Parts of a Comet

A comet has five different parts: the nucleus, coma, hydrogen envelope, plasma tail, and a dust tail. The nucleus, coma, ion tail, and dust tail are the four visible parts. At the same time, the hydrogen envelope is the only invisible part. Each of the parts with their characteristic features is given below:

1) Nucleus

It is the central solid part of the comet. The nucleus occupies the central position, also known as the core. It consists mostly of ice, gases, and dust particles covered with dark organic matter. The nucleus is often frozen, consisting of carbon dioxide, ammonia, carbon monoxide, and methane. It is sometimes composed of rocks.

The nucleus is usually 1 to 10 km in diameter but can sometimes be as big as 100 km. As the comet reaches the Sun, the nucleus heats up, and gases start escaping from it.

2) Coma

The coma is the most visible portion of the comet surrounding the nucleus. Along with the nucleus, they form the comet’s head. It is a spherical envelope of evaporated gases such as water vapor, ammonia, carbon dioxide, and dust particles. The coma forms when dust and gases sublimate from the nucleus, bypassing the intermediate liquid phase.

It is almost 1,000 times larger than the nucleus and can even become as big as planets Jupiter or Saturn.

3) Hydrogen Envelope

It is the only invisible layer of the comet found surrounding the coma. The hydrogen envelope forms when the comet absorbs ultraviolet light, releasing hydrogen, which escapes the comet’s gravity, forming an envelope of hydrogen gas. It is irregular-shaped, being distorted by the solar wind.

The hydrogen envelope comprises about 60 million miles in length and 6 million miles in breadth, which gets bigger as the comet approaches the Sun. It is invisible from the Earth because the light coming from this layer is absorbed by our atmosphere but is visible by spacecraft. 

4) Ion Tail

It is made of electrically charged molecules of carbon dioxide, nitrogen, and water. The ion tail is also called the plasma or gas tail. It forms when the charged molecules are pushed away from the solar wind’s nucleus, converting some gases into ions. It is not as enormous as the dust tail, but accelerates faster than them, moving in a straight line opposite to the comet and the Sun.  

The ion tail measures over 100 km long that sometimes disappears and later reappears when the comet crosses a region where the direction of the Sun’s magnetic field is reversed.  

5) Dust Tail

It is the layer away from the Sun. The dust tail is made of small dust particles that evaporate from the nucleus, pushing them away from the comet due to sunlight pressure. It is often curved due to the comet’s motion in fixed orbit at the same speed at which the dust moves away, much in the same way as water leaves the nozzle of a moving hose. This acceleration is relatively slow.

It measures up to 10 million kilometers in length. As their distance from the Sun increases, the dust tail fades and is found to diminish.