The Whipple Shields will protect the spacecraft when it encounters Comet Wild-2 in 2004. in honor of Fred Whipple. Today we know that they are black and cold, consisting of ices and dust that coalesced from an interstellar cloud as it collapsed to form the solar system. Their physical structure is poorly known.
Some astronomers guess that comets have low densities, perhaps less than half that of water ice.
This material would be crushable in one's hands. Even the ratio of ices to dust remains a subject of uncertainty. Probably the physical structure varies over small distances such as a few meters.