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

Question

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 experience? Would the Moon remain in an altered orbit, or would it precess and hit the Earth, or even recess out of Earth's orbit?

Answer 1

The Moon's mass is 7.34 × 10$^{22}$ kg. For an asteroid to have a noticeable effect, its mass must be in that ballpark.

• the asteroid mass needed also depends on its speed (kinetic energy).
• what time frame 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.

A very big asteroid would be needed to produce effects that would be noticeable 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 would stay on the Moon, so the total gravitational pull of the Moon would remain the same. The blast would exert some thrust on the Moon, so its orbit could be altered. But again that effect would be tiny, just going by the orders of magnitude we're considering 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 an asteroid the size of the Moon would be needed, 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.

Answer 2

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.

*I should say "up to 100%", because 100% is only if the asteroid starts the interaction with negligible relative speed. I'm talking about the optimal case, here.

Answer 3

A study in 2016 estimated that the diameter of the impactor creating Mare Imbrium on the moon was around 250 km, or 150 miles. There are only a few asteroids that big. The moon's diameter is almost 3500 km, more than 14 times as big. That's over 2500 times the volume (because the volume is proportional to the cube of the diameter). Larger rocky objects tend to be more dense, so it's reasonable to think the moon would have 2500 times the mass, or more. So this is a scenario of an extremely beefy asteroid (protoplanet-scale) hitting the moon. What would happen to the moon's orbit? A little, but not so much. With the impactor having 1/2500 the mass, the change in velocity of the moon would be 1/2500 the incoming velocity. (I am ignoring the mass of the impactor in estimating the combined momentum since it is so small.) A plausible high end estimate for the impactor velocity would be 50 km/s, so the change to the moon's velocity would be on the order of 50/2500 = 0.02 km/s. On the one hand, that's rather impressive, but the moon's orbital velocity around the earth is 50 times bigger. If this back-of-the-envelope calculation is correct, the ancients would have easily noticed the change in the length of the month, but I think the change in the orbital radius would be too small to see.

Of course, if that impact had happened while humans were around instead of billions of years ago, the humans of the day would have noticed a lot more about the moon than the length of the month changing.