How would the Voyagers finally die if allowed to transmit to the bitter end?

Question

According to this answer and this question the Voyagers will run out of power to "operate a single instrument" in 2025 for Voyager 2 and a little longer for Voyager 1. It is purported to be the end of mission.

But there is scientific data available even from a weak carrier, or from a 40 bit/sec transmission of internal engineering data, even if there are no scientific instruments running.

The Doppler shift provides a continued measure of velocity, and that can potentially provide some information about gravitational potential in the interstellar region. There could even be surprises if a body with some mass passed by and perturbed one of their trajectories slightly.

The frequency, amplitude, and phase of the are also affected by plasma, so if something happened unexpectedly there could be a change. If the spacecraft were bumped by something, the pointing would be altered and this would show up in the strength of the received signal.

So please, no arguments that there is no science to be had once the science packages are turned off!

Suppose there was funding and support to keep listening to the Voyagers as long as they could continue to transmit anything at all, and instructions were sent to configure them in such a way as to allow them to transmit some minimal signal for as absolutely long as possible.

What specifically would be the last straw for the Voyagers that would shut them down or silence them?

Would there be a low-power fault interrupt, or would something burn out, or would the computer's electronics get too cold to operate, or would the radio electronics get too cold to operate, or maybe something else?

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Per this answer and references therein, there is plenty of room in the link budget to receive signals from them for a long time to come, much longer than they are likely to last in terms of electrical power.

Answer 1

In this scenario, the last straw would be the hydrazine freezing which leads to loss of the thrusters. After a while, the loss of attitude control means Earth drifts out of view of the HGA.

The impression I get from reading a lot about Voyager's systems in the last few days is that quite a large part of Voyager's power budget seems to be going towards heating at the moment (270 W available, transmitter requires 70 W, instruments aren't using 200 W so a lot must be used to keep the spacecraft near room temperature). The DTR on Voyager 2 is kept powered up just for its internal heaters, for instance.

Switch off those heaters, and the internal temperature will start dropping, and eventually the hydrazine will freeze.

2024 update: this is what the team expects:

In the most optimistic scenario, the flight team is hoping to continue receiving data from the Voyagers into the 2030s. However, we may only be operating 1 or 2 science instruments by then. And both spacecraft have been in space almost 47 years and the hardware is getting old.

Both spacecraft are running low on power, and as a consequence some of the key hardware is getting colder. Some of their thrusters that control pointing the spacecraft at Earth are slowly starting to clog and we have already used all of our backup thrusters. As the spacecraft age, additional unexpected anomalies will continue to happen.

Once we are no longer able to collect science data, that will mark the end of the Voyager mission. Over their lifetimes, the Voyagers have extended our exploration of the solar system well beyond the neighborhood of the outer planets, and are iconic testaments to human curiosity.

Clogging of the hydrazine lines is a real possibility: the team recently switched thrusters again because for the set that was in use, the hydrazine lines were down to 1/10 of their original diameter due to clogging.

The clogged tubes are located inside the thrusters and direct fuel to the catalyst beds, where it is turned into gases. (These are different than the fuel tubes that send hydrazine to the thrusters.) Where the tube opening was originally only 0.01 inches (0.25 millimeters) in diameter, the clogging has reduced it to 0.0015 inches (0.035 mm), or about half the width of a human hair. As a result, the team needed to switch back to one of the attitude propulsion thruster branches.

Answer 2

We could not know for sure. There may be a failure of the transmitter, of the computer or of the attitude control. Or a malfunction caused by decreasing temperature when heaters must be switched off. The RTGs worked so long without problems, but there is a small probability for a total failure of the RTGs. But only if there is no failure we may see the effects of a gradual loss of power.

If there is a sudden final loss of signal, we could not know what happened and what was the reason.

Measuring the Doppler shift requires a very precise and stable transmitter frequency, but will the master clock circuit still work when temperature and operating voltage are dropping below specification?