Comet nucleus

A comet nucleus is the solid central body of a comet, composed largely of ice, dust, and rocky material. Heating near the Sun releases gas and dust from its surface, forming the comet’s coma and tails.

Cometary outgassing: The release of gas from a comet as its volatile materials warm and sublimate. Jets of escaping gas carry dust away from the nucleus and drive cometary activity.
Water ice: Water in its solid crystalline or amorphous form. Water ice is a major volatile constituent of many comet nuclei.
67P/Churyumov–Gerasimenko: A Jupiter-family comet visited by the European Space Agency’s Rosetta mission. Rosetta mapped this nucleus in detail and followed its activity over time.
Planetesimal: A solid body formed from smaller particles during the early stages of planetary-system formation. Comet nuclei are thought to descend from icy planetesimals formed in the early Solar System.
Rubber-duck model: A description of the bilobed shape of comet 67P/Churyumov–Gerasimenko, resembling two lobes joined by a narrow neck. Its shape prompted debate over whether the nucleus formed by a gentle merger or erosion.
Sublimation: The direct transition of a substance from solid to gas without passing through a liquid phase. Surface ices exposed to sunlight commonly vaporize this way.
Carbon monoxide: A colorless, odorless gas composed of one carbon atom bonded to one oxygen atom. Its ice is highly volatile and can drive activity far from the Sun.
1P/Halley: A periodic comet that returns to the inner Solar System about every 76 years. The 1986 spacecraft encounters provided the first close-up images of a comet nucleus.
Protoplanetary disk: A rotating disk of gas and dust surrounding a young star, where planets and smaller bodies form. The nucleus’s building materials originated in the Sun’s protoplanetary disk.
Contact binary (astronomy): A body consisting of two formerly separate objects that touch or are joined together. Some bilobed nuclei may record low-speed mergers of smaller icy bodies.

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