Re: How do comets form?

Hi Gerald,

That's an excellent question. Let's tackle it by looking at each kind of object separately to see how they differ:

• Comets are mostly composed of ices (water, methane, ammonia, etc.) and dust, which is the reason why some scientists call them "dirty snowballs". Comets, like the other objects in this question, were created from the same stuff that makes up all the planets. When the solar system was just forming, it consisted of just dust and gas orbiting around the early Sun. Much of this gas and dust stuck together and clumped to form the planets. However, there was some material left over. Some of this leftover debris consisted of small chunks of dust and ice. Today comets reside largely in two places, the Kuiper Belt and the Oort cloud. The Kuiper Belt is much like the Main Asteroid Belt, except that it lies outside the orbit of Pluto. The Oort cloud is a huge (spherical) halo of comets well outside the orbit of Pluto; the Oort cloud may extend nearly 1/5 of the way to the nearest star.

  Every now and then, comets get disturbed by gravity fluctuations (from the planets in the solar system or from passing stars). These gravitational fluctuations cause comets essentially to fall into the inner solar system. When that happens, they get closer to the sun and that ice turns into gas and vapor. We see a noticeable "tail" form. These orbits usually move through the solar system anywhere from a few years to as many as a couple hundred thousand years. After many orbits close to the sun, a comet will eventually lose all of its ices and basically become an object very much like an asteroid.

• Asteroids are often called minor planets. They are rocks that may be as large as a few meters all the way up to several hundred kilometers across. The largest part of all the asteroids are located in a region between the orbits of Mars and Jupiter, which is called the "asteroid belt." Asteroids are also formed from the "clumping" of dust and debris which is left behind from the forming of the solar system. In the case of the asteroids in the Main Belt, they are thought to be a planet that never formed. The gravitational influence of Jupiter was so strong that it prevented these asteroids from getting close enough to stick together to form a planet. Asteroids, like comets, can orbit inwards towards the sun and even towards us when other objects in the solar system give them a gravity "kick" that changes their orbit.

• Meteoroids are the smallest particles of this dusty material orbiting the sun. Meteoroids come from the dust and particles which break off comets or asteroids. Most of these particles are no bigger than grains of sand. They are very hard to see and can usually only be seen when they enter the atmosphere and burn up. When this happens they are now called meteors. Most meteors burn up before getting any closer than 60-80 miles from the ground.

I hope this helped you to understand some of the differences between these objects. Before I close I wanted to give you some links to two places where there are some great pictures for you to look at...enjoy!
ASTEROIDS!
COMETS!!!