# How would an extrasolar spaceship communicate with Earth?

Given modern technology, what kind of bandwidth could you expect for communication with an extrasolar spaceship? For example, is HD video possible? Is it safe to assume it would vary by distance? Or is that just the power requirements?

I'm actually hoping for low bandwidth where it takes a long time to communicate simple textual data. Part of the plot involves a mystery that can't be fully realized until the probe gets back to Earth.

As a note, power shouldn't be a concern, though, if it helps lower the bandwidth, power can be restricted. I don't care what kind of technology is used to communicate, radio waves or something else, though it should be something we have the ability to create today.

• When you say extra solar, what sort of distances are you envisioning? – Joe Kissling Apr 7 '17 at 3:52
• I haven't picked a solar system yet, but assume the probe is coming back from a mission to explore a nearby solar system. – Brian Apr 7 '17 at 5:12
• You are in luck, it will be years before a signal can be picked up because to broadcast at such distances requires a massive amount of power. So the probe will have to get close. – Joe Kissling Apr 7 '17 at 5:26

A ship at extreme distances from earth would communicate using the Deep Space Network, or something very much like it, which is an array of radio dishes spaced out across the globe 120 degrees apart. These communication stations broadcast in a designated set of frequencies (x and ka band) between at 20kw and 400kw of power.

The operating speeds for communication are from as high as is 150Mbps to as low as 10bps. Near Earth the communications are quicker the further out you get the slower the communications are. The New horizons probe communicates at 1kbit per second, which is plenty fast to send text information. A single picture takes several minutes so video with current technology is out of the question.

The good news is that there really is no limit to the maximum range that signals could be received, as long as they are being broadcast with enough power. This Document states that the DSN can pick up signals with a strength of just a billionth of a billionth of a watt of power. This puts the minimum signal power of $1\times10^{-25}w$

Now we can calculate the max distance that a signal could be received from using the Power Density Equation

$P_D=\frac{PG}{4\pi d^2}$

Where:

$P_D$ power density

$P$ is transmitter power

$G$ Gain, for new horizons it is 16000 so we will use that.

$d$ is the distance from the transmitter

So now we can make a guess at the max range a signal could be received for a given power output by solving the following for $d$ with New Horizons gain and a generous 100w transmitter:

$1\times10^{-25}w/m^2 = \frac{16,000\times100w}{4\pi d^2}$

Gives us a detectable range of $1.1\times10^{15} m$ or 8 thousand astronomical units or 47 light days.

I'm not actually an EE so if there is an error let me know. I also know that power density is not a 1:1 for being able to detect something, but this is a ballpark. *Also assuming lossless.

• I'm trying to hunt down the equations for directional antennas so we can guess the max range for a given power. – Joe Kissling Apr 7 '17 at 4:49
• This is perfect. Thanks. I'm probably not going to include all this detail (I don't understand it well enough to pull it off), but I want what little detail I have to be as accurate as possible so people that are smarter than me can nod sagely and think I know what I'm talking about :) – Brian Apr 7 '17 at 5:29
• @Brian the hard-science tag requires this much detail in an answer, I'm glad it's overkill. – Joe Kissling Apr 7 '17 at 5:33
• The detail is great. I've already started using it to figure out how much power I want the spacecraft to have in order to travel to the other solar system and back plus operate its sensors and communication. I was actually trying to figure out how to explain why it wasn't in contact all this time, so that part was great to know was real. – Brian Apr 7 '17 at 5:51
• @Joe Kissling Your range calculations look a bit optimistic. To make calculation easier, I'll convert everything in dB. So our minimum signal is -250 dBW (10*log10(1E-25)), our antenna gain is 42 dBi (I know that the i is not justified, but the documents don't to into details, so I'll assume it), our transmitter is 20 dBW. So the EIRP=62 dBW. This gives us the "fade margin" of 312 dB for the path loss, that is to say, we can lose 312 dB of signal strength along the path and the signal will still be detectable. – AndrejaKo Apr 7 '17 at 7:36

Quoting NASA on missions Voyager

Science data are returned to earth in real time at 160 bps. Real time data capture uses 34 meter Deep Space Network (DSN) resources with the project goal to acquire at least 16 hours per day of real time data per spacecraft. This goal is not always achieved due to the competition for DSN resources with prime mission projects and other extended mission projects.

Once a week per spacecraft, 48 seconds of high rate (115.2 kbps) PWS data are recorded onto the Digital Tape Recorder (DTR) for later playback. An additional 48 seconds are recorded each week on Voyager 1. These data are played back to Earth once every 6 months per spacecraft and require 70 meter DSN support for data capture.

While here you can find this data:

the standard values for Blu-ray Discs are:

MPEG-4 AVC Video at 18000 kbps for 1080p

MPEG-4 AVC Video at 8000 kbps for 720p

DTS-HD Master Audio

The bit rate of the audio codec is variable, but should stay under 4 Mbps. That's a total of 22 Mbps for 1080p and 12 Mbps for 720p. Lossy compression may result in much lower bit rates.

In comparison, some transfer speeds:

USB 1.1 (full bandwidth): 12 Mbps
USB 2.0: 480 Mbps
PATA: up to 1064 Mbps
SATA I: 1500 Mbps
Ethernet: 10 Mbps
Fast Ethernet: 100 Mbps
Gigabit Ethernet2: 1000 Mbps


So HD video is way out of the possibilities.