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View Full Version : If a large comet or asteroid struck the moon...

bunker9603
2011-Jun-25, 02:33 PM
...how would that impact the earth?

I have read other threads about such an event where earth is the target of a large asteroid and if big enough it could wipe out life on earth (ELE) or at least change it significantly.

How big would an asteroid or comet have to be to either knock the moon out of earth orbit or to remove a large chunk of the moon affecting life on earth?

If this would happen in the next 5 years do we have the technology now that could divert it's path?

swampyankee
2011-Jun-25, 05:14 PM
The orbital velocity of the Moon is about 1025 m/s, and and the escape velocity is √2 times that, so to increase the Moon's velocity to escape would require doubling the current kinetic energy of the Moon. Of course, its orbit would be changed by smaller impacts, but I have no real way of computing how much change would be required to have significant affects on Earth. I suspect that the most likely problems would be secondary impacts, from ejecta that achieve lunar escape velocity and strike the Earth and smaller ejecta that damage satellites in geosynchronous orbit and LEO.

chornedsnorkack
2011-Jun-25, 06:34 PM
Which proportion of ejecta ejected from Moon collide with Earth?

ngc3314
2011-Jun-25, 09:43 PM
Roughly 1/3 of ejecta close to or exceeding lunar escape velocity (http://www.sciencedirect.com/science/article/pii/S0019103585711931) from one study (everything else I could find pointers to remains behind paywalls). Other abstracts note fairly short typical transit times for Earth impact (thousands of years in contrast to a few milion for Martian debris).

JustAFriend
2011-Jun-26, 03:36 AM
If this would happen in the next 5 years do we have the technology now that could divert it's path?

Despite what you see from Hollywood, we can barely get up into low orbit.

The only thing we could do is party and pray....

bunker9603
2011-Jun-26, 03:59 AM
Despite what you see from Hollywood, we can barely get up into low orbit.

The only thing we could do is party and pray....

I was watching a show on Natgeo several months ago and there were several ideas about how it could be done, however I don't know if any of it was technology that we presently have?

ETA: I just did a search on "diverting asteroids" and found this on wikipedia:

http://en.wikipedia.org/wiki/Asteroid_impact_avoidance

It is lengthy so I have not had a chance to read it yet, but my answer can probably be found there.

danscope
2011-Jun-26, 04:44 AM
Now that we think about it....... the Moon was obviously struck by more than just a few good sized meteorites and asteroids..... surely...... and it's still there .

Cobra1597
2011-Jun-26, 06:24 AM
Now that we think about it....... the Moon was obviously struck by more than just a few good sized meteorites and asteroids..... surely...... and it's still there .

Indeed. The moon has a nice little gravity well itself. Hitting it with rocks, even big rocks, doesn't knock material off for very long. The stuff falls back down and the total mass of the moon is relatively unchanged. Well, maybe a little more because of the rock that hit it.

chornedsnorkack
2011-Jun-26, 10:35 AM
Roughly 1/3 of ejecta close to or exceeding lunar escape velocity (http://www.sciencedirect.com/science/article/pii/S0019103585711931) from one study (everything else I could find pointers to remains behind paywalls). Other abstracts note fairly short typical transit times for Earth impact (thousands of years in contrast to a few milion for Martian debris).

On the other hand, looking from Moon, Earth covers just 1/3600 of heaven.

How would Earth be affected by a large number of individually small impactors spread out over thousands of years, as opposed to single giant impact?

Hornblower
2011-Jun-26, 11:23 AM
On the other hand, looking from Moon, Earth covers just 1/3600 of heaven.

How would Earth be affected by a large number of individually small impactors spread out over thousands of years, as opposed to single giant impact?

That would depend on how big and how numerous the impactors are.

We have found pieces of ejecta from the Moon and beyond here on Planet Earth. Some meteorites found on the Antarctic glacier have been linked to Mars.

Nowhere Man
2011-Jun-26, 12:17 PM
For a fictional treatment of this idea, look for Moonfall by Jack McDevitt.

Fred

neilzero
2011-Jun-27, 05:02 AM
If the impactor is 10 cubic miles = 42 cubic kilometers and travelling 100 kilometers per second with respect to the moon, large amounts (10E24 = million times a million times a million times a millon tons of very hot fast plasma would be ejected that would condense to very fine dust in solar orbit, I think. Earth would clear it's orbit again over a few thousand years, making our atmosphere slightly more dusty. The larger grains would pit LEO and GEO satellites, but failures would be rare. Average meteor trails observed might only double as there would be comparatively few one microgram plus size ejecta. A lower impact speed of a larger impactor would produce about the same number of large and medium pieces, but much less dust. I'm guessing, so please tell me why I am wrong. Neil

NEOWatcher
2011-Jun-27, 12:00 PM
If the impactor is 10 cubic miles = 25 cubic kilometers...
Are you saying a 10 mile (=25km) across asteroid (which would actually be 10 mile =16km)?
Or are you actually saying 10 cubic miles (which would be 4173 cubic km)?
I suspect the former, but just wanted the clarification for the benefit of our loyal viewers.

JustAFriend
2011-Jun-27, 07:10 PM
I was watching a show on Natgeo several months ago and there were several ideas about how it could be done, however I don't know if any of it was technology that we presently have?

Yes, they have 'ideas', but:

As of next month, we will have no Shuttle and no heavy lifters for the next 5 years (probably 10)
The Russian capsules can barely make the ISS orbit.
The stuff we send to other planets weighs less than a ton.

Without many years notice and a lot of new tech, we're screwed in your scenario....

bunker9603
2011-Jun-27, 11:50 PM
Without many years notice and a lot of new tech, we're screwed in your scenario....

That's a comforting thought!

dtilque
2011-Jun-28, 08:38 AM
Are you saying a 10 mile (=25km) across asteroid (which would actually be 10 mile =16km)?
Or are you actually saying 10 cubic miles (which would be 4173 cubic km)?
I suspect the former, but just wanted the clarification for the benefit of our loyal viewers.

I suspect that neilzero got the exponents wrong in his calculations. 10 square miles ~= 25 square km (actually closer to 26)

chornedsnorkack
2011-Jun-28, 02:34 PM
If the impactor is 10 cubic miles = 25 cubic kilometers and travelling 100 meters per second with respect to the moon, large amounts (10E24 = million times a million times a million times a millon tons of very hot fast plasma would be ejected that would condense to very fine dust in solar orbit, I think. Earth would clear it's orbit again over a few thousand years, making our atmosphere slightly more dusty. The larger grains would pit LEO and GEO satellites, but failures would be rare. Average meteor trails observed might only double as there would be comparatively few one microgram plus size ejecta. A lower impact speed would produce about the same number of large and medium pieces, but much less dust. I'm guessing, so please tell me why I am wrong. Neil

Wrong because of grossly wrong orders of magnitude.

Is it even possible for a big rock to be ejected from Moon by any explosion?

Wouldn´t the shockwave of the explosion simply shatter any rocks into fine gravel, which would then settle down as smooth and featureless crater walls and ejecta blankets or escape into space?

What do the rims and ejecta blankets of fresh craters of Moon look like? Smooth because of fine gravel, or lumpy because of big rocks displaced in one piece?

Wyssa
2011-Jun-28, 03:25 PM
Looking for meteorologist who can theorize what will happen if the moon was knocked out of our gravity well. Without tidal forces to drive winds and currents, weather would be unpleasant at best I'd think.

danscope
2011-Jun-28, 04:28 PM
Er....shock wave..... in a vaccuum ? Hmmm.....

NEOWatcher
2011-Jun-28, 04:35 PM
No; in the surface.

danscope
2011-Jun-29, 01:15 AM
Yes, the surface. Correct, sir.

astromark
2011-Jun-29, 03:35 AM
I would think the most likely scenario would be as the thousands of craters on the moon indicate..

another crater would be added and we would give it a bright new name..

I would guess it would look very white and have lines of debris just as 'Copernicus' does.

and the view from Earth would be especially spectacular.. but only last a few days..

Without too much effort I think a white feather like protuberance would erupt from the impact site..

and settle back to the lunar surface.

Critical to any calculations of impact crater size, is dependant on the amount of energy imparted at impact.

The structure and density and its velocity and angle of impact..

Could we arrange 'Apophis' to give a demonstration of this maybe ?

Alienathome
2011-Jun-29, 01:55 PM
At what point (how far from Earth) the moon will be off the tidal lock? Speaking of tidal lock, could it explain the crater patterns and the relative absence of maria on the far side of the moon?