Asteroid hits moon → moon hits Earth...possible?

Count Iblis

Many gravitons, not photons, must be emitted. Classically, a system of two rotating bodies will emit continuous gravitational radiation. If you look at it more precisely (i.e. quantum mechanically), you see that the system makes jumps to lower energy states emitting gravitons. Selection rules apply to matrix elements <a|H|b> where H is the effective Hamiltonian of the gravitatonal field in second quantization. But to higher order in peturbation theory the amplitude for the transition probability contains terms

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

corresponding to emission of many gravitons....

JustVisiting

Please humor my curiosity.
Has anyone examined the concept of diverting a near-Earth asteroid towards the Moon? Would such an effort alter the orbital path of the Moon? It would seem to be easier to do, since the gravitational pull from the Moon would assist at some point.

russ_watters

Welcome to PF. Except as you just did, with a speculative hypothetical, no. 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!

Researcher X

I wonder how fast Ceres would have to go to do it (being the largest asteroid/planetoid available), not that it would ever do so (mad scientist?)...

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 destroyed by an impact long before it was harsh enough to perturb its orbit significantly. Doesn't make sense to me.

Chronos

That would be a whopper of an asteroid, much larger than any known asteroid in our solar system. A smaller asteroid would need to travel very fast to knock the moon out of orbit. So fast that it would blast chunks out of the moon, which would pretty much spoil the whole effect.

Vanadium 50

You can't compare linear and angular momentum. They have different units.

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.

D H

In fact, Ceres hitting the Moon head-on at 80 km/s (Mach 230) (Earth-relative velocity) will do the trick. What would give Ceres a velocity of 80 km/s is of course a different trick.

Count Iblis

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.

D H

With respect to what body? Earth escape velocity at the Moon's mean orbital distance is 1.4 km/s. Solar system escape velocity at 1AU is 42.1 km/s. Add them together and you get 43.5 km/s, not 72.

Count Iblis

42 km/s plus 30 km/s of Earth in its orbit around the Sun.

Count Iblis

To see what would happen if Ceres were to collide with Earth with such speeds, see here:

http://www.lpl.arizona.edu/impacteffects/

Sir Pwn4lot

It's a superficial question since the chance of an asteroid of sufficient mass to have such an effect colliding with the Earth within the next few generations is almost negligible, the chance of one hitting the moon is astronomically minute.

Besides, for those who watch Stargate, you could make the moon simply go faster than the speed of light and go THROUGH the earth, I loved that ep lol.

I'd be more concerned about religious fanatics like Bush or Palin getting control of the launch codes and blowing us all to hell.......

ray b

would not any impact of that force level shatter both our moon and any object that hit it
in to dust and fairly small chunks

crossinggo

Fascinating thread! Revisiting a small piece of the discussion, could a large enough object pass by the moon without striking it, having the effect of destabilizing the moon's orbit around Earth? So that eventually it would spiral into the Earth, even over a long period of time? Allowing that the gravitational effects might ravage Earth well before the impact ever happened.

And if that is possible despite being incredibly unlikely, would such a fly-by that could disrupt the moon's orbit also create havoc on the Earth (from our point of view) so we might not be around when the moon finally smashed us?

DaveC426913

Sure it could.

DaveC426913

Speaking of near-misses...

Anyone hear about the 7metre asteroid that passed within 14,000km of Earth last week?

Yes, 14,000km. Twenty times closer than the Moon.

Skywatchers spotted it only days before closest approach.

qraal

Hi All

Of course the Moon could suddenly get a lot closer by losing orbit energy due to a flyby of a rogue Mercury. I read this rather disturbing paper that was just posted on the arXiv...

An Overview of the 13:8 Mean Motion Resonance between Venus and Earth

...which describes very rapid (~Myr) scenarios in which the eccentricity of the planet Mercury is chaotically amplified, sending it careening past the orbits of Venus and Earth. Venus's inclination needs to increase to only 7^o for Mercury to start getting pumped up in eccentricity.

Makes me wonder just where the planets were a few aeons ago. Could the "Faint Young Sun" paradox be explained by a more eccentric orbit?