So, the Voyager spacecraft are old, and their nuclear batteries are getting weak. Here is the official plan to end the missions:

https://www.jpl.nasa.gov/voyager/mission/science/thirty-year-plan/

According to this list, they plan or have already reached "Termination of gyroscopic operations". Of course, without the gyroscopes, the craft don't point at the Earth, at all, so communication shouldn't work. It seems the plan is to spin the gyroscopes down when not communicating. Then, prior to calling home, shut down the Bay 1 heaters (these are the main heaters, spacecraft dies without them), spin up the gyros, find Earth, transmit to Earth, shutdown the gyroscopes, fire back up the heaters, wait for the next programmed transmit window.

The relevant data seems to be:
Gyroscopes (+14.4 W steady state, +3.6 W turn on transient and maneuver)
Bay 1 heater power - unknown
Transmitter power: about 25W reported elsewhere.

This seems like a reasonably clever idea, which should get us many more years out of the spacecraft. Although it might limit the amount of data we can get back, since the heater can only be off so much per day. My second question is how they find Earth to point at it. I'm sure they just need to point at the Sun, since the Earth and Sun are merely rounding error apart from each other at this point. But I can't seem to find an actual description of the process. I suspect they have an optical sensor which knows when it is pointing at the sun. Of course, the sun is getting very dim nowadays, and with the gyros off, it very well may find itself pointing no where near the Sun when the search process begins. If the sensor has a narrow field of vision, the search space might be large, forced to just spin in one axis, while varying the other axis slowly, then halt once the Sun is actually found. If no one here knows, then I guess I'll have to dig into a book or two about the spacecraft to satiate my technical curiosity

8/10/2017 3:59:25 PM

https://www.amazon.com/NASA-Voyager-Owners-Workshop-Manual/dp/0857337750/

oh, this is a pretty good book. The voyager spacecraft had friggin' star trackers on it, so reasonably quick orientation should be pretty doable.

8/10/2017 8:40:57 PM

very cool!

8/11/2017 5:12:21 PM

Okay, so the scheme right now is to alternate running the heaters versus transmitting sensor data live back to Earth. This pretty much means any sensor data occuring when not transmitting back to Earth is lost, since the magnetic-tape data recorder cannot run slow enough to transmit, and none of the computers on board have memory to spare for data storage.

This will get us to about 2025, when aparently the next shoe will drop: there won't be enough power to be transmitting to Earth and run the sensors even with the heaters off, which means no more transmitting live data...so after that, we have a working spacecraft that can no longer be configured to tell us anything?

That just wouldn't stand. I bet they'll use some form of on-craft data processing, just storing the results for return to earth. The computer has the ability to write to its own volatile CMOS program memory, although there isn't much of it that isn't taken up by its own programming. With the gyros and transmitter off, run the sensors, process the data as best the data computer can, then when appropriate turn off the heaters and sensors, on the gyros, find Earth. Instead of turning the sensors back on and transmitting the sensor data live back to Earth, I say they can leave the sensors off too, just transmit the tiny bit of data analysis the data computer had generated and stored on itself, which might just be a dozen bytes. Better than nothing. This assumes the sensors survive being turned off, and don't just freeze. All the prior sensors turned off seem to be intended to stay off.

It seems this would get us to the 2040s, when the hydrazine runs out. At which point I suppose we won't need the gyros ever again, just transmit a constant tone in hopes that one day the dish will accidentally point at Earth when a radio telescope just happens to be looking. So, probably never.

8/13/2017 11:08:56 AM

by 2025 super intelligence will have invented FTL travel and we can just go scoop it up

8/14/2017 11:53:28 AM

8/14/2017 5:26:54 PM