The hull where they're able to manually inspect it shouldn't be going through quite that extreme of a temperature differential because it's wrapped in insulatio

The hull where they're able to manually inspect it shouldn't be going through quite that extreme of a temperature differential because it's wrapped in insulation and protective layers. But it's also almost 21 years old so even the smaller actual cycle is probably getting to it combined with all the other mechanical stresses from equipment and thruster firings.