Comets, Centaurs, and Damocloids- oh my! an introduction to the Solar System's wide range of comets of all shapes and sizes.

Since antiquity, comets have flown past Earth towards the Sun to the awe and wonder of observers. Early on, they associated the comets with significant events, like in 43 BC, when Caesar's Comet graced the skies. As the name implies, the Romans took it as a reminder of the dead Julius Caesar, who died the year before. By the 1800s, however, people began to understand more about what comets were and how they worked, and began to study more about them. They found several periodic ones, many of which come by occasionally today. (Although some of them were subsequently lost or disintegrated, like 3D/Biela (Comet Biela), which broke into 2 pieces after the 1852 encounter and probably disintegrated later, or 5D/Brorsen (Comet Brorsen), which was last seen in 1879 after patchy observations of it.) While most of these comets have predictable orbits, their orbit often intersects with Jupiter, leading them to often change their orbit. This happened with comet 9P/Tempel (Tempel 1) in 1881, when a close approach to Jupiter lengthened the period of it from 5.68 to 6.5 years. But while the comets you see are the most common type, though, there are some different types of comet that are created by gravitational interactions with Jupiter, close approaches with the Sun, or even interactions with other stars!

Tempel 1 while it's not out-gassing. Credit: NASA/JPL/University of Maryland

First of all, while most comets are unrelated objects of completely different orbits, some all have similar orbits because they originated from another comet long ago. One of these groups of comets is called the  Kreutz Sungrazers. This group is thought to be descended from a huge comet that came to Earth in 326 AD. The comet came unusually close to the sun (between 0.001 and 0.01 AU.) Later, it broke into 2 fragments that came on orbits back into the Solar System. These fragments came back in 1100 and 1106 respectively. The first comet broke further into what would later be C/1843 D1, C/1880 C1, and C/1887 B1. Meanwhile, the second comet, the larger piece, fragmented into many more comets than the first, with C/1882 R1, C/1945 X1, C/1963 R1 (Comet Pereyra), and C/1970 K1 (Comet White-Ortiz-Bolelli) being a few of its pieces. Now, comets with similar orbits appear to come near the Sun every few years now, making them the largest comet group in the Solar System.

A diagram of the Kreutz Sungrazers' fracturing. Credit: Sekanina, Zdeněk; and Chodas, Paul W.

Sometimes, close encounters with Jupiter will cause comets to enter brief orbits around it, sometimes colliding with it. The most famous example of such an encounter was Comet Shoemaker-Levy 9, which entered an orbit around Jupiter between 1960 and 1970. The comet later fractured and collided with the planet. This doesn't usually happen, however. Often, Jupiter will simply change the orbits of the comets, leading them to become Jupiter-family comets, a group of comets whose aphelion is near where Jupiter orbits.


While those are the regular 'unusual' types, there are a few more odd objects which are truly odd!


Extinct comets are simply comets that have run out of all of their ice. After numerous orbits around the Sun, they've burned off all of the H2O they had to offer. A few suspected extinct comets are 3552 Don Quixote, which will have a close encounter with a windmill or two in the next hundred years, 14827 Hypnos which seems to be sleeping right now, and 2101 Adonis, a particularly young asteroid. This type of comet is hard to find, because it's difficult to tell if an asteroid used to have ice it burned off, or if it was an asteroid the entire time, so it's likely there are several asteroids that used to be comets that we don't know about yet.


A sort of 'extinct comet' orbits further out. Named after 2060 Chiron (a centaur in Greek Mythology), Centaurs are comets that were thrown out of their orbit near the sun, and orbit in the inner Kuiper Belt and between Uranus and Neptune. These comets, unlike extinct comets, have a lot of ice still in them to burn off, but orbit too far away from the sun for this ice to vaporize and form a tail. This group has many members, including 10199 Chariklo, the largest known Centaur, and 7066 Nessus.


In between these two groups exists a type of comet called a Damocloid. Named after 5335 Damocles, these are believed to be extinct Halley-type comets. Many objects belong to this group, with semi-major axis ranging from 2 AU (2009 HC82) to over 1200 (2013 BL76 (article by me!)) These are different from extinct comets in that they have extreme orbits that make them more Halley-type or even long-period comets. Speaking of long period comets, let's get on to one of the weirdest type of comet.


Interstellar comets are-well- comets that aren't from the Solar System at all! Like rogue planets, they wander interstellar space, sometimes being pulled into a hyperbolic orbit by another star. These types of comets are rare, and there aren't any likely comets in the Solar System that could be one. Usually, this type of comet has an extreme eccentricity of 1.2 or greater (anything smaller than 1 is a recurring orbit, but 1 or greater is a parabolic orbit, or an orbit where the object won't come back again.) No comets of eccentricities of more than 1.1 have been found, making it unlikely any are interstellar comets. However, gravitational perturbations could easily disrupt their orbit. The only way to know for sure is measuring a comet's spectrum. Comets with odd emission lines, and, therefore, different chemical makeups, probably came from another star. However none of these have been found yet.

Comets are an interesting astronomical phenomenon, and are both beautiful and odd. And, even if the comet is from our own Solar System, you could say that comets are truly 'out of this world'!