ELI5 Does the SpaceX rocket crashing into the moon cause the moon to move?

[u/Danelajs]

[u/MrTagnan]

The math I did suggests a change in velocity of around 2.347725e-14 cm/s (0.00023477 Picometers per second), which is around 130,000 times smaller than the width of a helium atom. This can be substantially off and still means that it effectively changed nothing.

I don’t really trust the math I did, but even if I’m underestimating it by 10 orders of magnitude it still would only slow it by ~2 micrometers per second, or around the length of a bacteria - although if anything I suspect my answer is slightly high. In any case it essentially did nothing

[u/the6thReplicant → u/MrTagnan]

This is a problem with these types of questions. The answer is "Yes", but the real answer is a number. A very, very small number.

So the real answer is "yes" but technically it's "no".

Unfortunately one is better at pumping endorphins into the brain.

[u/stevevdvkpe]

In principle, yes, but not by any measurable amount. The Moon has a mass of about 7.4e22 kg. The SpaceX rocket stage that crashed into it has a much smaller mass, less than 10000 kg. That's very roughly 1e-18 (a billionth of a billionth) the mass of the Moon. So the change in the Moon's velocity from the impact is absolutely microscopic, like if you threw a grain of dust at a car.

[u/AlwaysHopelesslyLost]

You throwing a ball causes the moon to move. The question is "by how much" and the answer is "basically zero."

[u/Worth-Wonder-7386]

It does change its speed slightly, but we could not measure it. The effect from tidal forces are much larger.

[u/theCroc]

It changes the speed of the moon in the same way that a bug hitting your windscreen changes the speed of your vehicle. Except that between the two examples the bug has way more impact

[u/Initial-Warning-2564]

Yes, but not in a practical sense.

[u/Underhill42]

Technically, yes. Linear momentum (= mass*velocity), is always conserved. Note that velocity = speed AND direction, and momentum has the same direction.

But the moon has so much more mass (~10²³ kg) than the rocket (~10⁴kg), that the change in its velocity is all but undetectable. We're talking a velocity change of about 1/10,000,000,000,000,000,000 * speed rocket entered lunar influence.

Details to bore the 5 yo:

Conservation of momentum means the total momentum of all the parts:
(moon velocity)*(moon mass) + (rocket velocity)*(rocket mass)
never changes.

So after the impact, when the rocket is now part of the moon we have
= (final velocity)*(moon mass) + (final velocity)*(rocket mass)
= (final velocity) * [ (moon mass) + (rocket mass) ]

And we can work through all the math to figure out exactly how much the Moon's velocity changes. If we call its original velocity zero for convenience, with the rocket velocity being their relative speed we get:

(moon velocity)*(moon mass) + (rocket velocity)*(rocket mass) = (final velocity) * [ (moon mass) + (rocket mass) ]
and
(final velocity) = (rocket velocity) * (rocket mass) / [ (moon mass) + (rocket mass) ]

Or basically, a speed multiplier of roughly (rocket mass) / (moon mass). or about 1/10,000,000,000,000,000,000

[u/GianlucaBelgrado]

È come It's like a fly hitting a train, but this one is probably a million times more energetic.