re: It's a big mystery
The brush on the rat is made, I believe, of steel wire, the sort of thing you'd take rust off an old car part, or mud out of boots.
The Solar arrays have a thin glass cover iirc - and the combination of steel wire brush +glass cover = broken solar cell.
The IDD can reach up to the very very front of the solar arrays - but probably <5% of the surface area. To have designed it with more range would have been to design it too big to fit in the space available.
Doug