Figure 2: The surrounding encapsulant is more than 1-mm-thick in 259-UL913 fuses. They keep a good seal and limit the energy and temperature generated when the fuse operates. Dust is also prevented from entering the fuse body.
Having been on the same NASA trip, Steve, I can second your statement that the engineers and the programs there are astounding. If there is still time, I strongly encourage readers to take the few minutes to put their names into the contest. As we saw on our trip, there are no strings attached. Littlefuse treated the contest winners likes kings and queens.