Comet Neowise c2020 f3 will return in ~6800 years, how can we be sure Earth will be in the right place to see it then?

Question

I have read in several places that the orbital period of comet Neowise c2020 F3 is about 6800 years. In several popular media sources it is said that we will be able to see the same comet after ~6800 years.
How is it known that the Earth will be at exactly the right place in order for it to be visible?

Oscar Bravo · Accepted Answer

It is important to appreciate the scale of the comet's orbit compared to the Earth's orbit. The Earth goes round the Sun in a nearly circular path. The average distance is one Astronomical Unit (AU).
C/2020 F3 (Neowise) has an incredibly skinny elliptical orbit. It's closest approach to the Sun (perihelion) is at 0.3 AU, which is inside the orbit of Mercury. However, its furthest distance from the Sun (aphelion) is 500-700 AU.
By way of analogy, if Earth is like a boat that is sailing around a harbour, Neowise is like a ship that comes in from 600km out at sea, turns in the harbour and sails back out again.
As to whether we can see it or not, as @JamesK states, comets are usually visible for several months. It might be possible for a comet to approach, orbit the Sun and recede, entirely in the daytime sky. In that case, we would never see it. However, it is more likely that at least part of its orbit would be in our nighttime sky and so visible at some point. [n.b. by visible, I mean visible to astronomers with telescopes and image processing equipment - not necessarily to naked-eye amateurs].

Yakk - Adam Nevraumont · Answer

Simply because we can see the sun.
The comet will be swinging around the sun, getting closer to it than the Earth does by a large factor.  This path cannot always be in-line with the sun or other body orbiting the Earth or the Sun, so there isn't a way for our view to be blocked.
It is plausible that its approach could be mostly co-linear with the sun, or its departure, but not both; its change in angle compared to the sun will be at a different rate, almost certainly different inclination, than Earth's.
Imagine you are walking around a tall skinny (3' radius) pole in the middle of an empty field.  You know in 6800 years someone will run up to the pole from some unknown direction, run around it 90 degrees, then run out another direction.  You aren't walking fast enough for the pole to block both its approach and its exit from the field.

James K · Answer

The time of the orbit is not well known but the inclination can't change as much (small changes in velocity due to outgassing can significantly change the orbital period, but can't change the inclination much. So, while we have a good idea of it's orbital track close to perihelion, we just don't know when it will be there.
We don't know where the Earth will be in its orbit when it returns, but a comet is typically visible for several months. It is possible that the Earth will be particularly badly placed, but even then it will be visible
The Earth doesn't need to be in an exact position for the comet to be visible. It may not be a naked eye object next time, but it should still be visible, no matter where Earth is on its orbit.

Comet Neowise c2020 f3 will return in ~6800 years, how can we be sure Earth will be in the right place to see it then?

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