Asteroids, comets and moons are leftovers of planet formation. Studying them and their samples, including meteorites, can help us to learn how the Earth was made and acquired the ingredients for life, to obtain practical information for deflecting near-Earth objects (NEOs), and to access resources that would enable space habitats and voyages. Answers are hidden beneath their complex and evolving exteriors. A book by its cover In the 1970s, Mariner 9 and Viking 1 obtained the first geologic images of small planetary bodies 1 , Phobos and Deimos, the moons of Mars (Fig. 1). Since then spacecraft have visited an astonishing diversity of battered relics, providing images of their geology and data about their compositions, and sometimes interacting with their surfaces by collecting samples and making exploratory craters. Astronomers have characterized spectral, thermal and rotational properties of thousands of them, and discovered that many comets and asteroids are binary or triplesystems. Radio telescopes, larger than many of the NEOs they track, measure their orbits, rotations, surface roughness, and for the ones that come close, their detailed shapes 2. Yet beneath these observations their interiors harbor secrets 3. The most obvious clue is mass, measured by tracking a natural moon or orbiting spacecraft. The bulk densities of small bodies are quite low compared to the rocky or icy materials they are made of, implying they are porous. While this is consistent with the popular idea that they are loosely packed rubble piles, inspections by spacecraft show clear evidence of strength, at least on those that are larger than a few kilometers. Hence the also-popular idea that small bodies are monoliths riddled with fractures. Can both ideas be right? Interiors of comets Comets, once captured by the inner solar system, are not long for this world. They vanish by sublimation of their ices with every perihelion. Comet 9P/Tempel 1 experienced decameters of scarp retreat between 2005 and 2011, and 67P/Churyumov-Gerasimenko lost hundreds of millions of tons of gas and dust between 2014 and 2016. Comets are also destroyed by close encounters with planets and the Sun, the best-documented example being Shoemaker-Levy 9 (SL9) that was pulled apart by Jupiter's gravity field into a chain of 20 nuclei in 1992. It responded to the tidal stress as a cohesionless granular solid with bulk density 0.5 g/cm 3-one might think, an icy rubble pile 4. The shape of 67P resembles two spheroids in contact, and its bulk density is the same as SL9, so it might too be a rubble pile. Yet the breathtaking landscapes imaged by the Rosetta mission feature competent structures 5 at regional to global scales, deep chasms and pits (Fig. 2a) and a kilometer-high cliff riddled with fractures. The Philae probe banged onto a hard surface. On its way down, Philae looped behind the nucleus, recording a 90-MHz radio signal from Rosetta. The experiment ended early but was filled with surprises. The complex surface and kilometers of