Albedo
Albedo is the fraction of light that a surface reflects, from 0 for perfectly black to 1 for perfectly white.
Albedo is a measure of how reflective a surface is. Fresh snow has a high albedo, reflecting most light, while charcoal has a very low one. For asteroids and comets, astronomers use albedo values from about 0.02, darker than coal, to about 0.5 or more for the brightest icy bodies.
Albedo is a key clue to composition. Dark C-type asteroids and comet nuclei have albedos around 0.03 to 0.1, while stony S-types are around 0.15 to 0.3 and basaltic V-types can be brighter still. Because a dark asteroid reflects less light than a bright one of the same size, albedo must be known to work out an asteroid's size from its brightness.
Infrared telescopes help by measuring heat: a dark asteroid absorbs more sunlight and glows more warmly. Combining reflected light and heat gives both size and albedo, which is why infrared surveys are important for planetary defence.
Related terms
Level: Explorer Ages 10 and up