In order to better defend my position that trying to track Juno from Earth once it can no longer transmit would be Quixotic, I'd like to ask what the current record is for the farthest detection from earth (using radar, optical/IR, etc.) of an earth-launched spacecraft or other earth-launched object that was no longer able to actively transmit or rebroadcast (e.g. Doppler ranging).
The recent so-called 'WTF' object comes to mind, but there may be something farther, or otherwise more demonstrative of (at least non-military) capability.
The bold font phrase is meant to exclude detection of landers on the Moon or Mars from spacecraft orbiting those bodies. I'd also like to exclude lunar retro-reflector arrays from Apollo missions as well as LAGEOS because these can be monitored only because we already know exactly where they are supposed to be.