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There are discussions under the current answer to Will Curiosity and the Mars 2020 rover be able to communicate with each other via a Mars orbiter? about the feasibility of MRO receiving a message from one rover on Mars and forward it to another.

I seem to remember reading about some NASA (and perhaps NASA + ESA) effort several years ago to make future deep space spacecraft able to intercommunicate in a more flexible way than they did in the past, so that messages between two points can take different routes and various spacecraft can receive and later forward messages to other spacecraft in a straightforward way.

But I can't remember where I'd read this or what it was called.

Am I crazy, or is this actually a thing?

Can rovers play too?

Question: In addition to each's ability to phone home by itself, are NASA (and ESA?) deep space spacecraft now generally able to store and forward messages between themselves?

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    $\begingroup$ Are they able to phone home by themselves or are they commanded to phone ? $\endgroup$ – Cornelisinspace Aug 2 '20 at 16:52
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    $\begingroup$ This type of concept is known as a "mesh network". There is heavy work going into this for future lunar missions. $\endgroup$ – CourageousPotato Aug 2 '20 at 23:34
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    $\begingroup$ @CourageousPotato thanks, I know there are apps that people can put on their phones so that they can set up a messaging network based on WiFi-to-WiFi comms directly between phones and bypass cellular networks or even WiFi servers, but I don't know what they are and so didn't mention the analogy in the question. $\endgroup$ – uhoh Aug 2 '20 at 23:37
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Yes! It is know as the "Delay Tolerant Network" [1] (prior versions called this the "Disruption Tolerance Network" [2]). It is currently undergoing standardization by the CCSDS. [3] The protocol itself is called the "Bundle Protocol", and an implementation of the BP is available as an open-source Core Flight System application. [4]

An example use of the DTN between ESA and NASA has been done with ESA' Trace Gas Orbiter (TGO) as early as 2017. [5]

The short version of how it works is this:

  1. Emitting spacecraft establishes a planned link with a relay spacecraft.
  2. Relay spacecraft reserves some amount of data for storage of the emitter spacecraft.
  3. Emitting spacecraft may encrypt the data it needs to relay (this is not mandatory in the protocol).
  4. Emitting spacecraft sends the data over the radio link to the relay spacecraft. If I recall correctly, the relay spacecraft notifies the emitting spacecraft if it has failed to receive (or decode) some of the packets.
  5. At end of transmission, emitting spacecraft may decide to delete the data to make room for more scientific data storage (this is configurable).
  6. When the relay spacecraft establishes another link (with the ground or with another relay spacecraft), then it will forward that stored information.

During the Flight Software Workshop in December 2019, a team demonstrated transfer speeds of several gigbytes per second with the use of FPGAs.

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    $\begingroup$ Thank you for this clear, well-sourced, concise and yet thorough answer! $\endgroup$ – uhoh Aug 3 '20 at 5:17

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