Sound Science Bite: August 6 Perhaps the top science story today is the rendezvous of the spacecraft Rosetta with the comet 67P/Churyumov-Gerasimenko. (The "P" in "67P" stands for "periodic".) Rosetta has now achieved orbit about this comet, a space first. The spacecraft will now follow the comet has it passes closest to the Sun in its orbit (the perihelion) and for a while beyond. This will allow it to study the comet as it gets "active" (releases material - mostly dust and gas - and forms a tail). A lander named Philae is scheduled to touch down on the comet's surface in the fall. There is a lot of information about this online. (Just "google".) What I want to address here is the origin of such a comet.
Periodic comets appear in the sky on a regular basis, unlike comets that essentially appear only once. These "long-period" comets can approach the Sun from any direction, whereas periodic comets generally have orbits close to the same plane in which the planets revolve. When the long-period comets, using orbital calculations, were tracked back to where they came from, it was found there was probably a large region of cometary objects orbiting the Sun at a great distance, as far as one light year or more away, called the Oort Cloud. A comet from this region can have its orbit perturbed from a passing star and plunge gravitationally into the planetary region of the solar system, rounding the Sun, and then flying back into interstellar space, perhaps never to be seen again. These comets rarely become a member of the periodic set. Then where to the periodic comets come from? or, have they always been there?
Periodic comets approach the sun and begin to get active, meaning they begin to, in a sense, evaporate (the correct term is "sublimate") as their ices turn to gas. Dust and possibly other material is emitted as well. They can't keep this up forever - eventually they will "run of out gas", so to speak. Some of these comets have been observed to break up and even plunge into the Sun. So they can't have been where they are for the 4.5 billion year life of the solar system. They would all have been long gone by now. What, then, is their origin?
One clue has to do with their orbits. They mostly orbit close to the same plane as the planets. Due to the law of the conservation of angular momentum from the world of physics, this means they almost surely had orbits close to the same plane originally. Another clue is that a large number of them, such as 67P, have their aphelion (farthest point on the orbit from the Sun) close to the orbit of Jupiter. Finally, it is known that a large number of cometary objects orbit the Sun close to the planetary plane in a region beyond that of the planets. (Pluto is now a recognized member of that number and not the largest one for that matter.) These objects are in the so-called Kuiper Belt.
So here is the current accepted theory for the presence of a comet like 67P. A member of the Kuiper Belt has its orbit perturbed, probably by a close approach to another member, and enters the planetary region. There is a fair possibility it will get close enough to Jupiter to have its path altered to an orbit where its aphelion is near the orbit of Jupiter and its perihelion in the inner solar system where the Earth and other rocky planets are. (Gravitationally speaking, the solar system consists of two objects: the Sun and Jupiter.) The comet 67P is in such an orbit, which extends outward almost to that of Jupiter and to a perihelion just outside the orbit of Earth. So, not only will the Rosetta mission get to study a comet as it becomes active, but also it will likely be getting a close up view of an otherwise distant Kuiper Belt object