# Would an asteroid collision affect Moon's orbit, and what consequence would that have for Earth?

If an asteroid collided with the Moon, or an atomic bomb was blasted on it, would it affect the orbit of the Moon? The Moon's gravity plays a major role in ocean tides. If the Moon's orbit changed, what kind of consequences would the Earth suffer? Would the Moon remain in an altered orbit, or would it precess and hit the Earth, or even recess out of Earth's orbit?

• Have a look into nuclear pulse propulsion. It basically sets out a simple set of equations for the 'if a nuke goes off' scenario. Nov 2 '15 at 19:28
• Will it affect: YES. Will the change be detectable, nevermind troublesome: NO, Moon is big, asteroid is small. With bigger asteroid, moving the moon will still not be a worry until far past the point where debris from the impact becomes a problem. Dec 30 '21 at 9:37

The moon weighs 7.34 × 10$^{22}$ kg. For an asteroid to have a noticable effect, its mass must be in that ballpark.

• the asteroid mass needed also depends on its speed (kinetic energy).
• what timeframe do you want to consider? On a timescale of a billion years, a tiny asteroid will have measurable effects: the change in orbit will be tiny, but over millions of orbits that change will add up.

You'd need a pretty big asteroid to produce effects that are noticable in the short term. An asteroid big enough to affect the Moon's orbit would break off large chunks of Moon, which would result in a meteor bombardment on Earth (see Neal Stephenson's novel Seveneves for an example of that), so we'd have more immediate concerns than its influence on tides.

A bomb can displace ~100 tons of soil per ton of TNT. So detonating a 1 MT nuclear bomb would displace (optimistically, rock will be harder to move than soil) 100 MT or 10$^{11}$ kg of rock, or a fraction of 10$^{-11}$ of the moon's mass. Also, most of that mass will stay on the Moon, so the total gravitational pull of the Moon will remain the same. The blast will exert some thrust on the Moon, so you could alter its orbit. But again that effect will be tiny, just going by the orders of magnitude we're talking about here.

What would happen to the Moon really depends on the details: a retrograde impact slows the moon down and widens its orbit, a prograde impact speeds it up and tightens its orbit. To get the Moon to collide with Earth , I'd expect you need an asteroid the size of the Moon, or lots of patience.

As it is, the Moon recedes slowly: the tides rob orbital energy from the Moon. Small effects like detonating a bomb would be swamped by this effect.

A wider orbit means smaller tides.

• It would be nice if you could include in your answer the necessary velocirty and mass (or size, considering a certain density) of an asteroid that could have such an inlfuence. Nov 1 '15 at 18:24
• This answer doesn't seem to really go into the base of the question. Yes you can move soil, but if the lunar orbit changes is really based on the momentum exchange. Nov 2 '15 at 19:35
• I think you've got the speed versus orbital radius relationship the wrong way round. If an orbiting body loses kinetic energy it falls into a lower (tighter) orbit. If it gains energy, it goes "up" into a higher (wider) orbit. Oct 12 '20 at 10:06

Strangely enough, an asteroid missing the moon by a small distance might actually deflect the moon by more than a collision!

When a small body moves past a larger body, it's direction is changed by 100%, meaning that its momentum has been changed by almost double the original falling momentum. This momentum must be conserved, so the moon must gain twice the momentum of the falling asteroid.

You can think of it as the same effect as if the asteroid was made of rubber and bounced off the moon, as compared to being made of putty (or rock) and sticking to it. A putty (or rock) asteroid collision transfers its momentum, but loses kinetic energy in the form of deformation, heat, shock. An asteroid that bounces off without expending energy in the collision contributes more energy to the moon's motion.