What makes a comet different from an asteroid? Simple. Asteroids are located in the asteroid belt, while comets are beyond that. They are mainly found in the Kiuper belt as well as the Oort Cloud. But unlike asteroids, NASA has only be able to identify about 3,743 of them. If they orbit too close to the sun, their frozen structures will melt, resulting in the discharge of vapors and particles.

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Source: Overview | Comets – NASA Solar System Exploration