So, I have been wondering. Pioneer 10 and 11 went silent because there wasn't enough power left to operate the antenna.
But what happened after that? Was there still enough power to keep the computers on and do attitude control? Is it possible they still point their antenna to earth and listen, but can't send anything back?
Thanks!
There's a decent chart of RTG output on figure 2-14 of this paper. From 1985 to 2000, RTG output of Pioneer 10 fell from ~100W to ~62W as the RTGs degraded. (Radioactive material decays, thermocouples degrade, etc, etc.)
Extrapolating this out to 2020 you'd expect an output of ~33W.
Meanwhile, this paper on page D-4 gives a nominal power requirement of ~98W in cruise phase, and, more critically, also gives a power breakdown. Estimated of 98W required, subtract off 24W of experiment power and 2W for the prop heaters, gives ~72W required. Of that, ~28W was to power the TWTA (the amplifier for the transmitter).
So we have the following power estimates:
98W -> everything ok
72W -> no experiments, but everything critical still ok
62W -> power output when Pioneer stopped responding
44W -> power estimate for critical systems without transmitter
Going back to the first paper for a second, note figure 2-19 - the output power of Pioneer 10 dropped drastically over time. This makes sense. The TWTA 'wanted' 28W of power, but likely functioned somewhat with less. Pioneer likely stopped transmitting when the TWTAs undervolted to the point they stopped working altogether.
Unfortunately, even if the TWTAs stopped drawing power altogether - I'm not going to dig into the power system enough to see if the Pioneer's undervoltage protection system would do this - we'd need ~44W to power the critical systems less the transmitter, and we only have 33W.
(This is a little bit unfair. The power system estimates include things like % losses in the inverter and such, which should be scaled.)
In the case of Pioneer 10, it has been twenty years since the last signal was received from the spacecraft. While attempts were made to contact the craft afterwards, no answer was received. Without the ability to transmit any acknowledgmentvfrom the spacecraft, we would not know whether a signal sent out to it was ever received.
Wikipedia gives a history of the ending of the mission:
The last successful reception of telemetry was received from Pioneer 10 on April 27, 2002; subsequent signals were barely strong enough to detect and provided no usable data. The final, very weak signal from Pioneer 10 was received on January 23, 2003, when it was 12 billion km (7.5 billion mi; 80 AU) from Earth.[1] Further attempts to contact the spacecraft were unsuccessful. A final attempt was made on the evening of March 4, 2006, the last time the antenna would be correctly aligned with Earth. No response was received from Pioneer 10.[2] NASA decided that the RTG units had probably fallen below the power threshold needed to operate the transmitter. Hence, no further attempts at contact were made.[3]
Cited references
1.
"This Month in History", Smithsonian magazine, June 2003.
2.
Lakdawalla, Emily (March 6, 2006). "The final attempt to contact Pioneer 10". The Planetary Society. Archived from the original on June 16, 2006. Retrieved June 7, 2011.
3.
Angelo, Joseph A. (2007). Robot spacecraft. Frontiers in space. Facts on File science library. Infobase Publishing. ISBN 978-0-8160-5773-3. p. 221.
