# Reality Check - Collision of a Moon prevented by turning it into a ring

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.
• 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'... Commented Mar 16, 2018 at 1:53
• @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. Commented Mar 16, 2018 at 2:35
• 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. Commented Mar 16, 2018 at 2:38
• @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 Commented Mar 20, 2018 at 21:03
• @bendl You're right. I read 2000 (M), not 2000 (Km)! My bad. :D Commented Mar 20, 2018 at 23:37

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.

• 8~9 km/s would be Mach 25
– Eth
Commented Mar 19, 2018 at 18:01
• 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.... Commented Mar 20, 2018 at 0:35
• @Spencer thanks for that, I have adjusted the post accordingly. Commented Mar 20, 2018 at 12:17
• 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. Commented Mar 20, 2018 at 21:07
• @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. Commented Mar 20, 2018 at 21:49

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.

• 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. Commented Mar 20, 2018 at 9:28
• @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. Commented Mar 21, 2018 at 13:37