An asteroid, comet, or large meteoroid whose orbit brings it exceptionally close to the Earth, and which may therefore pose a collision danger. Most such objects are in orbits around the Sun with peri
helion distance less than 1.3 astronomical units.
A term frequently used in climate model applications to refer to permafrost at depths close to the ground surface (typically down to 3.5 m). In modelling studies, near-surface permafrost is usually di
agnosed from 20 or 30 year climate averages, which is different from the conventional definition of permafrost. Disappearance of near-surface permafrost in a location does not preclude the longer-term persistence of permafrost at greater depth. See also Active layer, Frozen ground and Thermokarst.