Comets & Cometary Activity: Physical Mechanisms

An inactive comet consists mainly of a solid nucleus containing ice, dust, and refractory material. Near the Sun it develops a coma, a curved dust tail, and an ion tail that points more directly away from the Sun under the influence of the solar wind and magnetic field.

As the nucleus warms, volatile molecules sublimate and drag dust from the surface. Radiation pressure pushes dust away gradually, while ionized gas couples to the solar wind and is carried anti-sunward. Repeated perihelion passages erode and alter the nucleus.

Spacecraft such as Giotto, Deep Impact, Stardust, and Rosetta have imaged nuclei directly and sampled comet material. Spectroscopy identifies water, carbon-bearing molecules, and other volatiles in comae, while tail geometry confirms different forces acting on gas ions and dust.

Sublimating gas drags dust from the nucleus. Solar radiation pressure bends the dust tail, while ionized molecules couple to the solar wind and magnetic field, producing a straighter anti-solar plasma tail. Repeated perihelion passages erode the nucleus and can split or deactivate a comet.