[ibrow]

There's a thought. What would happen, hypothetically speaking, if an asteroid large enough were to hit the moon and spin it out of its orbit? What would happen to life here on Earth? Obviously tides would be affected, and I am assuming that this in itself would probably affect a lot of animals, fish, birds etc?

[derobert]

I'm pretty sure that said hypothetical impact can't exist. The object either has far too little energy to significantly change its orbit, or alternatively has enough energy but there is nothing moon-like left after that collision.

The moon has a mass of 7.349×10²² kg. Its mean orbital velocity is 1023m/s. So, with a perfectly-aligned impact, and perfect efficiency, to add velocity, you'd need to add energy equal to difference in kinetic energy. For 1m/s, that'd be 10²⁵J (assuming the change in mass is negligible). That's an incredible amount of energy, about 20 times greater than the Chicxulub impact (believed to have triggered the mass extinctions at the K-T boundary).

That's 1 m/s. You need far more than that. Earth escape velocity is 11200m/s. So, that would seem to need 10³⁰ J. And an impact would deliver that in well under a minute. Which would be a problem, since that's an order of magnitude greater than the gravitational binding energy of the moon.

Conclusion: impact required would fully obliterate moon.

(Moon mass and velocity, and escape velocity from http://nssdc.gsfc.nasa.gov/planetary/factsheet/moonfact.html. Gravitational binding energy and estimate of Chicxulub from Wolfram Alpha)

[Achshar]

well moon moving out of its trajectory means alot more than tides (which in itself is pretty darn serious) IMO. The nights would be absolutely dark, also earth's rotation speed (AFAIK) is also affected by moon's gravity and bulges in both earth and moon's softer crust. Plus maybe we can adapt to such changes but our culture will change and it will be alot more difficult to live through that, imagine explaining to your kids/grandchildren what moon is (or rather was) and how it looked when it was closer/farther.

PS if moon gets a major hit, its highly unlikely earth stays 'physically' unaffected. The aftermath of the collision would leave numerous chunks of huge rocks in earth's near space and it would be almost certain that some of then then hit earth. And the effect of decent sized rock hitting the planet is better known to our long lost friends, the dinosaurs :)

[ibrow]

Thanks for that. Quite a thought. Hopefully this will never happen - am assuming that the Earth has more chance of being hit than the moon (the moon being smaller and faster)?

But would the nights be totally dark? I've experienced a lot of nights out on a boat on a moonless night and the stars provide a (relatively) large amount of light on their own.

[T-hawk]

The Moon isn't faster. The velocities you care about for Solar System impacts are sun-relative. The Earth and Moon have essentially the same velocity relative to the sun, since they orbit it together. The Moon moves a bit slower at new phase and faster at full phase because its Earth-relative velocity subtracts from or adds to its Sun-relative velocity, but the former is 1 km/sec and the latter is 30 km/sec, small potatoes.

As for who gets hit; Earth's mass is 81 times that of the Moon so it attracts objects 81 times as strongly. Really, the Moon would only get hit if Earth already pulled an object into a collision course or near-miss and the Moon just happened to get in the way at the right time. Earth has 4x the diameter so presents 16x the cross-sectional target that the Moon does.

[Achshar]

hmm well they do give some light but it would not be nearly as bright as our beloved moon :)

[nitrogen]

An asteroid that could move the moon would have to be moving insanely fast, or be close to the size of the moon (due to conservation of momentum).