The Near-Earth Asteroid Rendezvous (NEAR) spacecraft determined that the bulk density of C-class asteroid Mathilde was 1.3 g cm (Veverka et al. 1998; Yeomans et al. 1998). The surface of Mathilde, however, has the same spectroscopic signature as carbonaceous chondrite meteorites, whose typical density is somewhere near 2 g cm (Wasson 1985). The only plausible way to explain Mathilde's low density is to assume its interior contains large void spaces or that it consists of small fragments with substantial interparticle porosity. Either way, Mathilde can be considered a rubble pile rather than a monolithic fragment.