1

There is a comet in my world which passes my world's version of Earth every 25 years. I'm having difficulty figuring out how to make the comet fit my requirements.

Requirements:

  • Passes my planet only once every 25 years.
  • Stays long enough to be visible for every part of my planet.
  • During its stay it always remains on my planet's night side.
  • Appears to be a clearly noticeable event in the night sky.

What I want to know:

  • What would the orbit of this comet look like?
  • What is the minimum circumference needed for the comet to fit my requirements?

Additional Information:

  • Try to keep the comet as small as possible.
  • There is magic in my world some hand waving is okay as long as it is minimal.
  • It glows by magical means.

Note: English is not my primary language so please excuse any grammar or spelling errors

Fallenspacerock
  • 899
  • 2
  • 20
  • How can it possibly pass by "around midnight"? Midnight is a time on a very specific part of the planet. For practically all of the planet, it has to pass by at a different time. – jdunlop Mar 22 '24 at 18:32
  • That is why i said around midnight. To be more specifically it is mostly visible deep in the night. – Fallenspacerock Mar 22 '24 at 18:33
  • 1
    You're asking that it have the same magnitude as the moon. It will be visible to half the world for twelve hours, barring clouds. It cannot be isolated to "deep in the night". – jdunlop Mar 22 '24 at 18:36
  • 2
    One problem is that the comet orbits the Sun, not the Earth - there is basically no chance the comet will have a orbital period that's an integer number of years without some odd resonance pattern. This means every time the comet comes back along its fixed path, the Earth will be in a different spot in its orbit. Even if the comet appears as big as the moon during one particular pass, it will be a tiny dot during another pass when the Earth is up to 186 million miles from where it saw the moon-sized comet. – Nuclear Hoagie Mar 22 '24 at 18:51
  • How much energy is required to keep the comet in a perfect orbit then? Also could it just be that only every 25 years the comet is clearly visible? – Fallenspacerock Mar 22 '24 at 18:53
  • VTC:Needs More Focus. I voted to close a previous question of yours for the same reason. One and only one question per post. Also, I removed all the bold text because, at least for me, it makes the question harder to read. Please stop emphasizing things that don't need it. We've been doing this a long time and know how to read questions. Thanks. – JBH Mar 22 '24 at 19:54
  • 1
    Now to the details: Your comet magically glows, which means that requirement #4 is irrelevant since magic controls the brightness. It also fundamentally negates the question about comet size for the same reason. Requirement #3 and #2 are in opposition. The only way to keep it on the night side is to pass the planet during one rotation of the planet. Any slower and it'll be seen on the day side unless it's dim enough to not be seen during the day, which might be contrary to requirement #4 (noticeable event). Finally, why does it matter how small it is? – JBH Mar 22 '24 at 19:59
  • 1
    One last question: Why is it important to minimize handwaving? What consequence do you expect if you don't? Only an astronomer would know the difference without thinking about it, and they want to suspend their disbelief for a good story, too. People have become over-sensitive to being "as realistic as possible" and in many cases (like yours) it's getting in the way of creating a cool world for a cool story. So, if you don't mind my asking, what's the point of this question? Why are you asking us to crowbar science into magic? – JBH Mar 22 '24 at 20:05
  • What do you even want from me? You to most of my post and try to close them down. Not only that but you also try to ridicule me like i am completely incompetent. Also there is only 1. question where is there even supposed to be a second one. If you really want to help please just tell me what i should change about these question and don't rant about me in 3 separate comments. I have already deleted the 2 other questions if that is what you wanted to archive. – Fallenspacerock Mar 22 '24 at 21:04
  • 1
    @Fallenspacerock I won't know that you've posted a comment unless you tag me in your comment. See example at the beginning of this comment. What do I want from you? To follow the rules of Stack Exchange and this Stack, which are found in the [tour] and the [help]. Do you honestly believe you're asking but one question? I see two bullets and they're asking different questions. I don't want to drive you away... I want you to learn to use this service appropriately - which includes responding to questions about your post. – JBH Mar 23 '24 at 02:14
  • They are but 2 sub points belonging to same question which is what properties this comet should have to fit my needs. What you are suggesting is that I post what Circumference this would have in an extra question. Such a thing is in my mind completely unnecessary and a very extreme interpretation of the 1. Question rule. And besides that literally almost every other question i have seen does the same thing or something very similar. And your interpretation of the rules the one might as well not post any questions to begin with. – Fallenspacerock Mar 23 '24 at 06:28
  • 1
    To give you an example here: https://worldbuilding.stackexchange.com/questions/27231/what-would-a-battle-between-supercomputers-be-like-with-the-battleground-being-t – Fallenspacerock Mar 23 '24 at 06:30
  • There are probably even more questions asked on this one than in mine. And yet it has 41 up votes. So somehow this becomes suddenly ok once you get enough up votes? Doesn't seem fair to me though this exactly what i see rules being ignored without anybody acting on it. So i would say the rules than in such an instance merely bad luck that strikes those that suddenly have those with certain interpretations of their rules stumble upon such questions. No to insult you or anyone but such treatments fell to me as rather unfair. – Fallenspacerock Mar 23 '24 at 06:41
  • Let us continue this discussion in chat. – Fallenspacerock Mar 23 '24 at 07:05

2 Answers2

4

You Either Need an Enormous Comet or (a Lot of) Magic

The real sticker here is "the size of the moon". The apparent size of the moon from Earth is about 1900 arc seconds. Its diameter is about 3500 km.

So all we need to do is get closer, right? Let's use Halley's Comet as an example. It's 11km in diameter. How close would it have to get to appear to be the size of the moon? $\frac{11km}{3500km}=0.00314$ So we'd need to be at 0.3% of the distance from the Earth to the Moon.

That's 1153km... which is inside Low-Earth-Orbit.

Most of our (artificial) satellites orbit higher than that. It's also waaaay inside Earth's Roche limit, but it (probably) won't be sticking around long enough for that to be a problem.

Solar Orbits Have Some Zip

Also using Halley's comet as an example, its orbital velocity is 55 km/s. So if it rocketed in close enough to Earth to be the same apparent size as the moon, it would only be that close for a timespan measured in minutes. At 55 km/s, that's 3300 km/minute, so it would have doubled its distance from Earth (and halved its apparent size) in thirty seconds.

So, Bigger Comet?

Doesn't seem that likely. While we don't have a ton of experience with other stars, we have a plethora of comet nuclei, courtesy of the Kuiper Belt. And the biggest one we've ever seen is about 130 km in diameter.

That gives you a slightly longer close-approach, but not nearly long enough for everyone on the planet to take a gander at the moon-sized comet.

So What Do We Do?

At this point, "magic". You want a smallish comet, you want it to be at an enormous apparent magnitude, and you want it to stick around. So the magical field of your planet, more puissant than the gravitational field of same, seizes the magical field of the comet and radically slows it while in proximity to the planet. As it begins to exit this interaction, the change in magical field densities flings the comet back on its way with its initial velocity. Given that you're compelled to do magical things to its orbit, the rest of its orbital structure is kind of immaterial - a high orbit out of the ecliptic could easily have 25-year periodicity.

Edit - as others have pointed out, no natural orbit would a) intersect with Earth at that kind of proximity at Earth-year intervals or b) survive the close interaction with Earth. So we're back to "a lot of magic" on that one, too.

jdunlop
  • 32,004
  • 5
  • 76
  • 119
  • I should note that with the enormous number of Starlink satellites now in orbit, "most" of our artificial satellites may no longer be a valid statement. – jdunlop Mar 22 '24 at 19:05
  • Thanks for your answer. The size of the moon was just an suggestion as long at it's a significant effect in the Sky it can be smaller. Also yeah there is probably a lot of magic involved in the orbit of the comet. – Fallenspacerock Mar 22 '24 at 19:11
  • 1
    @Fallenspacerock - this is a consistent problem you seem to have with your questions, where "requirements" don't turn out to be requirements. If you want "noteworthy in the sky", make sure that's the requirement, not "the size of the moon". – jdunlop Mar 22 '24 at 19:12
  • @Fallenspacerock - as I see you've made an edit, "half the size of the moon" has exactly the same problems as outlined in my answer. It lets you either halve the size of the comet with the same trajectory, or let it be twice as far away, which still doesn't matter at orbital velocities. – jdunlop Mar 22 '24 at 19:13
  • Seems like you are not including the puffed out size of comet due to out gassing. but I may be mistaken. Not sure if there is a easy way to calculate that. – Gault Drakkor Mar 22 '24 at 19:14
  • The comet is magical so it probably works a little differently in this way. Also i will correct the size requirement. It is that just commonly find out about the problems of my requirements after i wrote them. Also Gault Drakkor if there are any other problems with my requirements please tell me i am always willing to edit them. – Fallenspacerock Mar 22 '24 at 19:18
4

No such orbit exists. Unlike your moon question, for which a small amount of magic could stabilize a possible but unstable (L2) orbit, this can't be an orbit at all. If we set up such an orbit by contrived or technological means, it would happen once and then the comet would be ejected onto a very long, very eccentric orbit that didn't pass near the Earth again for billions of years. This trajectory can only be pure magic (or Sufficiently Advanced Technology) all the way.

Maybe the guardian spirit of the solar system visits Earth once every 25 years to die and be replaced by the Chosen One of the next generation, after battling the forces of evil that stalk the Kuiper Belt, and people just think it's a comet because they haven't figured out Newton's laws, yet, and it looks kind of similar.

g s
  • 4,764
  • 1
  • 10
  • 20
  • Some background information about this comet is that is that it is partially sentient. So yes it can in fact if that doesn't require to much energy self correct it's orbit. – Fallenspacerock Mar 22 '24 at 19:14
  • 2
    There's no orbit to correct, so the power requirement is huge and neverending. We aren't nudging the bicycle so that it stays on the track around the Energy Hill at constant height (an orbit). We're riding the bike all the way down the Energy Hill, stopping, turning around, and riding it all the way back up again, on a precise schedule. There are parts of the path that we can coast on (when we're going downhill and not hitting the breaks, or when we're going uphill and happen to want to be slowing down), but every loop costs us a huge amount of energy in both directions. – g s Mar 22 '24 at 19:24