Plural of asteroid: large rocky objects in space that orbit the sun, much smaller than planets.
From Greek 'asteroeides,' combining 'aster' (star) and 'eidos' (form/appearance). William Herschel coined the term in the late 1700s because through telescopes they resembled stars rather than planets.
An asteroid 10 kilometers wide ended the age of dinosaurs 66 million years ago, and today we track near-Earth asteroids because the impact probability is real—it's one of the few natural catastrophes humans can actually do something about through science and engineering.
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