Why is a comets orbit elliptical?
It’s true that all real orbits are (at least slightly) elliptical. I assume what you’re asking is, “Why is a comet’s elliptical orbit so eccentric (long & skinny) compared to most other things that orbit the sun?” This has to do with where comets come from. The ones with the most eccentric orbits are assumed to come from the Oort cloud, which is much, much farther than the orbit of Pluto. At that distance, the pull of the sun’s gravity is very weak, and a comet needs only a very tiny sideways motion to resist being pulled in. But every so often, a very slight gravitational disturbance (due to the changing positions of the other comets in the cloud, or the planets, or even another star) will give a comet a very slight nudge, perhaps robbing it of its already tiny sideways motion. When that happens, there is nothing stopping the comet from falling in toward the sun. Since the comet is barely moving sideways at all, the trip is nearly a head-first dive straight toward the sun. That ends u
