rickman wrote in :

The reason why cooling something gets harder as it approaches
absolute zero is because the heat flow is proportional to the difference
in temperature. Even if your pump is perfect and acts as if you put the
thing being cooled in contact with a heat sink at 0 °K, the rate of heat
flow decreases as that temperature delta diminishes.


Ok, that helps. It's close to what I had in mind, though my reasoning may
still be bad. For what it's worth... if a superconductor is very cold,
needing to be so, then because there is no way to go below zero K, there are
more things hotter, than colder, so they have more effect than the shaded
space conditions. That balance might favour a need for forced cooling just to
play safe in many cases, but I accept that isolation might be fairly easy to
do, and I also accept that 77K is likely far enough above shaded space
conditions that it gives a wide margin to prevent small leaks from nearby
heat sources causing failure.