Author Topic: The Moon really protect us ? (Read 30630 times)

APODman · « **on:** February 19, 2009, 04:35:22 AM »

Often read that the Moon would be a kind of natural Earth's “shield”, protecting us of the impact of (really) large asteroids, especially in the period of formation of our planet.

Although the probability of an asteroid of considerable mass can be attracted by the moon is low (in fact it only will happen by luck, just because the moon was in its way), I decided to do some simulations to test this argument.

Than I created, an object ( with 0.15 lunar mass and a velocity of 12 km/s ) in direct route of collision with the moon in various angles:

If the body collided on dark side of the moon ( opposed the Earth ) speed ejection of the fragments (about 12 km/s ) would they spread toward contrary direction to the Earth, few fragments would finish being attracted by our planet and coming to clash with us:

But if the crash occur in an angle that reach any part of the region visible of the moon then fragments would ejected at a speed of collision in direction of our planet:

Well, the mass of material ejected would be approximately 0.03 lunar masses ( cause part of the mass was accretad with the moon ) or 2.204E21 kg . The Earth occupies a small fraction of the area of the surface of the sphere (that can be drawn and the moon at the center of sphere ) of dispersal of fragments, than I conclude that only a small percentage of this material definitively reach our planet. This quantity would be sufficient to create a global disaster !

The percentage of the fragments that reach the Earth would be (radius of the Earth/ radius of the orbit of the moon)^2/2, or 0.014% and the mass would fall in the Earth would be 3E17 kg, comparable to a asteroid of 72 km in diameter.

The energy of the total impact of all fragments that reach us are 8E29 joules, equivalent to 380 trillion megatons or the conversion of 8 billion tonnes of matter/antimatter. Do not know how much energy would be transformed into heat, but it seems more than enough to transform the moon into a ball of metals and silicon in fusion, thousands of degrees. It would lead some decades to come back to solidify and 6 thousand years for cool until its "normal" temperature, if we applying this formula:

- http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/cootime2.html

Conclusion: If a large object were to collide with the moon we should have luck to this colision occur in the correct angle to avoid a global disaster because otherwise...

"This is the end, Beautiful friend
This is the end, My only friend, the end
Of our elaborate plans, the end
Of everything that stands, the end"
Doors - "ItÂ´s the End"

note: Thanks to Antonio Luiz of the Orkut community "Astronomia!" by the help with the calculations.

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Andreas · « **Reply #1 on:** February 19, 2009, 06:17:18 AM »

very interesting calculations. But you forget one thing. You see the moon as shield in the way of a body. Like if something will hit earth and the moon is in its way. But its not that way. Earth has a very big and massive moon. Astronomers call our system a double planet.
Only Pluto has a bigger moon compared to his size.

Our moon is a scouce of much gravity. The moon can change massive the orbit of an asteroid.

Andreas · « **Reply #2 on:** February 19, 2009, 06:32:29 AM »

I just make an experiment with Ceres heading straight for earth and cross the way of the moon in a distance of 40.000 km. I just reacord it. Its pretty impressive, the moon change the orbit of Ceres enough to get it away from the impact curse. Infact, it miss earth with few thousand km, orbit it one time and then is fired back in space.

Here it is:
http://www.youtube.com/watch?v=4-8hId_R3oI&feature=channel_page

APODman · « **Reply #3 on:** February 19, 2009, 08:32:19 AM »

Quote from: Andreas on February 19, 2009, 06:32:29 AM

I just make an experiment with Ceres heading straight for earth and cross the way of the moon in a distance of 40.000 km. I just reacord it. Its pretty impressive, the moon change the orbit of Ceres enough to get it away from the impact curse. Infact, it miss earth with few thousand km, orbit it one time and then is fired back in space.

Here it is:
http://www.youtube.com/watch?v=4-8hId_R3oI&feature=channel_page

Yes, the moon have gravitational interactions with objects taht pass close of it, but the moon donÂ´t have the capacity of the gravitational atraction of Earth, the object continues ahead in direction of Earth

And maybe the attraction of the moon is not responsable by the missing trajectory. You try running the simulation deleting the moon ?

I've made simulations with this situation and the body is affected by the moon only in its hills radii limits and teh body crosses this region vary fast ( the mean velocity for NEA bodies are 10-12km/s ) , the time that it is remain in the gravitational influence of the moon is very short to made considerable alteration in your trajectory.

But even the moon can affect the trajectory of some body it can change the trajectory pointing the body straight ahead to the Earth !

My simulations are to testing the argument that the moon are a shield of Earth, ie, if a body collide with the moon we are not affected. The simulations shows that this argument is not true.

In any cases ( even in yours ) the presence of the moon is very dangerous in this scenario.

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avadod · « **Reply #4 on:** February 19, 2009, 08:43:01 AM »

if the moon was gone the whole earth will be destroyed by thosands of asteroids and planets

APODman · « **Reply #5 on:** February 19, 2009, 09:20:25 AM »

Quote from: avadod on February 19, 2009, 08:43:01 AM

if the moon was gone the whole earth will be destroyed by thosands of asteroids and planets

ItÂ´s not true, see the simulation, the Earth are not be protect in all.

The moon can change the asteroid trajectories directing the asteroid to the Earth or, in the case that the asteroid collide with the Moon, a large mass of fragments certainly hit us causing ( depending of the original mass os asteroid ).

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avadod · « **Reply #6 on:** February 19, 2009, 10:46:05 AM »

but if there isn't a moon life on earth would die

FGFG · « **Reply #7 on:** February 19, 2009, 12:30:23 PM »

The real problem is how probably is an impact on the moon/on the Earth compared to a deviation from the impact path.

We should make an enormous number of tests:

We have to track how many time an asteroid hit the Earth without, and with the moon (changing position, approach angle, speed, dimension of the asteroid, position of the Moon and so on...), and then decide what is better.

The problem is that it is a very long task and Universe Sandbox is not accurate enough

APODman · « **Reply #8 on:** February 19, 2009, 12:42:22 PM »

Quote from: avadod on February 19, 2009, 10:46:05 AM

but if there isn't a moon life on earth would die

Maybe some kind of life would be extint, but surely not all.

The moon stabilizes the axis tilt of the Earth, without it the Earth would have a cyclical oscillation of its axis between the 27o and 60o in a few tens of thousands of years, which certainly affect the whole climate of the our planet and consequently the life on the surface of our planet.

BUT some bacteries lives far behind the surface, in fissures in the rocks. For this bacteries no matter what is happening on the surface of the planet:

- http://en.wikipedia.org/wiki/Endolith

But this topic is not intended to discuss the importance of the Moon for stabilization of the the axis tilt of Earth, but if it serves, or not, as a effective shield against objects on a collision course with Earth.

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APODman · « **Reply #9 on:** February 19, 2009, 02:16:28 PM »

Quote from: FGFG on February 19, 2009, 12:30:23 PM

The real problem is how probably is an impact on the moon/on the Earth compared to a deviation from the impact path.

We should make an enormous number of tests:

We have to track how many time an asteroid hit the Earth without, and with the moon (changing position, approach angle, speed, dimension of the asteroid, position of the Moon and so on...), and then decide what is better.

The problem is that it is a very long task and Universe Sandbox is not accurate enough

Really,

But the focus of this simulations is to show that even an asteroid collide with the moon we are not secure, cause the fragments ejected of the colision still offer risk to our planet.

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HallowedError · « **Reply #10 on:** February 19, 2009, 07:21:42 PM »

It would take very peculiar situations for the moon to protect us from a collision. If it gets hit there will almost always be debris that comes down which is pretty bad. $:-\$

atomic7732 · « **Reply #11 on:** February 19, 2009, 07:27:52 PM »

Yes, but that's where gravity comes in. It is more likely an object will be slingshotted bu the moon then the moon getting hit.
Remember, they are far apart. Not like it seems
Earth...Moon
|..........0
It's more like
|...0

HallowedError · « **Reply #12 on:** February 19, 2009, 07:30:35 PM »

Compared to Earth the moon has very little gravity, I'd guess that as the asteroid came closer the moon would just speed it up slightly. I doubt it would change the trajectory very much. But I'm not a rocket scientist either so I can't say anything for certain.

Andreas · « **Reply #13 on:** February 20, 2009, 12:41:30 AM »

Quote from: HallowedError on February 19, 2009, 07:30:35 PM

Compared to Earth the moon has very little gravity, I'd guess that as the asteroid came closer the moon would just speed it up slightly. I doubt it would change the trajectory very much. But I'm not a rocket scientist either so I can't say anything for certain.

Compared to earth, the moon has much gravity. Tell me a planet, that has a bigger moon then earth compared to its size.

avadod · « **Reply #14 on:** February 20, 2009, 11:44:48 AM »

the moon has great gravity when it's closest to earth.But the gravty is still strong

FGFG · « **Reply #15 on:** February 20, 2009, 12:07:56 PM »

Quote from: APODman on February 19, 2009, 02:16:28 PM

Really,

But the focus of this simulations is to show that even an asteroid collide with the moon we are not secure, cause the fragments ejected of the colision still offer risk to our planet.

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I don't deny that, but we don't know if this would be as frequent as an impact on the Earth. Maybe the Moon changing slightly the path on the incoming asteroids, makes them pass between Earth and the Moon itself more frequently. Maybe not. We don't know.

avadod · « **Reply #16 on:** February 20, 2009, 12:09:47 PM »

well if there was a ring around the earth the moon will mess it up

atomic7732 · « **Reply #17 on:** February 20, 2009, 03:28:39 PM »

Quote from: avadod on February 20, 2009, 12:09:47 PM

well if there was a ring around the earth the moon will mess it up

It depends, if it is out of the moon's Hill sphere. You would be wrong.

avadod · « **Reply #18 on:** February 20, 2009, 05:35:45 PM »

sometimes it can happen it has to be strong to do that

it might because of the gravity

Dan Dixon · « **Reply #19 on:** February 25, 2009, 04:03:01 PM »

"A Hill sphere is, roughly, the volume around an astronomical body (such as a planet) where it dominates in attraction of satellites to that body, rather than to a larger body (such as a star) which it orbits. Thus, for a planet to retain a moon, the moon must have an orbit that lies within the Hill sphere of the planet."
http://en.wikipedia.org/wiki/Hill_sphere

So actually I'm not sure that a Hill sphere would apply to to the Earth since the Earth's Hill Sphere would be determined by the Sun.

But either way, NeutronStar, your meaning is correct in that the ring would only be messed up if it were too close to the moon.

atomic7732 · « **Reply #20 on:** February 25, 2009, 07:34:02 PM »

Oh I noticed that things can be tugged out side the hill sphere, but it cannot like you said, retain a moon, and cannot capture anything, but can tugg it. Too many commas...

Chaotic Cow · « **Reply #21 on:** February 26, 2009, 06:29:24 PM »

I Love this thread so much I decided to do some tests. Note these test are NOT accurate.

I've only gotten the moon to deflect once! It almost did it twice but not enough.

It seems based on Universe Sandbox to deflect (Change trajectory) of a Universe Sandbox Asterioid at Default settings...it has to be REALLY close...not sure how close but you will see.

http://www.youtube.com/watch?v=Lje1v3oZz7U

osezno_ · « **Reply #22 on:** February 28, 2009, 01:56:34 AM »

This an excellent and enriching post! I very much enjoyed it!

I also had the perception that the moon indeed protected us somehow, but this post is making me thinking it twice. [On the other hand, is it likely that the moon has been good for life formation due to tides, that (a) somehow serves to "mix up" the initial "soup of life" and (b) that has certainly influenced on the creation of land-based life]

One my proposals for US evolution was to add an "event log", so that you could have a text file with all that has happened. In this case, the event to record would be "collisions". If it were the case, it would be pretty iteresting to run a simulation of the whole Solar System and create a large number or "menacing asteroids" (let's say they are trans-neptunian, but all with velocities and trajectories pointing the inner Solar System) and run the system for a long while and count how many of those have impacted the earth, with and without the moon, with and without Jupiter, etc.

I always thought that Jupiter acted like a Solar System Sweep and also protect as from many impacts, but I will have to re-visit my pre-conceptions.

Re-thinking about it, I guess we could just "count" the collisions without having to watch the whole animation (which would take hours, a little time-step would be advisable) by carefully measuring the mass of the earth at the end of each run.

I am not proficient enough with the xml editing to create the system myself (it should have several dozens "menacing asteroids", perhaps hundreds) but probably any of you SU-power users can do it for us.

Regards

Osezno_

Chaotic Cow · « **Reply #23 on:** February 28, 2009, 07:05:33 AM »

I would love for someone to make a system where multiple objects come at the earth.

I wish I knew XML.

Dan Dixon · « **Reply #24 on:** March 03, 2009, 04:38:30 PM »

Quote from: Chaotic Cow on February 28, 2009, 07:05:33 AM

I would love for someone to make a system where multiple objects come at the earth.

Could you describe (in more detail) the system that you'd like to see?

Chaotic Cow · « **Reply #25 on:** March 03, 2009, 08:45:11 PM »

Pretty much the Moon and Earth System but with a barrage of asteroids...maybe have them pass near the moon at different speeds.

Probably could be done manually but I would like to watch it just all unfold. xD