Four watts, and what you turn off next

NASA announced this week that Voyager 1’s cosmic ray subsystem was switched off on 25 February. Voyager 2’s low-energy charged particle instrument is scheduled to follow on 24 March. Both spacecraft lose roughly four watts of available electrical power each year. ^[1]

Four watts is an awkward amount to explain at home. My desk lamp is an unreliable comparison because I keep changing the bulb. In a machine whose spare capacity has already been used up, though, the number does not need a household analogy. It is another subtraction from a budget that has no new income.

Heat and electrical power age differently

With a Pu-238 half-life of about 87.7 years, the isotope-only fractional heat loss in one year is 1 − 2^−1/87.7, or about 0.79%. That does not directly predict the loss of electrical watts at the spacecraft bus. Conversion efficiency and operating temperatures have their own history. ^[2]

It is tempting to take an original electrical rating, apply the radioactive decay law, and announce what the spacecraft “should” have left. That calculation is useful only if you state that you have held the conversion process fixed. Otherwise the discrepancy becomes an invented mystery.

Turning an instrument off also changes the thermal balance. Some of the electricity it consumed was heating the local structure. Whether that matters elsewhere depends on the actual paths for heat flow. I cannot see those paths in a press release, and I would be wary of claiming to know an easy saving the mission team missed.

The question that disappears

NASA’s account says the Voyager 2 instrument’s rotating platform has a stepper motor tested for 500,000 steps; by its planned shutdown it will have made more than 8.5 million. The same account describes hopes of keeping at least one science instrument operating into the 2030s, subject to the spacecraft continuing to cooperate. ^[1]

I like that motor detail because it does not turn reliability into magic. A component exceeded a test requirement by a large margin. We can admire the result without pretending the original test guaranteed it.

The sadness is less numerical. A powered instrument can answer a particular family of questions. After shutdown, the spacecraft may still be reachable, and another instrument may still have something to tell us, but that family of questions has gone quiet.

This is dated deliberately. The second shutdown is still a plan as I write, and the team’s estimate is not a promise of a final transmission date.