Comets have historically been a subject of suspicions and bad omens. They were long considered a symbol of devastation and evil warnings. However,  as the advancements in scientific theories took off, understanding the nature and composition of comets became a challenging mystery to ponder upon. And amongst all the questions regarding these objects of nature, the most intriguing is the one associated with the long beautiful tails of comets that appear when they come close to the sun.

Composition of comets

There are approximately billions of comets orbiting our Sun in the neighboring Kuiper Belt and even the more distant Oort Cloud. All the comets are basically rocks with hints of metallic dust and the frozen dust of volatile materials. In other words, comets are more like dirty snowballs that are rolling around the universe.

Image Credits: Wisegeek

A comet has three main visible parts: the nucleus, the coma, and the tail. The nucleus is the solid body containing volatile ices, silicates, and organic dust particles. Then comes the coma, a loosely bound atmosphere that hangs around the nucleus that forms only when the comet comes close to the Sun. It contains the traces of the volatile ices that sublime with the sun’s heat and dust particles mixed with the frozen ices in the nucleus. And after these two, the tails dominate.

In 1682, British astronomer Edmund Halley critically observed the most famous Halley’s comet and deduced that comets actually appeared periodically. Moreover, he correctly predicted that it would return in 1757, which further solidified his argument. Over the years, many new comets have treated the sky-gazers worldwide, and as per the latest calculations, the number of known comets is nearing 4000.

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So what causes a tail?

As already mentioned above, comets contain some volatile elements in them. Well, volatile substances are those that have a great affinity to get evaporated. So, as a comet comes closer to the sun in its orbit, the Sun’s heat vaporizes some of the comet’s trapped volatile material, thereby releasing the dust particles that were otherwise trapped in the ice. This is what leads to the formation of a coma.

Ever Wondered Why Do Comets Have Tails? 2
Credits: NASA

Later, a combination of solar radiation pressure and the solar wind blows away that gas and the dust particles from the comet’s nucleus, and this is what leads to the formation of tails. Surprisingly, there doesn’t appear a single tail; rather, two separate tails are formed due to solar radiation pressure and the solar wind.

How can radiation exert pressure in the first place? Well, you might be surprised to know, but even the light being emitted by the light bulb in your room is exerting pressure on you right now, although it is negligible to be felt. But how?

A light beam consists of infinite numbers of photons. So when these photons hit an object without being absorbed, they transfer their momentum to that target, which acts as pressure. On average, the radiation pressure of sunlight on one square meter of the earth is equivalent to the pressure exerted by accelerating a one-gram object at seven millimeters per second. Although this pressure seems to be quite insignificant, it is actually the main reason behind the appearance of the comet’s tails.

Types of tails

Generally, two types of comet tails are associated with a comet: the ion tail and the dust tail.

Ever Wondered Why Do Comets Have Tails? 3
Credits: NASA Ames Research Center/K. Jobse, P. Jenniskens

When Ultraviolet photons interact with the neutral gas released by the comet’s surface, they ionize it. Later, the solar wind carries these ions, and they form the ion tail. The ion tail is also known as a plasma trail. The motion and direction of the ion tail are also affected by the magnetic fields associated with solar winds. An ion tail typically glows with a bluish hue and draws a straight line like a broomstick.

On the other hand, the dust tail is neutral and is composed of small micrometer-scale dust particles, which are similar in size to those found in cigarette smoke. The pressure from the Sun’s radiation pushes these particles away from the comet’s nucleus, and these particles continue to follow the comet’s orbit around the Sun. The dust particles form a diffuse and curved tail that typically appears white or pink from Earth.

Since Sun’s radiation pressure plays a crucial role in the formation of a tail, comets act as completely frozen balls of dust when they are far from the Sun, and the mystical tails appear only when a comet is near the host star of a solar system.

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