We know the RTGs will fail one day. Assuming Voyager probes have a full transmitter output power of 22 watts can/will this be scaled down as the need arises?
tl;dr: Voyager's X-band traveling wave tube amplifier (TWTA) can be set to either 18 or 12 Watts of output power. These correspond to 71.9 and 48.3 Watts of electrical power, so the short answer is Yes.
Since there is so much talk about the Voyager's end of mission around 2025 due to lack of power for any instruments (beyond communications) it is possible that they've already switched to the lower power setting.
But I don't know, so I've asked separately:
- Are the Voyager spacecrafts' X-band TWTAs currently set to high or low transmit power? What are their change histories?
- How would the Voyagers finally die if allowed to transmit to the bitter end?
- Currently there is definitely enough power for a downlink rate of 160 bits/sec according to this answer to How to calculate data rate of Voyager 1?
- DSN can combine received signals from multiple dishes to improve signal to noise ratio, according to this answer to Why does DSN sometimes uses two dishes at the same time to receive Voyager-1?
- There's plenty of room to boost the uplink power from DSN, which is usually a nominal 20 kW, but at least the DSN 70m dish at Goldstone is equipped to go to 400 kW. For more on this see Has DSS-43 ever been used in high power mode (>>20 kW) for an emergency situation?. For Voyager 2 coverage, higher power wold have to be added to the 70m dish in Canberra in the southern hemisphere. For more on this, see this answer to What if the Voyagers had remained within the plane of the ecliptic?
The go-to source for information on deep space communication is the DESCANSO Series (See also When was the idea behind the DESCANSO Book Series first conceived, by whom, and what is it's “mission”?).
In particular the DESCANSO Design and Performance Summary Series Article 4; Voyager Telecommunications by Roger Ludwig and Jim Taylor .
Here is an interesting table and quote in Section 1 on page 6:
The duration of the VIM is limited primarily by the decreasing spacecraft electrical power (from the two radioisotope thermoelectric generators [RTGs]) and telemetry link capability. Table 1-1 provides life estimates for electrical power, telecommunications, and hydrazine (for attitude control). With Voyager 2 now far south of the ecliptic, it is not visible from the northern hemisphere stations. The table shows telemetry data rate limits for two Deep Space Station sizes at Goldstone, California for Voyager 1 and near Canberra, Australia for Voyager 2. Limits for the third site, near Madrid, Spain, are similar to those at Goldstone for Voyager 1.
Voyager contin(u)ously reviews, updates, and consolidates processes in order to increase efficiency and improve its return on public investment. During VIM, Voyager has reduced its flight team staffing by 97%, from approximately 300 in 1989 to 10 in 2002. Reduced staffing increasingly constrains VIM in the areas of non-routine activity planning, execution and analysis, and anomaly response.
The allocations of VIM telemetry rate to types of data are as follows. At 160 bps or 600 bps, the different data types are interleaved.
- This was published in March of 2002, so it's based on all of the engineering data available over the 25 years starting from their launch in 1977. It doesn't reflect anything that's been learned about the conditions of the Voyagers after 2002.
- VIM = Voyager Interstellar Mission, surely a sign of optimism!
- With predicted 40 bits per second downlink capability of 2045 and 2050 for Voyagers 1 and 2 respectively, it was felt at that time that with the best estimates of the Pu-239 RTG degradation (half-life of the radioisotope plus loss of efficiency of the thermocouple converter itself due to radiation damage and other aging effects) the systems would still be able to operate their transmitters at least through then.
Electrical Power System
What else do we know about the power system?
When there is more than enough power available from the RTG, the Voyagers maintain a constant supply voltage by shunting a variable amount of power to a set of shunt resistors which radiate heat into space, according to @Hobbes's answer to Why did Voyager have to shunt unused electrical power and radiate as heat?
But what happens when there isn't excess power?
X-band Transmit Power
3.3.4 X-Band Exciters
The X-band exciter converts the frequency at the output of the S-band exciter to X-band to drive the X-band TWTA. Comparable to the S-band exciter function, the X-band exciter phase modulates the RF signal with the composite telemetry signal from the TMU and, if the X-band ranging channel is on, the ranging signal detected by the receiver.
3.3.5 X-Band Power Amplifiers
Only one X-TWTA can be powered at a time. Further, a control input from the CCS ensures that the X-TWTA is powered off when the X-exciter is off. As is the case for S-band, whether powered on or not, the X-TWTA power level is selected to either of two levels4 by CCS control input.
In October 1987, the Voyager 1 X-TWTA-2 failed, as annotated in Figure 3-1, legend item 3. The primary downlink was switched to X-TWTA-1. In November 1998, Voyager 2 switched from X-TWTA-2 to X-TWTA-1 .5 For both spacecraft, legend item 1 in the figure flags the changes from X-TWTA-2’s right hand circular polarized downlink to X-TWTA-1’s left hand circular polarized downlink.6
4The low-power and high-power RF levels to the HGA for the X-TWTA are 12 W and 18 W.
5The switch to the backup X-TWTA is in status report http://vraptor.jpl.nasa.gov/voyager/pressrel/vg981117.html
6The relationship between X-TWTA selection and the resultant polarization of the X-band downlink is described in the SXA section that follows.
Foot note #4 indicates that there is at least one method to decrease the transmit power of Voyager built into the system. A command from the ground can instruct the X-band High Gain Antenna (HGA) Exciters to change between 18 W and 12 W of transmit power delivered to the antenna. But what does that mean in terms of electrical power?
Figure 3.1 from Article 4; Voyager Telecommunications (click for larger), discussed further in the question Frequency responses of Voyager's S-band high gain antenna's feed-horn and receiver front end & IF?
According to Table 3-3. Voyager spacecraft input power and mass summary that reflects a change between 71.9 and 48.3 Watts of electrical power, a difference of over 23 Watts which is a sizable chunk of Voyager's total power budget!
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.
However, simply having the radio carrier itself allows one to do at least a little bit of science based on the Doppler shift and signal strength. So I am not yet sure if they will really shut it down because there's no data coming from other instruments (just internal engineering data), or just let it continue to send a signal towards the Sun and Earth so that it can be picked up from time to time. Remember the optimism of the Voyager Interstellar Mission with dates of 2045 and 2050 for the 40 bps data rate !
I think it is safe to say though that so close to a potential end of mission due to low power that the X-tube amplifiers are already set to the lower power setting.