We will be losing contact with Voyager 1 and 2 in a few years due to the decreasing power output from their RTGs. But if I could wave a magic wand so that the RTGs would maintain their current power level indefinitely (but change nothing else), how long could we continue to communicate and get scientific data from the spacecraft?
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asked Apr 30, 2019 at 14:24
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1$\begingroup$ Nice question. Since the launch of the Voyagers decades ago, we've made considerable improvements on the Earth side of the communications link. Does your question presume that we can continue such improvements? $\endgroup$– DrSheldonApr 30, 2019 at 16:09
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$\begingroup$ Yes, within reason - I agree that improvements will occur, but no "magic bullets" are allowed. $\endgroup$– Eugene StyerApr 30, 2019 at 18:02
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$\begingroup$ The plutonium-238 in the RTGs have a half-life of 87.7 years. They are currently operating at ~72% full power. I suspect keeping them at full power will only extend the communicable lifetime of the Voyagers a little bit. $\endgroup$– IngolifsMay 1, 2019 at 4:02
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$\begingroup$ They were down to 72% power in 1998 (nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1977-076A), power level also depends on thermocouple degradation so it drops off faster than Pu decay. $\endgroup$– HobbesMay 1, 2019 at 14:45
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$\begingroup$ Wanted to note that one of the answers to space.stackexchange.com/questions/36006/… also applies to my question $\endgroup$– Eugene StyerMay 8, 2019 at 13:04
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The Descanso series has most of the information we need, specifically volume 4. On page 44 we find this plot that shows performance predictions using one of the DSN 70-m antennas.
The tooth-shaped lines show the power/noise ratio in dB for each year. The horizontal lines are the minimum P/N ratio (threshold) needed to communicate at a certain speed. In 2020, there's still enough P/N to communicate at 600 bit/s.
When the P/N drops below the threshold, they can switch to lower speeds (not shown in this plot). Voyager can handle speeds down to something like 40 bit/s. Unfortunately the thresholds for these lower speeds are not given. They can probably be calculated from the data in the PDF, but I don't have time to do that right now.
The DSN can be configured as an array, combining a 70-m dish with one or more 36 m dishes. Even a radio telescope like the VLA can be used, resulting in an extra 5.6 dB of signal strength.
As of 2007, the Voyagers have around 280 W available. Power levels are dropping by about 4W/year. The transmitter requires ~72 W at full power so available power is enough to run the transmitter at full power for the time being.
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$\begingroup$ I'm in a library right now so I can't tell if this is paywalled or not, but you may find it interesting. It's from 1980 JAMES WAlT INTERNATIONAL GOLD MEDAL LECTURE, THE VOYAGER SPACECRAFT, RAYMOND L. HEACOCK, Voyager Project Manager, The Jet Propulsion Laboratory, Pasadena, California, USA journals.sagepub.com/doi/pdf/10.1243/PIME_PROC_1980_194_026_02 $\endgroup$– uhohMay 8, 2019 at 12:59
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$\begingroup$ @uhoh the article seems to be paywalled from my home. $\endgroup$– UweMay 8, 2019 at 13:35
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$\begingroup$ @Uwe rats, okay thanks! I'll see what I can do... $\endgroup$– uhohMay 8, 2019 at 15:12