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? 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?
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.