https://metro.co.uk/2024/09/29/see-a-once-80-000-years-comet-uk-october-2024-21699657/
I’ve seen a few posts and articles about this comet and how the last time it was this close to earth was about 80,000 years ago. I think that’s super cool but I don’t understand how we would know that.
In: Planetary Science
In general, objects in orbit around the sun are pretty predictable. All you have to do is plug the comet’s current speed and trajectory into Newton’s equations for orbital dynamics and you’ll get both the shape of the orbit and its period.
The only tough part is figuring out what that speed and trajectory are. For that, you have to have multiple observations of the object. By looking at the difference in its position and brightness (caused by how much of the sun’s light it reflects) between observations, you can get a good idea of how fast it’s going. Nowadays, we can even shoot radar at objects in space to get exact distances and tangential speeds, something that wasn’t available a century ago. The more observations you make of the objects, the easier it is to pinpoint their exact orbit, but even a ballpark calculation from two observations can give you a rough estimate of its orbital period.
You might be thinking, “What about all of the planets out there? What keeps them from changing its orbit?” Planets and other celestial objects tend to be really far apart and don’t affect each other very much, if at all. There are dramatic situations, such as the mini-moon Earth will hold onto for the next couple months, or Shoemaker-Levy 9, where a planet does have a major effect on something’s orbit, but those are rare exceptions. The odds of A3 having had a planet do something that changed the period of its orbit are astronomically small (see what I did there?), so we’re safe in assuming that the 80,000 year figure that was calculated is accurate.
Where objects in space are at particular times can be calculated with equations. They’re really complicated, but if you have a good computer and you know where the object is and how fast it’s moving, you can recreate where it was in the past and also predict where it will be in future.
Comets like that don’t just roam around the universe at random – they are *orbiting the sun* just like everything else in our solar system. They simply have really, *really* big orbits.
Orbits are predictable; calculating when an orbiting object will be here again (or when it was here last) is pretty straightforward math, relatively speaking.
We can calculate orbits very precisely. The physics and math to do it is well established and thoroughly validated.
We can make observations of far dimmer objects very precisely. Comets are by their nature very easy to track.
So we can figure out the orbit of a comet, and how long it takes it to make a round trip, fairly easily.
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