A planetary surface is where the solid (or liquid) material of the outer crust on certain types of astronomical objects comes into contact with atmosphere or outer space; planetary surfaces can be found on terrestrial planets, dwarf planets, natural satellites and planetesimals. The study of planetary surfaces is referred to as surface geology but also a focus of a number of fields including planetary cartography, topography, geomorphology, atmospheric sciences and astronomy. Land (or ground) is the term given to a planetary surface that is not covered by liquid, the term "landing" is used to describe the collision of an object with a planetary surface and is usually at a velocity in which the object can remain intact and remain attached.