerous than a high-power radio transmitter. $\endgroup$ – 2012rcampion Sep 22 '15 at 20:41
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    $\begingroup$ Oh, I do realize that you even said so in the answer. But people aren't always swayed by logical arguments based in science and engineering. If someone were to propose a scheme like this in the real world, lots of people would just hear "x-rays" and go on to chant "dangerous!", no matter how much you show them that it's perfectly safe. (Experiments can be rigged, you know; how do we know that this receiver is sound?) Even more so since we're talking interplanetary communications. $\endgroup$ – a CVn Sep 22 '15 at 20:43
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    $\begingroup$ @Michael ... just as many people today here "radiation" and chant "cell phones cause cancer!" $\endgroup$ – 2012rcampion Sep 22 '15 at 20:44
  • $\begingroup$ Indeed, that's one relevant comparison. $\endgroup$ – a CVn Sep 22 '15 at 20:44
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I can't cite the specific example, but this is a classic communications/information theory problem. Basically the lower your signal to noise ratio the less bandwidth you have, but the easiest way to increase your signal (with minimal effect on noise) is power. The only reason New Horizons is so slow is its power budget.

A solution I consider worth looking at is a laser based link, it can have incredibly high bandwidth, and a very tight beam.

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We should very quickly be able to get TCP/IP (the core protocols of the internet) running on such a link- though it is likely to require some modification of the Data Link Layer (see OSI Model) and probably those higher up the stack, to allow for massively increased packet and connection timeout durations.

The OSI Model, upon which TCP/IP is based, describes a standard way of handling communication between two points and deals adequately with packet errors, packet timeout delays, out of sequence packets etc.

Obviously this assumes we have:

  • made contact
  • have established a common language
  • have physically transmitted the comms specs to the other end (or vice versa!)

Having done that, all that would remain would be to introduce the aliens to Facebook, Twitter and Instagram et al :)

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    $\begingroup$ Plain TCP wouldn't be a good fit, since it does basic things such as congestion control only based on responses from other side; with a 9-hour round trip, you'd want to optimize for sending things already in anticipation so that you don't waste multiple days just because your application protocol handshake requires a few extra roundtrips to negotiate some insignificant detail. It would be of no benefit to use TCP/HTTP/etc unchanged since the rest of the internet devices will have timeout values measured in seconds, not days, so you would use custom protocols and custom data transfer anyway. $\endgroup$ – Peteris Sep 22 '15 at 16:55
  • $\begingroup$ I didn't suggest "plain" TCP, I suggested a tweaked version, which is exactly what is outlined in the TCP-Planet paper suggested by Green above. Talk about nit picky! $\endgroup$ – Marv Mills Sep 22 '15 at 18:44

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