If a comet hit the moon.

[Undesired Walrus]

If a comet hit the moon, how big would have to be for us to see the impact from Earth? Would we see a shadow approach the moon, and then a massive explosion?

If it was big enough to tear a chunk off the top corner -for example- would that prove to be a signifigant problem for us on Earth? How long would it take for the moon to become round again?

[Roboramma]

If it was big enough to tear a chunk off the top corner -for example-
The moon is spherical. :P

Actually, interesting question, I just don't have anything intelligent to respond with.

[nathan]

If a comet hit the moon, how big would have to be for us to see the impact from Earth? Would we see a shadow approach the moon, and then a massive explosion?

Do you really mean comet? IIUC comets are always visible to us when they are that close -- their tail gives them away. If they are so small the tail would be too tiny to see, they'll have already evaporated by the time they get this close to the sun.

[RecoveringYuppy]

We have already seen small meteroids impact the moon, so yes it would be visible. (Google moon and Leonids, for example). And anything large enough to remove a visible (to the naked eye) portion of the moon would be dangerous to Earth.

A shadow would be seen if the comet was between the Sun and the Moon.

[Small Town Jesus]

It would rain cheese for days!

[Undesired Walrus]

Let's say it was a 5 mile deep impact hole. How powerful would that look and how dangerous would it be for Earth?

[Bikewer]

We know that we have bits of Mars here on Earth as a result of collisions, presumably with a large asteroid or similar body.
So a really serious impact of the moon could certainly result in debris being ejected with sufficient force to make it here.
Just off the top of my head, unless we're talking about a really huge asteroid, it doesn't seem likely we'd get anything much larger than smaller meteorites.

[XBoxWarrior]

Let's say it was a 5 mile deep impact hole. How powerful would that look and how dangerous would it be for Earth?

Very "Painfull", and soooooo very "Dangerous".......

(sounds like a porn title?)

[Reality Check]

Let's say it was a 5 mile deep impact hole. How powerful would that look and how dangerous would it be for Earth?
It would look as powerful as a 5 mile deep hole. The danger to Earth varies from zero to a lot depending on where the impact was and what happens to the ejected mass.

But getting back to the impact of a comet:

If a comet hit the moon, how big would have to be for us to see the impact from Earth? Would we see a shadow approach the moon, and then a massive explosion?

If it was big enough to tear a chunk off the top corner -for example- would that prove to be a signifigant problem for us on Earth? How long would it take for the moon to become round again?

The answers are:
Any size so long as someone is looking at the time of the impact.
We will see a "shadow" (the comet I guess) approach the moon and then a massive explosion. This assumes that the impact is on the side of the Moon facing us or that we just happen to have a satellite observing the other side.
If a chunk of Moon was torn off and fell to Earth then that would be a problem for us ranging from minor to devastating depending on the size of the chunk and where it landed.
The Moon will remain as round so the time is zero. If you mean a really really big chunk (e.g. a quarter of the Moon) then the Moon would take a long time (maybe millons of years) to become round again. Or maybe the heat of the impact would be enough to melt the entire Moon and so it will be a lesser time (maybe hundreds of years).

[Beausoleil]

We know that we have bits of Mars here on Earth as a result of collisions, presumably with a large asteroid or similar body.
So a really serious impact of the moon could certainly result in debris being ejected with sufficient force to make it here.


And indeed we do have small meteorites - lunar meteorites sample bits of the moon not sampled by the Apollo and Luna missions and so are changing our picture of lunar evolution quite a lot.

[CapelDodger]

We know that we have bits of Mars here on Earth as a result of collisions, presumably with a large asteroid or similar body.
So a really serious impact of the moon could certainly result in debris being ejected with sufficient force to make it here.
Just off the top of my head, unless we're talking about a really huge asteroid, it doesn't seem likely we'd get anything much larger than smaller meteorites.

I think we can safely say the Moon took the bullet for us. And not for the first time, I'm sure.

[davefoc]

I take the OP question to be something like:
Assume that the moon is struck by a very large object that is big enough to substantially change the shape of the moon, but strikes the moon in such a way that significant ejecta doesn't strike the earth. Would there be significant consequences to the earth anyway?

The only consequences that I can see would be tidal changes that might be very severe if the moon's orbit is changed significantly. Severe earthquakes are conceivable to me if age old patterns of lift and fall from the moon's gravity are changed.

On how long the moon would take to return to its roughly spherical shape:
Reality Checks estimate seems way too long to me. The forces to return to a roughly spherical shape would be enormous. I would have thought that it would happen very quickly.

[Undesired Walrus]

If something the size of the asteroid that destroyed the dinosaur's were to hit, what would the explosion look like? Obviously not something big enough to tear off a huge chunk of the moon, but signifigant nonetheless.

Would that cause any problems down here?

(forgive the careless phrasing of this question, I think I'm coming down with flu)

[RecoveringYuppy]

The Chicxulub crater is over a hundred miles across. The same asteroid wouldn't hit the Moon with the same force (due to the Moon's lower gravity) but it would be huge and imposssible not to notice from Earth. And I don't know what you mean by "huge" but it would certainly lift pieces of the Moon in to orbit.

[Dancing David]

The effects would vary on what and where they hit (on earth), I like the idea of green cheese falling. :D

On a serious note it is believed to have been something fairly large that hit the earth and made the moon.

[I Ratant]

I expect an object large enough to be of "interest" on Earth would be visible before the Moon got in its way.
An Earth-orbit crossing asteroid, for example.
Many (most?) of these are known.
And it isn't the gravity of the Earth or Moon that would be a contributor to the force of the impact, but the velocity of the object itself. Overtaking from behind, or coming in head-on... big difference in energies.
Most stuff in space is moving right along, relative to any speeds we're used to on the surface of the Earth.. 10's of thousands of miles per hour. A slight change in any gravitational attraction as it nears the Earth wouldn't do much to alter its direction of flight.

[RecoveringYuppy]

And it isn't the gravity of the Earth or Moon that would be a contributor to the force of the impact, but the velocity of the object itself. Overtaking from behind, or coming in head-on... big difference in energies.

Any object that hit's Earth is going to gain Earth's escape velocity prior to hitting. Any object that hit's the Moon gains the Moon's escape velocity prior to hitting.

[casebro]

What with Earth';s higher gravity to attract the projectile, and being a bigger target, it seems like the chance of earth being hit by the same size bullet is waaaay higher than the moon getting it. So the question ought to be "What will happen when the Earth get's hit with that big of a rock?"

[Cuddles]

Any object that hit's Earth is going to gain Earth's escape velocity prior to hitting. Any object that hit's the Moon gains the Moon's escape velocity prior to hitting.

You have to take atmosphere into account as well though. An object striking the Moon will gain the Moon's escape velocity, or close to it at least. An object striking the Earth is likely to slow down rather than speed up due to friction with the atmosphere - most meteorites have slowed to terminal velocity before they hit the Earth. So actually, the Moon is likely to be hit with much more force than the Earth.

Another issue is water. The majority of the Earth's surface is covered in water, so an impactor is likely to hit there. This will obviously have rather different results from an impact on land. From the point of view of life, either way is going to be pretty bad, and an impact in water may well be worse than one on land. However, from a geological point of view, an impact in water will be pretty much irrelevant. Instead of a massive impact crater and bits of rock and dust being thrown all over the place, you'd just have a load of water being sloshed around and vapourised and then flowing and falling straight back to where it started. Obviously the Moon doesn't have this, so an impact there is much more likely to leave lasting visible changes.

[Denver]

If something really big hit the moon, I think the weirdest thing would be watching the spectacle in its complete silence. I would have at least wanted a boom, like in most scifi.

[shadron]

This page tells about one meteor collision and shows a video of the occurrence. The object was estimated at 10" diameter, and created a 45 foot-wide crater; estimated energy release was 17e9 joules, about 4 tons of TNT equivalent.

http://science.nasa.gov/headlines/y2006/13jun_lunarsporadic.htm

[Undesired Walrus]

If something really big hit the moon, I think the weirdest thing would be watching the spectacle in its complete silence. I would have at least wanted a boom, like in most scifi.

I suppose we would see the rock illuminated in the nights sky for a few days? Or would we see it shoot across in a matter of minutes?

If it were the size of Shoemaker Levy (when it hit Jupiter, it hit with the force of 600 times the world's nuclear arsenal), would it be a fireball? Or does a planet need an atmosphere for that result?

Any boffins able to calculate the size of the destruction on the surface?

[The Atheist]

I suppose we would see the rock illuminated in the nights sky for a few days? Or would we see it shoot across in a matter of minutes?

Going by how long comets are visible for and how far they are away, usually, I imagine we'd see it lining the moon up for several days at least, getting bigger and bigger.

I just hope it hits at night in the southern hemisphere if it's big enough to be spectacular. Some of the craters extant on the moon are pretty impressive, but obviously the chances are vastly higher that the side facing away from us will be hit, because the side facing us faces us and not too many comets are likely to come from this direction.

Maybe we could launch a decent-sized nuke to check it out?

If it were the size of Shoemaker Levy (when it hit Jupiter, it hit with the force of 600 times the world's nuclear arsenal), would it be a fireball? Or does a planet need an atmosphere for that result?

Alas, no gas, no flames.

[RecoveringYuppy]

Alas, no gas, no flames.

There would be a "fireball". Plenty of plasma from such an impact. No flames per se, but plenty of energy.

[shadron]

One thing you'd need to do is clean up all the feces here on earth, as hitting the moon represents a real near miss of our own planet. Particularly if it hit the visible side.

[marting]

The velocity of most meteors from outside the solar system is far greater than the the Earth's (or Moon's) escape velocity. Depending on the direction of the meteor impact, it's velocity will be incremented - or decremented- but by a delta far less than "escape velocity." Imagine a ball dropped from 16 ft. It will hit with a velocity of 32 ft/sec. Now imagine it thrown downward at 130 ft/sec. from the height of 16 ft. It will hit at a velocity of approx 132 ft/sec, not 162 ft/sec.

[I Ratant]

Any object large enough to be of interest would be thru the atmosphere so rapidly there would little diminution of its velocity.
The effect of its speed and mass on the atmosphere would be more than significant, talking with my buddy T. Rex, who said "You don' wanna be around when that happens... and then again, you won't!"

[RecoveringYuppy]

@marting, What kind of velocity are you suggesting? And how many meteors are known to have come from outside the solar system?

[marting]

I was using large differences with a thought experiment to illustrate the point that incomng meteors did not change their velocty by escape velocty but rather a small fracton thereof. For normal, intra system meteors, the difference in velocity is still a sgnificant multiple of Earth let alone the Moon's excape velocity and so velocity would change by considerably less than escape velocity.

[RecoveringYuppy]

OK. So I agree that objects with high initial velocity relative to our system don't gain full escape velocity. But isn't that a rather rare case? I was under the impression that most (90+ %) of impacts are with objects in prograde somewhat circular orbits where our gravitational field(s) would dominate the impact energy. I'm also under the impression that percentage is even higher for larger objects.

[marting]

OK. So I agree that objects with high initial velocity relative to our system don't gain full escape velocity. But isn't that a rather rare case? I was under the impression that most (90+ %) of impacts are with objects in prograde somewhat circular orbits where our gravitational field(s) would dominate the impact energy. I'm also under the impression that percentage is even higher for larger objects.

Not AFAIK. IMy understanding is that most meteors are on highly elliptical orbits and generally passed the earth at pretty high speed compared to escape velocity (3x or more for most of them).

[Dancing David]

I suppose we would see the rock illuminated in the nights sky for a few days? Or would we see it shoot across in a matter of minutes?

As far as seeing a small rocky body, nope, really small and dim. If you had the right scope with the right tracking equipment then you could see it. You might get lucky and see it shortly before impact, especialy if it was reflective.

Comets on the other hand eject gas and dust that is much more visible from a distance.


If it were the size of Shoemaker Levy (when it hit Jupiter, it hit with the force of 600 times the world's nuclear arsenal), would it be a fireball? Or does a planet need an atmosphere for that result?

There would be the vaporization of material, so 'fireball' as great mush of hot expanding stuff. Yes.


Any boffins able to calculate the size of the destruction on the surface?

The angle of impact and such will create variations. Destruction implies something to destruct, so if you had a surface lunar settlement, maybe. Otherwise, just another addition to the impact covered surface of the moon.

[soylent]

If a comet hit the moon, how big would have to be for us to see the impact from Earth?

Less than 25 cm. (see http://science.nasa.gov/headlines/y2006/13jun_lunarsporadic.htm)

edit: Scratch that, less than 12 cm(see http://science.nasa.gov/headlines/y2005/22dec_lunartaurid.htm).

[Cuddles]

Any object large enough to be of interest would be thru the atmosphere so rapidly there would little diminution of its velocity.

It depends what you mean by "of interest". The kind of meteorite that can wipe out dinosaurs may not slow down much (although it would still slow down and not speed up), but things on the scale of Tunguska will slow down a lot, and most people would consider an impact capable of wiping out large cities to be of interest.