By Phillip Chappell
Updated Mar 24, 2022
The nucleus sits at the heart of a comet’s head and remains perpetually frozen. It is composed of a mix of ice, dust, and rocky material, with volatile gases such as carbon monoxide, carbon dioxide, methane, and ammonia trapped within. Typical diameters range from about 0.6 to 6 miles, but can exceed this size in some cases. The nucleus accounts for most of a comet’s mass and is renowned as one of the darkest objects in the sky, reflecting only a few percent of sunlight.
Surrounding the nucleus, the coma is a cloud of gas and dust that expands outward, often reaching diameters of up to 600,000 miles. It is formed when the comet’s ices sublimate, releasing water vapor, carbon dioxide, ammonia, and other neutral gases. The coma, together with the nucleus, constitutes the comet’s head and is the portion most easily visible from Earth.
A comet’s tail is actually a collection of three distinct streams that trail the nucleus and coma. The ion (plasma) tail, composed of charged particles, is pushed directly away from the Sun by solar wind, creating a sharp, bluish streak that can extend over 60 million miles. The dust tail, formed from microscopic solid particles, is broader and follows the comet’s trajectory, bending gently as the comet moves. Between these lies the envelope tail, a thin layer of hydrogen gas that spans roughly 6 million miles across and can stretch for about 60 million miles. When the comet is near the Sun, the envelope tail appears more pronounced.
Due to their modest sizes, comets rarely form spherical shapes; instead, they display irregular, often lumpy forms. Their visibility from Earth increases dramatically as they approach the inner solar system and brighten under the Sun’s light. The nucleus reflects merely about 4% of incident sunlight—one of the lowest albedos recorded—compared to asphalt’s approximate 7% reflectance.
