Comets come from two major areas of our solar system: the Kuiper (pronounced KY-per) Belt and the Oort (pronounced OR-t) Cloud. Each of these regions contains billions of comets, but they have so much room in these vast rooms of space that they get no closer to each other than we on Earth do to the sun.
The Kuiper Belt
The Kuiper Belt is about two billion miles wide (3.2 billion km); it begins just past Neptune’s orbit, which is about three billion miles (4.8 billion km) from the sun, and stretches to about five billion miles (eight billion km) from the sun. We have never sent a spacecraft that far. The Kuiper Belt appears to be a flat disk of icy debris that lies in the same plane as the orbits of the planets (we call that the ecliptic plane), but it may grow thicker in the other regions further from the sun. Scientists think that Kuiper Belt comets formed close to where they are now - outside the planetary part of our solar system.
In addition to comets, the Kuiper Belt contains much larger objects, called Kuiper Belt Objects (KBOs). Some would be large enough for us to call them moons if they orbited a planet. Scientists do consider some of them to be “dwarf planets” (for example, Pluto, UB313 or Eris, and Ceres).
The Oort Cloud
The Oort Cloud is way out there, at the extreme edge of our solar system - about 18 trillion miles (30 trillion km) beyond the sun! That’s more than a light year away from the sun, and more than a third of the way to the nearest star, Proxima Centauri. The Oort Cloud is not disk-shaped like the Kuiper Belt; it’s round like a ball, and huge! It completely envelops the sun and the rest of our solar system. Billions of icy bodies pass through this enormous celestial ballroom as they weakly orbit our sun. When a passing star pulls one of these bodies out of its orbit, it can fall into that star’s inner solar system and become a comet. These comets, called long-period comets , have very large orbits of greater than 200 years.
Swinging away from the planets
Strange as it seems, some scientists think that Oort Cloud comets formed closer to the sun than the Kuiper Belt comets. They may have originally formed amid the orbits of Jupiter, Saturn, Uranus, and Neptune. Then they were possibly catapulted out to the Oort Cloud, as a result of the gravity assist provided by one of those large planets. Rosetta will use gravity assist from Earth and Mars to gain the momentum it needs to reach comet C-G. If, as a kid, you ever held both hands with a friend and swung each other around and then let go, you’ll have a sense of how gravity assist works. For the Rosetta mission, it’s a clever way to save money on costly and heavy rocket propellant! In the same way, billions of years ago, vast numbers of comets may have broken from their orbits of Jupiter and other giant planets to sail clear out of the solar system. Those that didn’t quite make it formed the Oort cloud. That’s one theory, anyway.
Another theory is that comets originally formed in dense molecular clouds of interstellar space and were captured by the sun to form the Oort Cloud.
Are they Oort or Kuiper comets?
We generally believe that comets with orbits of 200 years or less, traveling in the ecliptic plane, come from the Kuiper Belt. Those comets with longer orbits, or that travel at angles to the ecliptic plane, come from the Oort Cloud. The Rosetta scientists hope that this and future comet missions will help us find out if comets differ in other ways.
Or are they a Jupiter comet, such as C-G?
There are other comets from the Kuiper Belt or Oort Cloud that are corraled by Jupiter’s massive gravity into its planetary family. These comets develop very short solar orbits - from three to 20 years. Rosetta's target, C-G, is one of these. It currently travels in 6.57-year orbit beyond Jupiter's orbit to a point between the orbits of Earth and Mars.
We still have questions
We still have much to learn about comets. Do the comets from the Kuiper Belt, the Oort Cloud, and Jupiter’s orbit contain different stuff? We want to find out.
For more information on what we are trying to learn from comets, see Science Goals.