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So is such a scenario even possible? Could an asteroid collision with the moon send it out of orbit and careening into us? Would such a collision be a global-killer?

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So is such a scenario even possible? Could an asteroid collision with the moon send it out of orbit and careening into us? Would such a collision be a global-killer?

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You can rest easy as the asteroid would have to have momentum equal to that of the Moon's orbital momentum. Since the Moon is many orders of magnitude more massive than the largest near-Earth object, the asteroid would have to have a relative velocity many orders of magnitude larger than the Moon's orbital velocity. Such an asteroid would escape the solar system.

So what about some maurauder from outside the solar system? We would see it coming. If it were big enough we would not be able to do a thing about it. Such is the stuff of multiple science fiction stories ...

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Chris Hillman

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Of course, if an asteroid or meteor strikes the Moon, it might well happen that a *small piece* of the Moon might be ejected and eventually happen to strike the Earth. Similarly for Mars (indeed, certain rare meteorites are thought to have originated in just this fashion.)

But if you want to have more realistic nightmares, dream about something like the next global pandemic. Or dying of thirst. Or the economy crashing. Or a combination. That chances of a major asteroid strike occuring in your lifetime are extremely remote.

But if you want to have more realistic nightmares, dream about something like the next global pandemic. Or dying of thirst. Or the economy crashing. Or a combination. That chances of a major asteroid strike occuring in your lifetime are extremely remote.

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Astronuc

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Geez - I saw "Asteroid hits moon " and thought - wow what an event. Then when the thread opened, I saw that it was a hypothetical.

The consequences of an asteroid/Moon collision of course depend on the size of the asteroid. It would certainly make a crater, which is how the craters got made way back when. The would be some ejecta coming out of the impact area.

That would make an interesting calculation. Take the moon and Earth released from rest at their nominal distance and determine how long it would take for them to come together. Acceleration is not constant.D H said:It would still take days for the Moon to plummet to the Earth.

The consequences of an asteroid/Moon collision of course depend on the size of the asteroid. It would certainly make a crater, which is how the craters got made way back when. The would be some ejecta coming out of the impact area.

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tony873004

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5 days 6 hours 6 minutes from apogeeThat would make an interesting calculation. Take the moon and Earth released from rest at their nominal distance and determine how long it would take for them to come together. Acceleration is not constant.

4 days 19 hours 19 minutes from the Moon's average distance

4 days 7 hours 44 minutes from perigee

This assumes an initial velocity of 0. Of course, the asteroid could knock the Moon towards or away as well yielding an initial radial velocity other than 0.

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I wrote a simple velocity verlet integrator in Excel. Nothing much of interest happens for the first day. After one day, the Earth-Moon separation drops by a mere 2.6% and the relative velocity reaches 0.24 km/sec. It takes 3.3 days for the velocity to climb to 1 km/sec, at which time the Earth-Moon separation has dropped by nearly a third. Things get interesting on the final day. After 4.8 days or so, the Earth and Moon finally collide with an impact velocity of about 9 kilometers per second.That would make an interesting calculation. Take the moon and Earth released from rest at their nominal distance and determine how long it would take for them to come together. Acceleration is not constant.

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Just out of curiosity, how did you calculate the time it would take to impact?I wrote a simple velocity verlet integrator in Excel. Nothing much of interest happens for the first day. After one day, the Earth-Moon separation drops by a mere 2.6% and the relative velocity reaches 0.24 km/sec. It takes 3.3 days for the velocity to climb to 1 km/sec, at which time the Earth-Moon separation has dropped by nearly a third. Things get interesting on the final day. After 4.8 days or so, the Earth and Moon finally collide with an impact velocity of about 9 kilometers per second.

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I used numerical integration techniques, specifically a velocity Verlet integrator.Just out of curiosity, how did you calculate the time it would take to impact?

Herein I place an upper bound on the time. For simplicity, assume the initial lunar orbit is circular with an orbital radius of [itex]a[/itex] and period [itex]T[/itex]. Now suppose the Earth and Moon are point masses rather than solid masses. I don't have to worry about the actual collision with this assumption. If all but a tiny fraction of the Moon's initial tangential velocity is canceled at some point in its original circular orbit, the point mass Moon and Earth will now orbit each other in a highly elliptical orbit with some semi-major axis [itex]a_e[/itex]. By Kepler's third law, the period of this new orbit will be [itex]T_e = T\;(a_e/a)^{3/2}[/itex]. The time from apogee to perigee is half the orbital period. If a collision between the real Earth and real Moon occurs, it will be sometime before this half period interval: [itex]T_c < T_e/2 = 1/2\;(a_e/a)^{3/2}\;T[/itex].

In the limit that the tangential velocity becomes zero, the semi-major axis of this new orbit is simply half the semi-major axis of the original orbit. Thus

[tex]T_c < \left(\frac1 2\right)^{5/2} T[/tex]

Since the moon's orbital period is 27.32 days, the collision will occur in less than 4.83 days. Note that this agrees with the result obtained by numerical integration. The upper bound is very close to the collision time as the Moon's velocity is awfully high at the time of collision and would grow even higher if only the Moon and Earth were point masses.

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Thanks for that explanation. I appreciate it.I used numerical integration techniques, specifically a velocity Verlet integrator.

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But if the same asteroid hit the earth, it would wipe us out no matter what the angle was. And the earth is a much bigger target. And even a comparatively much smaller (and slower) asteroid would still be enough to do this.

So rest assured that it is far more likely that civilisation will end due to a direct impact (with little warning at all since, realistically, it just isn't easy to spot an asteroids at long distances) rather than by some improbable trick-shot that gives you the luxury of watching the moon grow for some number of minutes.

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yes dying of thirst is bad news but it was just a thought a possible aesteroid moon collisionOf course, if an asteroid or meteor strikes the Moon, it might well happen that asmall pieceof the Moon might be ejected and eventually happen to strike the Earth. Similarly for Mars (indeed, certain rare meteorites are thought to have originated in just this fashion.)

But if you want to have more realistic nightmares, dream about something like the next global pandemic. Or dying of thirst. Or the economy crashing. Or a combination. That chances of a major asteroid strike occuring in your lifetime are extremely remote.

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The Moon could also make a spontaneous quantum jump to a lower orbit.

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DaveC426913

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Man, what would the energy of *that* released photon be?

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<a|H|b1><b1|H|b2>...<bk|H|b>

corresponding to emission of many gravitons....

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Has anyone examined the concept of diverting a near-Earth asteroid

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russ_watters

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Except as you just did, with a speculative hypothetical, no.Has anyone examined the concept of diverting a near-Earth asteroidtowardsthe Moon?

Certainly. But how much? Answer: not enough to matter. Consider that all those craters we see on the moon were caused by near-earth asteroids hitting it. And since the far side isn't shielded by earth, it looks even worse!Would such an effort alter the orbital path of the Moon?

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I looked this up, and amusingly, the momentum of ceres even at a blistering nonsensical speed of Mach 1000 (about the speed of solar wind) is less than the moon's orbital angular momentum by a factor of 10 billion, despite having 3.4 times the kinetic energy needed to scatter the moon permanently (approx 4.053e29 joules).

So, really, even if the conditions were right, it looks like the moon would be

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Chronos

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Vanadium 50

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You can't compare linear and angular momentum. They have different units.I looked this up, and amusingly, the momentum of ceres even at a blistering nonsensical speed of Mach 1000 (about the speed of solar wind) is less than the moon's orbital angular momentum by a factor of 10 billion, despite having 3.4 times the kinetic energy needed to scatter the moon permanently (approx 4.053e29 joules).

Ceres weighs about 1.2% of what the moon does. It will be difficult to make a comparison where there is a factor 10 billion difference between them.

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There can be a Ceres sized asteroid kicked out from another solar system that would enter our solar system. It could have the required velocity, because an object arriving from rest at infinity can already have a maximum relative velocity w.r.t. Earth of 72 km/s.

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