Astronomy - Tale of the Comet

Tale of the Comet

        There are many objects in our solar system: the Sun, the moon, the eight planets, Pluto, asteroids, and many more objects, but one of the most astonishing objects is the comet. Up until the seventeenth century, comets were generally accepted to be “… vapors or dry exhalations rising from the Earth” (Fernandez 39). Newton challenged this idea with his view of comets as simply a body that is made up of a small nucleus that shines as a result of light being reflected from the Sun, with a tail that is formed by vapors being released from its nucleus (Fernandez). Newton’s idea was correct, but the true physical attributes of comets continued to be debated for about three centuries. Now, astronomers have a much more clear idea of what exactly comets are, how their tails are formed as well as how their tails relate to meteor showers seen on Earth, and the ways in which comets can die.

What are Comets?

        Comets aren’t too complicated when it comes to their structure and composition, but like most things, they’re made up of different parts. The main part of the comet- the nucleus- is made up of ices and rocky dust (Bennett). This part of the comet is usually only a few kilometers in diameter. The nucleus is the only part of the comet that is a permanent feature that survives during the whole lifetime of the comet (“comets”). When a comet gets near the Sun, a coma forms around the nucleus (Bennett). This is basically an atmosphere formed around the nucleus caused by the increase in surface temperature of the comet due to it being in close proximity with the Sun. The increase in surface temperature causes the ices to sublimate. Sublimation is the process in which materials go directly from a solid to a gas, without ever becoming a liquid (Cochran). The gas that is formed escapes the comet’s gravity, dragging dust particles away from the nucleus, thus forming the coma (Bennett). The coma’s size compared to the nucleus is a lot larger. As the comet gets closer and closer to the inner solar system and the Sun, the coma grows. At the same time, some of the gas and dust that make up the coma gets pushed in the opposite direction of the Sun, forming tails. Comets are actually low in density although their heads and tails have rather large volumes (Fernandez). Once the comet passes the Sun and starts its journey away toward the outer solar system, the coma begins to dissipate and the tails begin to disappear as sublimation ceases (Bennett). This process repeats itself each time the comet orbits the Sun.

        Although comets are repeatedly seen in the inner solar system, they actually originate from the outer solar system (Bennett). Comets come from the Oort cloud and the Kuiper belt. The Oort cloud is a “… vast, spherical region of space…” that is presumed to contain around a trillion comets (Bennett 269). The Oort cloud extends out to around 50,000 times the distance between the Earth and the Sun (Bennett). This region of space is “… gravitationally bound to the solar system, which it follows in its orbit around the Milky Way Galaxy” (“comets”). Comets within the Oort cloud cannot be observed as they don’t develop comas or tails since they’re nowhere near the Sun, and they are too far away to be observed in general (“comets”). The orbits of comets in this region aren’t uniform, rather, they’re pretty randomized (Bennett). When the jovian planets were formed, the planetesimals that escaped imminent collision with these new planets were flung in all directions. Some of these planetesimals ended up drifting through interstellar space as they gained enough speed to escape the solar system completely. However, most of these planetesimals ended up forming orbits around the Sun. This is how the Oort cloud was formed. The other source of the comets in our solar system is the Kuiper belt. The Kuiper belt is located just beyond Neptune’s orbit, at 30-55 times the distance between the Earth and the Sun. The comets that are located in the Kuiper belt follow patterned orbits and journey around the Sun in the same direction that the planets do. Both of these regions combined are the sole sources of the comets in our solar system.

        Some comets repeatedly return to the inner solar system; these comets are called periodic comets (“comets”). There are short-period comets, intermediate-period comets and long-period comets. Short-period comets on average have a seven-year period but can have periods up to twenty years. There are 135 short-period comets in our solar system. These comets have a perihelion distance of about one and a half times the distance between the Earth and the Sun. Short-period comets revolve around the Sun in the same way that the planets do. Intermediate-period comets have periods between twenty and two hundred years. There are 20 intermediate-period comets in our solar system. These comets revolve around the Sun in the opposite direction in which the planets do. One of the most famous intermediate-period comets is Comet Halley. This comet has made thirty perihelion passages. Long-period comets have periods of more than two hundred years. There are 655 long-period comets in our solar system. These comets are spread out at random in all directions. About half of the long-period comets revolve around the Sun in the opposite direction that the planets do. These periodic comets are the comets that are observed on Earth whether it is once in a lifetime or once a year.

Comet Tails

        Most comets remain in the outer solar system, forever frozen; however, some comets manage to enter the inner solar system, where they can form tails (Bennett). There are two types of tails that comets can have: a plasma tail and a dust tail. It is possible for these tails to reach as much as 108 km in length (Fernandez). The plasma tail is bluish in color and it stretches out to larger distances in the opposite direction of the Sun. This tail is made up of atoms, molecules and ions. Some ions present in the plasma tail include carbon monoxide, nitrogen, and carbon dioxide (“comet”). The dust tail is curved and made up of dust particles that scatter sunlight, causing the comet to glow (Fernandez). The sublimation of ices that occur as the comet approaches the Sun creates the gas molecules that form the plasma tail (Brandt). The same sublimation process releases dust particles that were imbedded in the nucleus to form the dust tail. The Sun exerts radiative pressure on the comet’s dust as it orbits the Sun, causing the dust grains to be pushed away from the Sun (Cochran). The combination of the Sun’s radiative pressure and the motion of the dust particles as the comet orbits around the Sun is what makes the dust tail curve away from comet. The plasma tail is a straight line, pointing directly away from the Sun because of the Sun’s solar wind pushing the ionized gases of the comet outward. In essence, comet tails are a direct result of being in close proximity of the Sun.