# Would a sun-blocking asteriod field be created if something big was to smash into the moon?

I'm wondering if it is plausible that a large asteroid (interstellar in origin? Or, perhaps launched from the Kuiper Belt by something big with a lot of gravity?) would be able to smash into Luna and tear out significantly large amounts of rocks from it, enough to form a field of rocks orbiting the earth that is dense enough to cool the planet significantly for a few hundred years.

While I realize that an apocalyptic amount of debris would fall to earth from such an event, would it at all be possible for humanity to survive such an event though they would have to do without civilization of course. How much would this depend on the angle it hit the moon at?

How long would it take for the field of rocks to decay enough and fall to earth, to such a degree that the sky becomes mostly clear? Decades? Centuries?

What would happen to the orbit of what remains of the moon? Would it have to leave the Earths Sphere of Influence or could it simply become more eccentric? Or would it not have much effect at all?

Thank you in advance for any responses to my questions!

• Possibly related: Conditions needed for a Fractured Moon – Alexander Jun 8 '18 at 17:07
• Neil Stevenson wrote a book with a scenario similar to this, complete with apocalypse, and subsequent recovery. en.wikipedia.org/wiki/Seveneves – Nate White Jun 8 '18 at 17:11
• Not a full answer, but through the power of math I just found out that if the moon became a torus at the same distance as the moon it would have a radius of only 168km. Probably not enough to block much sun. – Giter Jun 8 '18 at 18:26

1. Cooling the entire planet would require a seriously thick ring. A ring on the order of 60% of the planet's diameter in thickness. The moon has no where near enough mass for this. Also, I'm willing to bet there's some math on this that says a thick ring wouldn't last very long as orbital forces fight between pulling the ring down and spinning it off into space. Like a big ol' pizza in-the-making, it would flatten and spread.

2. As a side note to (1), If the ring were seriously tilted off-axis such that the ring needn't be thick, but broad, and that shadow were cast on the Earth... but such a ring would not always be facing the sun to create the effect, so no, this wouldn't work, either.

This really all boils down to mass. We think the moon is big because, compared to other planetary moons, it's ginormous. But it doesn't really have all that much mass, and you're suggesting spreading that mass more-or-less uniformly around the planet. In the end, other than destroying our ecology by ruining tide, weather, probably even tectonic movement, the final result would be no intrinsic cooling effect.

Unless what hits the moon has serious mass and hits it just right to bring most of that mass into orbit. Note that all those biblical events (tide, weather, tectonics, etc) would happen, ruining the ride, but you might end up with enough mass to cool the planet.

# There is plenty of material....

The orbital radius of the moon is $3.85\times10^{8}$ meters. In order to block all sunlight from the Earth, we would need to transform the moon into a solid shell (basically, like one of the Halo rings). Since the Earth's axial tilt is significant, the Moon-shell will need to extend roughly 23 degrees north and south of the Equator.

The surface area of a sphere is $4\pi r^2$. The surface area is a polar cap is $2\pi r^2(1- \cos \theta)$, where $\theta$ is the polar angle of the cap. In this case, that polar angle is $\theta = 63^\circ = 1.1 \text{ radians}$. Therefore, the overall surface area of the shell-like debris field must be $$4\pi r^2 - 2\cdot2\pi r^2\left(1 - \cos\theta\right)=4\pi r^2 \cos\theta.$$ Plug in the radius and polar angle and we get $8.4\times10^{17}$ m$^3$.The mass of the moon is $7.3\times10^{22}$ kg. If you spread the entire moon out over the required area, then you have about 86401 kg available to block the sun for every m$^2$ of surface area.

The moon's density is 3300 kg/m$^3$; but that material is somewhat compressed by gravity. Lets say the moon's density is 2500 kg/m$^3$ when not in a gravitationally compressed ball. This is slightly less than the density of the Earth's crust, so probably not a bad estimate. If we use that figure, then you can form a solid shell 34 meters thick around the Earth. That will definitely block out the sun.

If you decide to only block 1% of the sun's mass into space; there is still enough for 30 cm of solid rock and iron (over a foot!) blocking the sun from the entire surface of the Earth.

# ...but it will not be in the correct place...

This is going to be a big spherical cow estimate, but let us say that the Moon is destroyed completely and turns into a structure the shape of Saturn's C ring (the innermost bright ring). A mass of $1\times10^{18}$ kg is the distributed around an inner circumference of about 135,000 km. This is four order of magnitude less massive than the moon; but then twice as close to the main planet. Also relevant is the density; Saturn's ring is water ice but the Moon is about 2.5 times more dense, as estimated in the previous section. Taken all together, let us estimate that the Moon turns into a ring with length dimension $10000\cdot\frac{1}{2}\cdot\frac{1}{2.5} = 2000$ times larger than Saturn's C ring.

Unfortunately, Saturn's C ring is only 5 meters thick, so our Moon ring is estimated at only 10 km thick. To cover the 23$^\circ$ north and south fo the equator, it really aught to be 300,000 km thick. That isn't going to cut it. A mere 10 km is going to block less than 0.001 % of the light blocking Earth; this won't have much effect at all on the climate.

# Conclusion

There is plenty of mass, but naturally forming ring structures are nowhere near thick enough to block the Sun; certainly not at the distance from the Earth that the Moon already is.

Unless you engineer a solution, like blowing up the moon, moving all the rocks into a closer orbit, and then using some mechanism to disperse them from a naturally forming ring, you simply aren't going to get appreciable blockage of the sun.

Unlikely.

A belt like that is going to be pretty thin. At any given time the asteroid belt isn't getting in the way of very much sunlight at all. More of a concern would be the short-term cooling if a bunch of the pieces dropped into the atmosphere and created significant dust cloud effects. That'll lead to some ugliness for a year or two, but it'll recover relatively quickly thereafter.

Effects on the orbit are kind of an "it depends" situation. Orbital mechanics are complicated, and collisions are even moreso. It could have any one of a wide variety of effects - knocking it out of orbit entirely, causing it to crash into the earth... all sorts of possibilities.