# How to fragment a moon to form a ring around a planet

Given a planet about the size of Earth, and a moon the Size of the Moon, how could one destroy the moon without flinging the fragments onto the surface of the planet or into outer space?

How can one make the moon break up and form a ring around the planet, like Saturn did with his moons?

• One chunk at a time? Anyway, it would be a hard time since, as you disgregate it, the center of mass of the Earth-Moon system changes and you will have to readjust the orbits continuously all over the ring. Note that, most likely, the rings of Saturn contain only a tiny % of the fragments that did end in other places of the solsar system and/or Saturn itself. Nov 28, 2014 at 8:53
• If the core of the moon was turned from rock directly into gas, wouldn't that readjust the center of gravity of the moon in the fashion that the breaking up fragments follow the old periphery around the planet? Nov 28, 2014 at 9:17
• @RobertBoettcher If you change a solid to a gas you don't move the center of gravity. The resulting effects of the gas spreading out wouldn't do either...although external effects on that gas may then do so. Nov 28, 2014 at 9:23
• @SJuan76 The Question is about the following: You have a moon, you want to break it up. You dont want the moons pieces to fly into outer space. And you dont want them to crash on the planet either. How to go about it? Nov 28, 2014 at 9:38
• @TimB Indeed the moon has a solid core surrounded by molten and semi-molten spheres. Kindly see following diagram: en.wikipedia.org/wiki/File:Moon_diagram.svg Nov 28, 2014 at 13:36

## 4 Answers

"All" you need to do is move the moon closer in towards the planet (without sending it into the planet or out of orbit).

Once the moon gets close enough and moves inside the Roche Limit it will be ripped apart by tidal forces and you can just sit back and let it happen. The fragments will form a ring by themselves. This is believed to be one of the main ways existing rings formed.

• +1 Good idea, but I would not bet on no fragments falling on Earth. Nov 28, 2014 at 9:14
• There would almost certainly be a few fragments fall but not as many as you think. Because of the way orbital dynamics works as things "fall" inwards they move faster until they start moving away again. It's only if they hit the atmosphere that they start slowing down and then will impact the surface. Nov 28, 2014 at 9:16
• Note that the roche limit of earth/moon is ~9500km, with earth radious ~6400km and moon radius ~1800km, meaning you have ~1300km space between those which would probably fun for satellites, and makes me wonder if its really just a few fragments that would drop Nov 28, 2014 at 13:24
• The atmosphere only extends up for 100km, plenty of room :) Nov 28, 2014 at 13:26

Moving the moon in to the Roche Limit, as Tim B suggested, is an excellent start. In the case of Earth, however, that involves lowering the moon's orbit to between 10 and 20,000 km (when it currently orbits at 360-400,000 km). Since this is low enough to interfere with the orbits of artificial satellites, it may be preferable to try to keep the debris ring higher than that.

To do that, we need to counteract some of the gravitational forces binding the moon together. That's straightforward enough - it just takes a really big bomb. Note, however, that in this case we don't want the bomb to be big enough to blow the moon apart properly. Instead, what we want to do is to make the explosion big enough to turn it into a cloud of rock fragments, but small enough that the cloud's gravity would eventually pull it back together again. If we get it just right, as the cloud gets close to its largest size tidal forces from the planet it orbits will kick in and start dragging off the inner layers. Once that process starts, the internal gravitational forces are no longer in control and the moon should disintegrate nicely.

There are a couple of ways to fine-tune the process. Moving the moon in closer to the planet is one - the closer you get, the stronger the tidal forces become, and the easier it is to prevent the moon from recoalescing. Secondly, you can play with the positioning of the explosives; your local astrophysicist or registered terraformer should be able to advise you on what shape debris cloud will be most useful.

Ok, I'll make that an answer.

Disclaimer: I am not a registered terraformer, consult your local registered terraformer before actually performing astroengineering works.

You cannot do. At least, you cannot say: "I will put my electroscientimagic device at point P, push button B at hour H and then I can call it a day".

What your elecroscientimagic device will do will be applying energy. That energy (minus inefficiences) will go into breaking the Moon (which by itself is not a big change) and giving the fragments enough energy to leave the Moon's gravity and get into Earth orbit.

Say you only make two halves and want one of each half in opposite points of the Moon's orbit. For the fragment you move, you need to:

• accelerate to Moon's (well, half-Moon) escape velocity.
• put in the desired point of the orbit.
• accelerate (o deccelerate) to the needed orbital speed to keep that orbit.

Now, by doing that change, you have changed also the center of masses, so you will have to adjust the orbit parameters of both halves of the moon.

To do your project, you will need to begin a long time process that will need constant supervision and readjustments. And of course, you will need some reason that justifies such effort.

You could certainly try.

A less sensible method, if your local astrotech store sells them, is to get hold of a mass transporter and a matter inversion matrix (though in many timelines the latter comes in around the year 4121). This would enable you to move dense objects such as small neutron stars around. If you can then vaporise the Moon's core as suggested in the comments (usually by introducing some intense heat to vaporise the rock inside), you would be able to move your neutron star into the core of the moon. This would collapse the moon. Soon after this (15 seconds or so), you use your matter inversion matrix to destroy the neutron star, causing a small explosion due to the lack of gravity (this is why you need to make sure to use a small neutron star).

Alternatively, if you don't have a ready source of intense heat, you could detonate a big bomb in the moon such that it expands for long enough to insert the neutron star, then the matter is pulled together again.

Disclaimer: This method does not guarantee all the fragments enter a ring, though most will. No responsibility can be accepted for damage to nearby planets from flying Moon fragments.

If you want to be more sensible, I suggest looking at Tim B's answer. The Roche limit is always a good way to destroy celestial objects. In fact, I think I remember the Varga using it to destroy Mercury in 3112...

Or, of course, just a really big bomb.

• I dont understand. Doesnt a neutron star have mass? Wouldnt introducing it into the system of celestial bodies not increase the entropy in the system? The targets are to keep the investment of energy low, so moving other celestial objects costs probably more than creating them from virtual particles. Please elaborate the core vaporization. You said first vaporize the moons core and THEN insert the Neutron star. I dont understand? Nov 28, 2014 at 13:34
• A neutron star is incredibly dense. Introducing it then removing it instantly would cause a disastrous loss of gravity and disintegration. Nov 28, 2014 at 14:05
• Doesnt that emit a lot of x- and gamma rays? I dont want to fry the planet. Nov 28, 2014 at 15:06
• They have been known to pulse low-intensity X-ray bursts, but it's only there for 15 seconds or so and it's inside the moon, so... Nov 28, 2014 at 19:15
• This answer doesn't make much sense. the question asks for science-based answers; this one is really sci-fi-based. Nov 29, 2014 at 1:21