3
$\begingroup$

In my current fantasy story, I have an earth-like planet with one large difference: it has a planetary ring. I would like this ring to be large and visible. I've been using Saturn's rings as a template, and have decided that the rings should be roughly the same composition and ratio in size as Saturn's rings are. So basically scale Saturn-and-rings down to Earth's size, and those are the rings I want, in terms of size and composition.

In addition to having a planetary ring or rings, a very important part of the story is how they formed. Back in the distant past, something was going to collide with the planet. A comet, a moon - something big enough to be a serious problem for all life. I'm excluding asteroids because Saturn's rings are primarily icy, and asteroids aren't.

There were extremely powerful mages on this planet, and they all banded together to stop the collision. Exactly how they did it isn't too important. They might have literally torn it apart before it could reach them, or just slowed it down enough so that it disintegrated inside the Roche Limit before it could reach the surface. Assume they are powerful enough to do whatever you need them to do to stop the collision and form the ring.

The point is that a large body came within the Roche Limit of the planet (because as I understand it that's where rings form), was stopped somehow, and pulled apart until it became a ring.

Question: What would be the effects on the planetary inhabitants of such an event?

I'm trying to avoid a scenario where possible side effects include massive earthquakes, volcanic activity, worldwide floods, meteors, or the eradication of all life on the planet. I'm fine with a few temporary effects, but the less deadly they are the better. Ideally, the inhabitants of the planet would be able to watch the formation of the new ring from the comfort of their own home, with nothing else being different. Is such a scenario possible?

Some notes:

  • Assume the planet is identical to Earth.
  • Assume the planet already has a moon identical to the one we have now.
Improve this question
$\endgroup$
8
  • 4
    $\begingroup$ Earth's Roche Limit if you consider the moon as the relevant mass you want to create, looks to be a trifle close. Technically (if I've read this right) it's inside the height of the Himalayas. To quote Deep Thought; 'Tricky'... $\endgroup$
    – Tim B II
    Commented Mar 16, 2018 at 1:53
  • 1
    $\begingroup$ @TimBII Realistically the mages would need to smash the moon into pieces of few hundred kilometers size first. That is fairly realistic side effect of stopping a large body from colliding with the planet, though. It would probably be very hard to apply enough force without breaking the moon. $\endgroup$
    – Ville Niemi
    Commented Mar 16, 2018 at 2:35
  • 1
    $\begingroup$ Well, not really stopping, diverting to an orbit instead of collision. Requires less force and is safer. Still would probably involve enough force to break the moon though. $\endgroup$
    – Ville Niemi
    Commented Mar 16, 2018 at 2:38
  • 2
    $\begingroup$ @TimBII While it is quite close, I think you did misread that. The number given in the answer you link is 2000km surface to surface, before a little extra math. That's definitely higher than the Himalayas, unless I missed a big earthquake recently $\endgroup$
    – bendl
    Commented Mar 20, 2018 at 21:03
  • 1
    $\begingroup$ @bendl You're right. I read 2000 (M), not 2000 (Km)! My bad. :D $\endgroup$
    – Tim B II
    Commented Mar 20, 2018 at 23:37

3 Answers 3

Reset to default
3
$\begingroup$

Edit: it's been six years or so since I answered the question. To honor the votes I got so far I will leave my original answer below, I just want to say that I think a bit differently now.

A ring around the Earth and anything that could even make it would be the cause of a global level extinction, but the rings might be stable for maybe millions of years and life could "restart" (geologically) quick after the rings formation, so the scenario in this question seems feasible.

This video about the Moon nearly colliding with the Earth, but becoming a ring system instead is great food for thought. It's from my favourite popular science channel.

I am writing a story with similar premises, so from one hopeful writer to another, good luck OP.

My original answer follows below:

Earth's Roche Limit is within the exosphere. This is too close. Over time, collisions within the ring would cause its pieces to fall onto the Earth. The orbital speed at those heights is around 8~9 kilometers per second, about Mach 25 - that is, 25 times the speed of sound at sea level - and once they get into a suborbital path they will only get faster relative to Earth.

To make some comparisons... The asteroid that killed the dinosaurs hit us at four times that speed, and it was 14 kilometers wide. If your rings are in scale related to Saturn's, their mass would be well within 1/100 to 1/1,000 of most of the largest known asteroids. Given the mass and speed the debris just might not cause a global mass extinction, but you should still prepare for super earthquakes and tsunamis. No matter how much we downplay it, if you bring anything large enough into the Roche Limit, life would never be the same.

You may then think, "ok, let's just break the expletive thing in space then. Form a ring around Earth."

Doesn't work. Take Saturm for example. The rings are quite stable, but they do decay:

These particles often collide with one another and are affected by the gravity of their parent planet. A combination of collisions and other such forces mean that Saturn’s rings tend to spread out. Particles that are closer in to the planet can often be found falling into the planetary atmosphere.

In other words, constant bombardment.

I know this is not the answer you would like to see, but the tag is often cruel towards life when combined with astronomy.

If you are writing a story and some wizards did create the ring, it's no further stretch to say that the original spell/cantrip/ritual keeps the ring there. But that would be within the realm of the tag.

Improve this answer
$\endgroup$
18
  • $\begingroup$ 8~9 km/s would be Mach 25 $\endgroup$
    – Eth
    Commented Mar 19, 2018 at 18:01
  • $\begingroup$ I'm not sure where you got "within the stratosphere" out of the article you linked to, which places the surfaces of the Eath and Moon 2000-3000 km apart at the Roche limit. The stratosphere only goes out to about 60 KM. 2000-3000 KM is within the exosphere.... $\endgroup$
    – Spencer
    Commented Mar 20, 2018 at 0:35
  • $\begingroup$ @Spencer thanks for that, I have adjusted the post accordingly. $\endgroup$
    – The Square-Cube Law
    Commented Mar 20, 2018 at 12:17
  • $\begingroup$ Is it possible to have the thing hit a moon and not the planet? If this earth-like planet has several moons, I believe it would be possible for them to shepherd the debris in to a ring. $\endgroup$
    – bendl
    Commented Mar 20, 2018 at 21:07
  • $\begingroup$ @bendl sure, but then it would be outside the Roche limit. The OP asked for the debris to be contained within that limit so as to form a ring. $\endgroup$
    – The Square-Cube Law
    Commented Mar 20, 2018 at 21:49
2
$\begingroup$

I can see only one way this could possibly happen.

The basic problem is that deflecting it still leaves it on a hyperbolic path--if you get rubble it heads back out into deep space.

To actually capture the object means you need a far greater velocity change than is needed for deflection. Thus you need some scenario that causes major overkill.

The only way I see this is they guide some smaller object into the path of the big one. The deflector object will be chosen by how much energy it will take to get into the right spot--a large object that needs a small deflection might be chosen over a smaller one whose orbit doesn't work well.

Improve this answer
$\endgroup$
2
  • $\begingroup$ Right idea but you are missing that deflection would break the object. Most of it would indeed continue on to deep space but some of it would slow down in collisions with other fragments. Less massive fragments would be more likely slow down enough so you'd end with bulk of the mass going on and probably being drawn back together while some smaller remnants get caught in orbit. This is actually the exact scenario we want. I know your answer is missing all that but I am too lazy to write an answer for the remainder, so I'll +1 anyway on assumption you are willing to edit. $\endgroup$
    – Ville Niemi
    Commented Mar 20, 2018 at 9:28
  • $\begingroup$ @VilleNiemi How much is captured vs how much goes back out depends on the masses of the objects and the impact speed and any secondary encounters the object might have. $\endgroup$
    – Loren Pechtel
    Commented Mar 21, 2018 at 13:37
0
$\begingroup$

Impactor Threat

  • Instead of asteroids, you probably want a comet, also known as "dirty snowballs". Consists mainly of ice and sand, mud, small rocks etc. making them perfect for shiny, sparkly rings.

--> Advantage: Because of the tail, it can be seen from much greater distance (and probably during daylight also) than an asteroid (that doesn't reflect light most of the time, let alone producing a tail).

  • A comet would just not be that much of a threat, though. They would probably disintegrate/vaporize when entering the atmosphere (but will certainly cause some effect; see comment by @Gilgamesh).

  • Asteroids would be a real threat (but ruled out). It seems like a good motivator though having a deadly threat...

--> You can have an hybrid as well, like with a rocky core and an icy crust. Still needs a good explanation why the mages know the internal composition of that object or care about it and why they conclude that this is a cataclysmic event. A comet looks so pretty...

--> Even when using a "not direct impact course" (or even a hyperbolical course, making the comet leaving the system again for forever), a good motivation for mages might be "superstition", "bad omen" or the "assumed danger of the unknown celestial body", "end of the world" etc. (although it wouldn't be one; well, depends on the circumstances really when/if impacting)

Creating Rings

  • Shoving some big rock into the Roche limit is not the only way to create planetary rings.

  • Rings can also form when the solar system and the host planet form. This can lead to stable rings because the system "grew up together".

--> This would miss the effect of creating them, though. The people would be totally used to having them all their life, rather. Particles entering the atmosphere will (or plausibly can) be much lower in numbers.

  • You can have any debris forming the rings (also think many many small "satellites") orbiting a planet in basically any stable orbit higher than the planet's atmosphere. This needs their angle of attack, mass and speed to be "right".

--> Could be adjusted to match the explosion of the comet. This would again need some kind or orbit or tangential course, not a direct impact course. Forming of the rings would not be instantaneous, but arguably a lot (huge lot) faster and more plausible than "Roche grinding". Think maybe several years (or as you see fit. Also within a day would easily be "plausible" I think -> geostationary orbit then (or close to)). But because of the explosive nature, those rings would probably not last for too long (at least not for forever), because being spread thin over time and eventually dissolve.

Concerning Roche / The Roche limit seems to be popular these days ;)

  • Roche grinding needs a really slow approaching on an descending orbit. The Roche limit will not disintegrate everything that crosses it auto-magically! Especially not if crossing in a straight line (collision course). Roche grinding and collision course are pretty much mutual exclusive.

  • Roche grinding is awfully slow, especially if the object has much material to be grinded to tiny dust speckles (unless you pull something from the sleeve). Think in terms of thousands or millions of years for a "simple" asteroid or comet that was "stopped".

  • A moon takes more like a billion years (more rather than less) to be finished for good, and would break up into several large pieces multiple times during that process.

--> Meaning, the image in the sky would be most likely this new moon/asteroid, accompanying the planet for many generations/forever in human-terms, with an ever so slightly growing tail of debris forming the final state of the rings eventually (the asteroid/moon would also sweep the particles away constantly). Larger pieces of the moon will impact the planet on several occasions probably. Causing devastating effects at least locally (think of an radius of about 100km to 1000km and more, depending on size/mass and speed).

--> A moon is most likely so heavy that the gravity of the planet will have it pulled-in to impact long before it is grinded away. Let alone the increased tides (tsunamis) and other catastrophies that would be caused by that big/heavy object that close (near the Roche limit). A moon so close is like being able to be touched by hand, would appear much bigger than the sun in the sky, would be visible at the brightest daylight and cast a big shadow onto earth. Our moon for example is like 300,000km away from earth, not like <10,000km. Imagine our moon would be 30x larger. Also that moon has to be pretty damn fast on its orbit to maintain staying at the Roche limit and not crashing into the planet. Inducing winds in the atmosphere constantly, probably stronger than any hurricane we know. Living on the planet surface would pretty much be impossible.

--> A moon (not even a small one, I'd say) is not feasible at all for your purpose of "no harm to the planet" (Technically an asteroid orbiting earth is also considered a "moon", but I mean it in terms of size here. Moon being multiple times larger than an "average" asteroid).

  • Roche limit cannot be computed for real objects of actual mass, material composition etc. It is a theoretical value, based on heavily simplifying assumptions. A mage knowing the correct limit to park an asteroid there, will cause some very serious frowning on the reader's side, I guess (or they don't care). This has never even been computed on earth once, because it is virtually impossible. Ok, they are mages, but still... strange. This needs a good explanation to work.
  • Most people never heard of the Roche limit. You could not just throw in that term and everybody goes "ahaa, good thing they cared about that [in a fantasy world] acknowledging nod".

--> I deem it more plausible that the mages just let the asteroid/comet instantaneously disintegrate by magic power (can still cost them something), still a safe distance away before any harm can reach the planet, and only dust clouds, icy particles, etc. remain that sling around the planet to form the beautiful rings more or less instantaneously (or as you see fit) and everybody rejoices over it in awe.

--> If sticking with the Roche grinding, keep in mind: When entering the atmosphere, atmospheric drag would inevitably cause particles to be decelerated and to heat up because of the friction, causing either "micro-meteors/shooting stars" (glowing and burn up in the atmosphere) or even some that are able to survive (depending on material mostly) and impact the planet (named "meteorites" then). This would happen probably at a very high rate during the whole time. Having either very visible or almost no visible effect, depending on size, material composition and again mass, speed, angle (and maybe more factors, like day/night time, gravity of the planet, composition of the atmosphere, weather conditions etc.).

Also under special conditions, but possible, would be the creation of effects like the aurora borealis at the poles, when those particles enter atmosphere.

Fantasy vs. Physics

Bluntly speaking, if I may: I'd find it super odd, reading about mages (probably having all the power I can think of), but then they act totally bound to the laws of physics. Why didn't they summon a dragon and send him there to clear out the asteroid/comet instead of computing a Roche limit?

--> Maybe this is your intention. But I think, actually you are searching for an explanation to have rings on the planet and something that ties the mages together (not a rug ;)). Hope I could provide you with some ideas.

Improve this answer
$\endgroup$
2
  • 1
    $\begingroup$ Have to take issue with Comet "Not being that much of a threat". Recalling a part of a fantastic book, (Lucifer's Hammer) where it was explained in an interview the composition of a comet and the effects of a strike; A modest comet would deliver “two point seven times ten to the twenty-eighth ergs.” to the planet in just a few minutes. Thats “Six hundred and forty thousand megatons”. Excerpt here: goodreads.com/topic/show/… $\endgroup$
    – Gillgamesh
    Commented Aug 1 at 19:32
  • $\begingroup$ @Gilgamesh: True, I stumbled over that as well on rereading. I added ("well depends on circumstance") - it is open how large that comet is etc. But yeah, more probable is that some effect will happen instead of none. $\endgroup$
    – Antares
    Commented Aug 1 at 19:56

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .