Main page: Asteroids (theoretical models)
Comets are celestial bodies with elliptical orbits, that orbit a star and who lose matter through outgassing.
Comets originate in the Kuiper Belt or in the Oort Cloud. At some point, something interacts with their orbit and pushes them towards the inner Solar System, on a new, elliptical orbit. Once they are on the new trajectory, they are heated. Water ice and solidified gasses start to sublimate and are lost into space. The comet also loses solid particles (dust and salts).
Depending on gravity perturbations from the planets, comets can often change their orbits. Sometimes, they can enter a hyperbolic trajectory and will be ejected from the Solar System. In other cases, they can impact a planet or even the Sun. There have been reported many cases when comets have lost all their volatiles and became small asteroids or clouds of dust.
Generally speaking, comets from the Kuiper Belt are as solid as rocks and will resist many passes around the Sun. Oort Cloud comets, on the other hand, are fluffy and will easily disintegrate.
For sure, other stars also have comets. There are stars like Epsilon Eridani or Tau Ceti, who have larger Kuiper Belts then Sol. In such conditions, comets must also exist. Solar comets are small (a few km in diameter). However, there are records of far greater comets  .
The Kuiper Belt consists of small and large objects, some of them with the size of a planet. If an object with a diameter of 500 km would be sent towards the Sun, it would create a giant comet. These events are far more rare in the Solar System, but in Epsilon Eridani, it should happen more often. We can imagine even a dwarf planet, like Pluto, thrown towards the inner Solar System. If you take Pluto to Earth's orbit, its gravity will be far too weak to keep its atmosphere locked. Its nitrogen rich Sputnik Planum will boil and escape into the cosmos, together with its methane rich plateaus. Then, all its water will evaporate into a giant atmosphere. Soon, Pluto will become a giant comet and its tail will probably be visible as far as the orbit of Jupiter.
Many sci-fi movies present comets as very dangerous celestial bodies. In the movie, walking on a comet is like walking on a minefield, as they throw away with boulders and jets of gas. What both Rosetta and Deep Impact found, is completely different. Comets lose their volatiles through sublimation and sometimes might erupt, but not that violent. Eruptions can throw into orbit boulders, dust grains and can even break comets apart, but we must not forget the weak gravity of a comet. An adult human, weighting on Earth 60 kg, will weight a few grams on 67/P CG. Flying around a comet can be a tricky job, because of the existing dust. Rosetta lost its way many times  when its orientation camera confused dust grains with stars needed for orientation. The dust around a comet is moving slowly. Boulders, if they exist, also are moving slow. They are not a problem for a spacecraft on orbit, but they can damage a spaceship moving at a few km/s.
There are two types of comets.
Kuiper Belt Comets Edit
- Detection of exposed water .
- Detection of organics , including the amino acid glycine
- Detection of argon (one of Earth's noble gasses) 
- Detection of salt (sodium and magnesium) .
- Detection of methane .
- Detection of solid compounds .
- Detection of carbon dioxide .
- Detection of molecular oxygen , showing that the comet formed somewhere where temperatures are cold enough for oxygen to not sublimate.
- Comets have high differences of hydrogen - deuterium ratios and Rosetta's comet is very rich in deuterium .
After all these chemical discoveries, Rosetta has successfully made other analysis. They all give us ideas about what we should find on a Kuiper Belt asteroid:
- Comets appear not to be magnetized .
- Rosetta's comet has no large caverns inside and appears to be homogenous .
- Rosetta's lander, Philae, discovered that the comet is made of solid materials, as hard as rock . The lander could not drill into the ground at its final landing spot. Also, at its first landing spot, it recorded the sound of touchdown, proving that beneath a layer of dust, there is hard, solid ice.
However, the most amazing of all findings on 67/P is stratification . This is something that gives us an idea of how comets are made. 67/P is made of two lobes who slowly collided into one larger body. They are both made of an alternation of strata, each one with a slightly different composition. An interesting discovery happened when a hole appeared in one layer. Below it, the newly exposed layer started to sublimate and after a few weeks it pulled away the previous layer. This proves that the two layers had different chemical composition .
Rosetta's comet was analyzed in high detail, but similarities can be found on other comets. Stratification was proven to exist also on Deep Impact's Tempel 1 [. Also, the impactor that penetrated Tempel 1's surface, showed that the comet is made of solid materials.
Based on all these findings, we can conclude that Kuiper Belt asteroids are usually solid bodies and not piles of rubble. They contain dirty ice with many volatiles inside, but also with salt and solids. They have a homogenous composition (with stratification) and are not magnetized.
Oort Cloud Comets Edit
Oort Cloud comets are very fluffy. Their matter is held together by a weak gravity force. This is why a vast part of the Oort comets, once they come close to Sol, face their destruction. Some of them
Comet Elenin disintegrated at 0.5 AU from the Sun (half of the distance between the Sun and Earth), after it was hit by a corona mass ejection. It left behind a cloud of dust.
Another Oort comet, C/2006 M4 (SWAN), has dramatically increased its brightness from magnitude +7 to magnitude +4 (increased over 15 times). This massive outgassing is the result of its instability.
Industrial Uses Edit
Comets will be very important in future terraforming attempts. As we will try to terraform Venus, Luna, Mercury and probably also Mars, we will need water for the oceans and gasses for the atmosphere. The most easy way to do this is by diverting Kuiper Belt objects on collision trajectories. Basically, this means that we will create new comets.
Kiuper Belt objects move around the Sun with speeds of only 4 km/s. Changing their trajectory will not require as much energy as diverting satellites from the giant planets.
In order to change trajectory of a Kuiper Belt asteroid, to transform it into a comet and to impact the targeted planet, a special space probe will be needed. The probe will have a nuclear generator (probably a deuterium fusion one, since comets contain deuterium). The probe will fire its engine, creating a weak and continuous thrust, so that the comet will follow a precise orbit and will not break apart.
Comet Colony Edit
Why should anyone attempt to create a colony on a comet? Well, comets have fascinated humans for millennia. We have sent more probes towards them then towards the outer gas giants. Comet Halley is so popular, that it won't be a big surprise to see a permanent research base on its surface. The colony will have all ingredients needed for life: carbon, hydrogen, oxygen, nitrogen, sulfur, phosphorus, sodium, magnesium and even argon. Silicates are also present.
Some scientists have proposed the creation of a Dyson Tree, a living habitat able to feed itself with the ingredients found on a comet and able to host an environment for humans to live inside.
A base will experience high seasonal changes, as the comet moves closer and further from the sun. Therefore, heating and cooling facilities will be needed. At aphelion, life might not be possible without the help of a nuclear generator, to provide heat and light, while at perihelion, the base will be exposed to comet outgassing. So, it must be fixed to the ground, but also allow gasses to flow beneath its basement. Underground building might not be a good option. Dust can set on solar panels and windows, reducing the amount of light received.
Comets are dynamic worlds. As they heat, they lose part of their surface layers. As their environment is full of dust and gas plumes, they might also be good touristic destinations.
Creating large greenhouses, like on asteroids, will not be possible, because comets don't offer a safe surface. Heat from human activities can make gasses to sublimate (which will sublimate anyway when getting closer to the Sun). Repeated outgassing and the loss of matter will affect the foundations.
Since comets are dynamic worlds, an industrial corporation will not be so excited on setting a factory on a comet. It would be more profitable to set a base on a Kuiper Belt asteroid, despite the distances involved. However, comets can attract both scientists and tourists.