AsteroidWatch — glossary

Name: AsteroidWatch live feed
Creator: AsteroidWatch

Plain-language definitions of the terms used across AsteroidWatch (NASA CNEOS + NeoWs).

Objects and approaches

Near-Earth object (NEO): An asteroid or comet whose orbit brings it within about 1.3 AU of the Sun, so part of its path lies near Earth’s orbit. The great majority are asteroids (NEAs).
Near-Earth asteroid (NEA): The common type of NEO — a rocky or metallic asteroid on a near-Earth orbit. Most are small, and most never come close to Earth.
Close approach: The moment an object reaches its nearest point to Earth on a given pass — a flyby. Despite the name, most close approaches happen millions of kilometres away.
Miss distance: How far an object passes from Earth at closest approach, given in lunar distances (1 LD = 384,400 km) and astronomical units (1 AU ≈ 149.6 million km).

Absolute magnitude (H): A measure of an asteroid’s intrinsic brightness. Combined with an assumed albedo, it is used to estimate the object’s diameter.
Albedo: How reflective an object’s surface is. Because a bright small rock and a dark large one can look equally faint, albedo must be assumed to turn brightness into a size estimate.
MOID: Minimum orbit intersection distance — the closest two orbits can ever come, regardless of where the bodies are. A core input to the “potentially hazardous” definition.
Lunar distance (LD): The average Earth–Moon distance, 384,400 km, used as a yardstick for flybys. A pass of several LD is routine; even 1 LD is farther than the Moon.

Potentially Hazardous Asteroid (PHA): An asteroid roughly ≥140 m across with a MOID ≤0.05 AU. A definition about orbit geometry and size — not a prediction that an impact will occur.
Torino scale: A 0–10 public scale combining impact probability and size into one colour-coded number. Almost every object sits at 0, meaning no hazard.
Palermo scale: A technical, logarithmic scale specialists use to compare a potential impact against the ordinary background risk. A negative value means the chance is below that background level.
Sentry: NASA/JPL CNEOS’s automated impact-monitoring system, which continuously scans known asteroids for any future impact possibility and maintains a public risk list.