According to the Lunar and Planetary Institute, the nucleus of a comet is usually 1 to 10 kilometers across, and the tail stretches tens of millions of kilometers. A comet’s nucleus is a stable, porous mass of dust, ice and gas.
The nucleus of a comet can become as large as 100 kilometers, explains Craig Freudenrich on HowStuffWorks. It is the primary, solid part of the comet, and it is composed of rock. A comet has various parts, including the nucleus, the coma, the hydrogen envelope, the dust tail and the ion tail. These distinct parts become more visible when a comet approaches the sun. The coma is a halo of dust and evaporated gas, which consists of water vapor, carbon dioxide and ammonia. The nucleus and coma form the comet’s head. As the comet warms up, it creates its coma, which is generally 1,000 times bigger than the nucleus. It can even become as large as Jupiter or Saturn, with a size reaching 100,000 kilometers.
An invisible layer of hydrogen, known as the hydrogen envelope, surrounds the coma. It becomes bigger as the comet draws nearer to the sun. The hydrogen envelope often has an irregular shape because the solar wind distorts it.