If we compare the amount of visible light to the amount of infrared light, we can establish a ratio of the amount of incoming sunlight an asteroid reflects to the amount it absorbs. When an asteroid absorbs light from the sun, it emits this absorbed light back into space in the infrared portion of the spectrum, in other words, as heat. Some of the sunlight is reflected and some is absorbed. The light astronomers see shining from asteroids is reflected sunlight. We determine reflectivity by comparing the brightness of light in the visible spectrum to the brightness of light in the infrared spectrum. We determine an asteroid's distance from its position in its orbit. We calculate physical size from the asteroid's angular size and distance. The brightness of an asteroid in our sky depends on its size, distance and reflectivity. The light we see shining from asteroids is actually reflected sunlight. Direct observations make it easy for scientists to research asteroids however, spectroscopic analysis has given us many clues about the composition and properties of asteroids.
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