Comets

Comets are sometimes called "dirty snowballs" which is a term coined by Fred Whipple to describe the composition of a comet. They are basically icy bodies with dust mixed in. As mentioned in the past two posts, comets generally come from either the Kuiper Belt or the Oort Cloud, depending on the period of the comet. Short-period comets (periods of less than 200 years) come from the Kuiper Belt and long-period comets (periods of greater than 200 years) come from the Oort Cloud, though some short-period comets come from the Oort Cloud.

A good rule of them to determine from where a comet comes, is to look at where the comet is in the sky. Since the Kuiper Belt is generally aligned with the ecliptic (and the solar equator), those comets will appear within the zodiac constellations*.

*Please do not confuse the zodiac constellations with the astrological zodiac (which is total bs, by the way). The zodiac constellations will be a topic for a future post.

Oort Cloud comets, however, can appear anywhere in the sky since the Oort Cloud encompasses the entire Solar System. So if you see a comet near the north celestial pole, you can be sure that that comet come from the Oort Cloud, and likely may not come back again.

Anatomy of a Comet

 

Comets have three main parts: the nucleus (also called head or body), the coma, and the tail. The comet nucleus is a permanent fixture as it contains all the ice and dust of which the comet is made. When the comet is disturbed and is pushed towards the Sun, there is a point when the solar radiation and heat will create the coma and the tails of the comet. The coma is the hazy envelope surrounding the comet and is caused by the outgassing of the ices on the surface of the nucleus. It can be thought of like a fog surrounding the comet. The tail has actually two parts: the gas tail and the dust tail. The gas tail is the tail that always points directly away from the Sun and is caused by the solar wind pushing the gas away. The dust tail is actually curved and follows the orbit of the comet.

 

What is unique about a comet's tails, is that they always point away from the Sun. In the above picture, the bottom portion of the path shows the comet leaving the solar neighborhood, and the tails are leading the comet, rather than following.

 

Comets are also believed that have brought water to the Earth. Water is considered a volatile element, which means it has a low boiling point and wouldn't be found in large quantities in the solar nebula when the Solar System first formed. Water may not have been the only thing comets brought to Earth: they may have brought complex organic molecules which could have helped lead to life on Earth. This is only speculation, however. No hard proof has ever been found.

 

Comets also can lead to meteor showers. As a comet travels around the Sun, it loses parts of its nucleus which stay in orbit around the Sun. As the Earth orbits the Sun, the Earth may go through the particles left and those particles may fall to the Earth and we see them as meteors.

Oort Cloud

Graphical Representation of Oort Cloud (obviously, not to scale)

The Oort Cloud is a spherical region of the Solar System that can effectively thought of as the boundary between our Solar System and interstellar space. Unlike the Kuiper Belt, the Oort Cloud does not lie mainly in the plane of the Solar System, but it does, however, contain objects similar to the Kuiper Belt. The Oort Cloud objects are mainly water ice and ammonia ice bodies.

The Kuiper Belt is home to short-period comets, which generally are comets with elliptical orbits and periods of less than 200 years. Oort Cloud comets have much longer periods (if they are periodic) of more than 200 years, and may have elliptical, parabolic, or hyperbolic orbits. The difference between elliptical orbits and parabolic and hyperbolic orbits, is that elliptical orbits are closed (comets with these orbits return periodically), while parabolic and hyperbolic orbits are open (comets with these orbits come towards the Sun once, never to return). When the eccentricity of an orbit equals 1, that is what defines a parabolic orbit, while eccentricities greater than one have hyperbolic orbits.

Different Orbits

There is a theory that the objects in the Oort Cloud formed closer to the Sun, but were bumped into the region of the Oort Cloud by gravitational interactions with the outer planets. It is about 100,000 AU in diameter with two main regions: the spherical outer Oort Cloud and a disk-like inner Oort Cloud (also called Hills Cloud). The cloud is named after Dutch astronomer Jan Oort and is sometimes referred to the Opik-Oort Cloud in conjection with Estonian astronomer Ernst Opik who postulated in 1932 that the long-period comets came from a cloud on the outer edges of the Solar System. Jan Oort revived this theory to help resolve a paradox on the stability of comets. Comets could not have formed in their orbits because the Sun's radiation would cause them to boil away.