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I have read in Wikipedia about Comet C/2020 F3 Neowise that it's orbit is near-parabolic. But in another section, it says that it is a non-periodic comet whlith orbital period ranging from 4500 years to 6800 years.

My doubt is how a comet returns back if it is in a parabolic orbit and why it is non-periodic?

Here is the Wikipedia link: https://en.m.wikipedia.org/wiki/C/2020_F3_(NEOWISE)

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  • $\begingroup$ It is a long-periodic comet. "Non-periodic" means that the comet wouldn't return for ten thousands of years and that we can't determine precisely when it would return. So the Wikipedia section is wrong. $\endgroup$ – Ioannes Jul 18 at 8:32
  • $\begingroup$ Isn't non-periodic means we can't predict when it comes back? Also the reference in Wikipedia page is NASA JPL website. It says about period of comet. JPL Horizons barycentric solution for epoch 1950 (before entering planetary region) Goto JPL Horizons Ephemeris Type: Orbital Elements Center: @0 (Solar System Barycenter) Time Span: 1950-01-01 to 2050-01-01 and Step Size: 100 years 1950-Jan-01 is "PR = 1.63 × 106/365.25 days" = 4462 years (For long-period comets on multi-thousand year orbits, asymmetric outgassing will affect the highly sensitive orbital period and eccentricity.) $\endgroup$ – Athul R T Jul 18 at 8:44
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    $\begingroup$ Link to the reference : ssd.jpl.nasa.gov/… $\endgroup$ – Athul R T Jul 18 at 8:45
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Supplementary answer:

"Near-parabolic" means almost or nearly parabolic, but not quite. Another term would be "highly elliptical". Technically speaking, no realistic orbit can really be exactly parabolic since that means eccentricity is exactly 1.00000000000.... and most of Physics (solar photon pressure, relativity, gravitational perturbations from other bodies, etc.) doesn't happen.

You can complain that in your linked Wikipedia article:

C/2020 F3 (NEOWISE) or Comet NEOWISE is a retrograde comet with a near-parabolic orbit discovered on March 27, 2020, by astronomers using the NEOWISE space telescope.

the term "near-parabolic" hyperlinks to Parabolic trajectory. Probably the "near" part should not be part of the link.

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    $\begingroup$ This reminds me of a question on another SE as to what meaning the phrase "near infinite" means. When parameterized by eccentricity, "near parabolic" means that it's close to 1, which doesn't seem all that strange of a thing to say. But if it's parameterized by period, then "near parabolic" means that it's nearly infinite, which is a bit weirder. It's either finite or not. If x is close to an infinite number, intuitively it seems like that should mean that x is infinite as well. $\endgroup$ – Acccumulation Jul 18 at 18:52
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Comet naming conventions limit "periodic comets" to those comets whose orbital period is less than 200 years. Comets in elliptical orbits with a period of greater than 200 years are classified as non-periodic. Comets in parabolic or hyperbolic trajectories are also classified as non-periodic (and they will never again approach the primary).

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  • $\begingroup$ Thank you for the answer. Can you explain how it comes back even having a parabolic trajectory? $\endgroup$ – Athul R T Jul 18 at 12:40
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    $\begingroup$ @AthulRT - ??? I didn't write anything like that. That said, eccentricity is computed by assuming things that are know to be not true (two body problem, no outgassing by the comet, no relativistic effects, ...) So even a comet with an apparent eccentricity slightly greater than 1.0 might well return after a long, long time. The only two objects that have been prefixed by I, for interstellar, have statistically significant hyperbolic excess velocities. $\endgroup$ – David Hammen Jul 18 at 15:30
  • $\begingroup$ I submitted an edit to try to make the similarity / distinction clearer to @AthulRT. $\endgroup$ – Ross Presser Jul 20 at 18:37

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