# Odd question

1. Dec 6, 2011

### MaverickBlue

How big would an asteroid or comet need to be to say, disturb the moons orbit, either by impact or gravitational attraction? And would this significantly alter the chances of "Death From Above"? :)

2. Dec 7, 2011

### Drakkith

Staff Emeritus
To do more than change things a tiny amount it would have to be a sizeable fraction of the moons mass and pass very close to it. Probably much closer than the Moon is to the Earth.

3. Dec 7, 2011

### DaveC426913

No.

The way you phrased your question suggests you're thinking of Death from Above via the Moon.

It would have to make a whopping change to the velocity of the Moon for it to collide with Earth. You would almost have to bring it to a halt. Anything less than that would merely put it in an eccentric orbit with a low perihelion. Of course, that would still cause quakes and the like...

No asteroid of any size could do that (since the Moon's momentum is far more than any asteroid).

4. Dec 7, 2011

### MaverickBlue

By "death from above" I basically mean any object hitting the earth, either directly, or or by changing the moons orbit. So if the moons orbit were disturbed, and I don't just mean "big rock smashes moon which causes moon to smack into earth like a billiard ball", are you saying that there's virtually no chance the moon's orbit would become irregular, thus leading to a possible eventual collision with the earth? It seems feasible to me that this could hypothetically happen, though the feasibility of a large enough object moseying along is questionable to me(as in, are there any objects this large wondering around the solar system), hence why I asked. :)

5. Dec 7, 2011

### DaveC426913

Well any object could mean the asteroid. Certainly an encounter with the Moon could cause that.

It is not ruled out, but it would be very, very difficult to contrive the circumstances for it to happen.

6. Dec 7, 2011

### Drakkith

Staff Emeritus
It is 100% possible if the right circumstances take place. I think our observations of the solar system have accounted for all objects of sufficient mass to do this however.

7. Dec 8, 2011

### MaverickBlue

Just how big would an object need to be exactly?

8. Dec 8, 2011

### DaveC426913

It's not as simple as that. An object of mass great enough to cause the Moon to collide with Earth would also be large enough (and close enough) to
- cause global quakes
- pelt Earth with extinction-sized debris as the object broke up, and
- alter Earth's orbit.

9. Dec 8, 2011

### Drakkith

Staff Emeritus
There isn't an exact amount. It depends on how close it gets to the moon, how fast it's traveling, and a myriad of other things. Let's just call it: Really freaking big.