Long period comets are comets which seem to appear once, and then
go away. Short period comets are those like Halley's, which appear over and
over again. Long period comets seem to have orbits that are parabolas.Return to my Astronomy/Space pages
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© Copyright 1997, Jim Loy
Long period comets are comets which seem to appear once, and then
go away. Short period comets are those like Halley's, which appear over and
over again. Long period comets seem to have orbits that are parabolas.
The earth's orbit is almost a circle. But, a circle would be a perfect shape. Things like that are never perfect. The orbit of the earth differs from a circle a little bit, it is an ellipse. Some ellipses are much flatter than the earth's orbit. The orbit of a short period comet is a very flattened ellipse. A parabola is essentially an infinitely long ellipse. A parabola is between a very long ellipse and a hyperbola. A hyperbola is essentially a nearly straight line path, which begins to curve near an object (the sun), curves around it, and resumes its nearly straight line in another direction.
A parabola is another of those perfect shapes (like a circle) which cannot be attained exactly. A comet which comes from some distant place, well beyond the Solar System, should have an easily identifyable hyperbola for an orbit. The Solar System is moving through space at a fairly high speed. Any comet from way out there should have a fairly high speed, with respect to the Solar System. An orbit which is a parabola should begin with a zero velocity, at an infinite distance from the sun. And so, no comet should exhibit a parabolic orbit. And ones coming from outside the Solar System should have distinctly hyperbolic orbits.
Even though a hyperbola looks something like a parabola, they are easily distinguished when examined closely. But, a very very long ellipse is often indistinguishable from a parabola. And so, we can assume that the orbit of a long period comet is not a parabola, but instead, a very very long ellipse. In that case, the long period comets are not from outside our Solar System, just visiting once, but instead are part of the Solar System. For that reason, the Oort Cloud of comets (at a great distance from the sun) has been deduced.
Also see Comets.