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I would like to know if the orbit behaviour of this comet is realistic and if not what could be altered to make it so.

The setting: The solar system
The system is quite similar to our own
A single G-type star
the orbits of the planets are stable
the system age is a bit less than 5 billion years
-planets from the inmost ones-
3 inner rocky planets A, B, C. C is Earth-sized. Also its orbit duration and distance from the Sun can be assumed the same as Earth: 1AU, 1 year. A and B could be any size but must be visible from C.
C has 1 main moon and 2 small moonlets.
Planet D is a large giant, very much like Jupiter with several moons. It's quite far from C and E so it does not perturb their orbits. Planets E and F are large giants, similar to Uranus (but could be altered if needed).
Other planets may be added outside the orbit of F but are not relevant to the story.

The system is visited by both periodic and non periodic comets.

The comet that this story is about should have a short period (less than 200 years). Ideally the period should be somewhere around 120 / 150 years. Long enough to keep the memory, too long for anyone to see the comet twice. In its orbit it is perturbed by E, F and especially D.
Because of these perturbations the period of the comet would change over time. Halley Comet is not regular as well but over the centuries has changed from 74 to 79 years.
Its perihelion should be inside C orbit.
it should perform a series of passages ever closer to C before hitting it (or doing a very close passage)
This 'series of passages' should be multiple, covering thousands of years.
Would the period of the comet thus be increasingly shorter?
It would be a fine addition if several periods before impacting C the comet were to partially fracture and a fragment were to hit the main moon of C. Would this be possible? Wouldn't fragments stay in the same orbit of the comet and so it would be all of it hitting / missing the moon?
I would also like it to be like the 'Daylight comet'. The little people on C should be in awe at it.

Corollary doubt: how would the closer and closer passages be realistic? Wouldn't a periodic comet have impacted C long before? (the system is almost 5 billion years old).

Additional plot idea I would like a planetary alignment (possibly of C, D, E, F) to have an effect on the orbit (for the good or worse)

I ask here in worldbuilding rather than Astronomy because in this question planet sizes, orbits, moons can be adapted to suit the scenario of a comet doing several ever closer passages before hitting (or not hitting). You can use D's influence or a very near miss with one of the moons of D to justify the orbital behaviour.

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    $\begingroup$ Short period comets in Solar system are NOT on their current orbits for 4-5 billion years. Typically, something perturbs the orbit of a body in Kuiper belt or Oort cloud, that body plunges into the inner Solar system and becomes a comet. After that, its lifetime won't extend for more than a few thousand revolutions, often much shorter than that. $\endgroup$
    – Alexander
    Aug 23 at 17:51
  • $\begingroup$ @Alexander fair point. A few hundreds revolutions of about 120 years would be fitting the story perfectly. $\endgroup$ Aug 23 at 18:01
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    $\begingroup$ If the comet breaks apart due to e.g. a collision or tidal forces or a close encounter with the star, the pieces might remain gravitationally bound to each other, but since comets have so little gravity, probably not. More likely, they'll spread out into similar but not identical orbits. It's entirely possible for some of the debris to impact a planet (or moon) later on, and for the rest to miss. But depending on how spread out the debris is at that time, the debris that misses may get scattered into wildly differing orbits. $\endgroup$ Aug 24 at 4:41
  • $\begingroup$ Could a gas giant like D cause tidal forces to break the comet apart in its passage? If not I can just change the comet's orbit to come closer to the Sun. My original idea was to have the perihelion inside C orbit, slowly creeping closer. $\endgroup$ Aug 24 at 7:30
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I think you can get all the results you want.

Recently Perturbed The non collision in the past could be explained if your comet is a recently* perturbed body from the system's outer system like our Oort cloud or Keiper belt (if your system has such) That's why we still have comets, otherwise over time they would have all been exhausted.

Effects from planets. Would depend on the comet's inclination. Is it on the stellar plane or at an obtuse angle. If it is on the plane the planets would have a greater effect and any of the large ones could push your comet to its collision fate.

Earlier Fragment hits Comets aren't necessarily unitary bodies, they can have satellites themselves. A pass by one of the larger outer bodies could move a comet's satellite such that it still follows a close trajectory parallel with the comet, (becomes a mini comet on its own) and that trajectory could impact the moon while the main comet flies by.

*in astrological scales

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  • $\begingroup$ What determines the comet's inclination plane? I assumed that being an object formed together with the solar system it would have an inclination similar to the planets. Is it correct? In our system with the exeption of Mercury (due to the Sun influence) and Pluto (because it's not a planet? ;) ) planets seem to have quite aligned orbits. Of course if the original perturbation of the comet was a collision or an externally passing star it's inclination could be anything.... $\endgroup$ Aug 23 at 18:08
  • $\begingroup$ If it comes from the Oort cloud it can have any inclination as this area is a sphere around the sun rather than a disc. $\endgroup$
    – Allan
    Aug 23 at 19:35
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Your comet reminds me of Halley's comet. It's perihelion is well below Earth's orbit and it's currently got a period of 75-76 years. It is also perturbed by Jupiter.

Funniest thing: it's got cyanogen in its tail, and Earth went through that tail in 1910. People around the world were freaking out back then because they thought everybody would be poisoned to death, but the tail is so thin that practically no cyanogen reached the Earth's surface. That was right around Carnival in Brazil... Cultural organizations there were competing to see which one would come up with the best Samba about the Apocalypse. The winning one was sang by Carmen Miranda, and tells the story about a woman who spent the doomsday in debauchery, only to wake up to regret and a huge hangover the next day.

The comet missed the Earth by hours to days, if I remember well. In an alternate reality, it could have hit the Earth and possibly lead humanity back to the stone age, if not wipe us out completely... Halley's comet is just one order of magnitude less massive than the asteroid that extinguished dinosaurs. And by all that we know, it would have impacted here with the same speed, with a good chance to impact at a similar angle.

Corollary doubt: how would the closer and closer passages be realistic? Wouldn't a periodic comet have impacted C long before? (the system is almost 5 billion years old).

That depends on an astronomical lot of parameters. It seem like Halley's comet orbital period is getting longer and longer due to Jupiter's influence during the last few passes. Also that comet is still out there, and who knows? It might kill us all in a few centuries.

Additional plot idea I would like a planetary alignment (possibly of C, D, E, F) to have an effect on the orbit (for the good or worse)

The barycenter of the solar system is sometimes inside the Sun, sometimes outside it, due to the influence of the planets (mostly Jupiter). If all the planets combined have that effect on the Sun, which concentrates 99% of the mass of the whole system, they sure have a lot more push on everything else.

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I think the only way you'll get your closer and closer approaches is if the comet is in resonance with an outer planet which is not quite in resonance with C. Lets say F has an orbital period of 149.99 C-years, the comet matches this and thus on each orbit it comes 1% of C's orbit closer until either the close approach (which substantially changes it's orbit) or impact.

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  • $\begingroup$ This is a very interesting idea. I wonder if there's a tool to run a simulation. I can only find simulators of the solar system as is. $\endgroup$ Aug 24 at 7:26
  • $\begingroup$ @DuncanDrake Universe Sandbox certainly can model this, but I don't know if it can do it with enough precision for things like resonance effects. Setting up the resonance will be tricky, also--it will let you specify the orbit (although I have hit bugs that have kept me from doing some simulations) but what I'm picturing isn't a normal orbit, it relies on the tug from F each time around. Note, also, that when you are talking comets no orbit is truly stable. $\endgroup$ Aug 25 at 3:26

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