> ... a captured comet?
Interesting. Unlike Tempel's size, Hyperion (according to the NASA site) is 204 by 162 by 132 miles, and it's spongy (in Jupiter's gravity) rather than powdery (in the Sun's gravity).
I don't know where to take it from here....
That would be a huge comet. But just how large could one be?
But as for the surface, "Spongy looking", yes. But it very well could be powdery...