21p

21P, designated 21P/Giacobini-Zinner, is a periodic comet in the inner solar system that completes an orbit around the Sun in roughly 6.6 years. It is categorized as a Jupiter-family comet, a group characterized by short orbital periods and orbits influenced by Jupiter’s gravity. The comet’s path is a highly elongated ellipse that brings it close to the Sun before it returns to the outer solar system.

Discovery and naming: The comet was first observed by Michel Giacobini at the Nice Observatory in 1900. It was later observed independently by Ernst Zinner, and the combined designation Giacobini-Zinner honors both observers.

Orbit and dynamics: As with many comets, 21P’s orbit is subject to perturbations from the planets, especially Jupiter. Outgassing—release of gas and dust as the comet approaches the Sun—can also produce small non-gravitational forces that alter its trajectory slightly from pass to pass.

Physical characteristics and observations: Like other comets, 21P develops a coma and a tail when near perihelion, caused by the sublimation of volatile materials. The nucleus is thought to be relatively small, and the level of activity can vary between apparitions. The comet has been observed across multiple perihelion passages since its discovery, contributing to studies of comet composition, structure, and behavior.

In astronomy, 21P/Giacobini-Zinner serves as a representative member of the Jupiter-family comets and provides data relevant to understanding how short-period comets evolve under planetary influences and solar heating.