There is post on a sub I frequent that said there is going to be a [comet](https://www.npr.org/2024/10/10/nx-s1-5148895/how-to-see-comet-c-2023-a3-tsuchinshan-atlas) that won’t be seen for another 80,000 years.
How do we know this particular comet has an 80,000 year long orbit? What are those observations that tell us something like that?
80,000 years ago, as fas as I know, people weren’t writing down astronomical observations. And even if we did, we aren’t relying on their observations today. The post says they discovered this comet in 2023.
What’s our justification for saying something so definitive that this comet won’t be seen for another 80,000 years? I’m not understanding how they would even begin to determine something like that after having discovered it last year.
In: Planetary Science
Remember “objects in motion stay in motion” from school?
An object orbiting in space will *keep* orbiting. And it turns out that the math to calculate orbits is relatively straightforward, as these things go. So we can calculate the comets orbit, and look forward or back at any point in the math to see where it will be or was.
The orbit angle tells you the speed, they are related. From that, math gives you the time of the orbit.
Comets orbit the sun. We just see them as they go past. If you know how big it is, how fast it’s moving and the direction in which it’s moving, you can calculate its orbit around the sun and work out how long it will take.
We have a very good understanding of orbital mechanics.
They aren’t making these statement because they watched the comet 80,000 years ago. They’ve watched the current approach and measured speed, distance, and direction. They then plug those numbers into orbital calculator and it shows the comet’s entire orbital path, including how long it takes for the comet to travel that path.
You can calculate the orbit of an astronomical object without having to see its entire orbit. We ‘discover’ comets and asteroids by making observations of them, mathematically predicting where they should go, and then following up later to see if they’re actually there. If they are, we know the orbit with a degree of certainty that increases with each subsequent observation. And once you know the orbit, you know how long it will be before it returns to the area it will be observable from Earth.
https://en.wikipedia.org/wiki/Gauss%27s_method?wprov=sfla1
That’s one way.
Lots of great answers so far. We two sufficiently accurate and precise position measurements, we can mostly find the orbit. By using Lambert’s problem, two position measurements and the time between them allows us to solve for two possible orbits. Most of the time you can intuitively select which of the two is accurate or, to be more thorough, a third position measurement would confirm it
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