Extract from introduction: There are a number of contexts in which space hardware may interact mechanically with the solid material present in a low-gravity environment
at the surface of an asteroid or comet, possibly penetrating to some depth and yielding useful information. These contexts range from low-speed scenarios, such as passive free-fall to the surface, to hypervelocity impact. Such penetrating devices may be classed as penetrators, anchors, impactors, 'moles', etc
