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Why do comets have low gravity?
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Comet
Comet McNaught with filamentary tail and the Moon over the Pacific Ocean, photographed from Paranal Observatory, Chile, January 2007.
Comets have low gravity primarily because of their small physical size. In the context of astronomy, the gravitational pull of a celestial body is directly related to its mass; because cometary nuclei are relatively small, they do not possess the significant mass required to generate strong gravitational fields.1
This low gravity has important consequences for a comet's behavior. Because the gravitational hold of the nucleus is weak, gases and dust particles released through the sublimation of ices can easily escape the surface and expand freely into the vacuum of space, forming the comet's characteristic coma.1