# What kind of event, if any, would knock the moon off its orbit, without destroying it?

I am recalling the Space:1999 nuclear waste containment explosion; this has been criticized because such an explosion would have actually destroyed the moon. I am curious for events that could happen directly on the moon, or elsewhere, and the natural effects that would impact the moon as well.

Also, I know about the moon orbit slowly spiralling away from Earth, but I am asking about some more specific event in time.

EDIT: I am not directly interested in effects of such event on Earth, actually I am not interested in Earth, if not because playing a role in the event that lets the moon go off orbit. My interest is in a feasible event that could generate that scenario, and the repercussions for the moon itself.

• "Simple! Change the gravitational constant of the universe!" Commented Oct 10, 2014 at 9:20
• a close pass of a heavy object (though that would also wreak havoc on the rest of the system) Commented Oct 10, 2014 at 9:20
• if TV is to be trusted, the explosion of a sizeable Uranium confinment area should move the moon at a very high speed... Commented Oct 10, 2014 at 11:08
• I haven't seen Space:1999. How far do you want the moon to get off its orbit and in what direction? Just change it a bit? Crash it into Earth? Get it on escape velocity from Earth? Commented Oct 10, 2014 at 12:46
• Not realistic, but also an interesting scenario: what-if.xkcd.com/13 Commented Oct 10, 2014 at 14:41

## Requirements

The speed needed to escape the Earth, from the Moon's distance, is given by:

$$ve = \sqrt{ \frac{2 \cdot G \cdot M}{a} }$$

where $M$ is the mass of the earth, and $a$ is the distance from the Earth to the Moon (the moon's semimajor axis). The speed of an object in a circular orbit around the Earth, at the Moon's distance, is:

$$vo = \sqrt { \frac {G \cdot (M+m)}{a} }$$

where $m$ is the Moon's mass. This can be expressed in Earth-masses as $0.012 \cdot M$. The easiest way (requiring the least energy) to get the Moon out of Earth's orbit is to give it a push in the prograde direction (i.e. in the direction it is already going). But how big is this push? Let's determine what the escape velocity is in terms of the Moon's current velocity:

$$\frac{ve}{vo} = \frac{\sqrt{\frac{2 \cdot G \cdot M}{a}}}{ \sqrt { \frac {G \cdot (M+m)}{a} }} = \sqrt{\frac{2}{1.012}} = 1.405$$

This tells us that, at a minimum, we need to add a bit more than 40% of the Moon's current orbital speed in order to knock it out of orbit. Since the Moon's average orbital speed is about $1.023 \text{ km}/\text{s}$, we need to create a change in velocity of about $dV = 414 \text{ m}/\text{s}$. As far as velocity changes in space go, that's not a lot, but due to the huge mass of the Moon ($7.35 \times 10^{22} \text{ kg}$), that's a lot of momentum to transfer.

Note: I intend to come back later and add some additional computations regarding the rocket equation and collisions, but this should provide a starting point.

• Nice answer. What about the spin instead? Would applying an angular momentum be feasible? Commented Oct 13, 2014 at 7:51
• You mean to spin the moon faster? I'm not sure how that would help. Commented Oct 13, 2014 at 14:13
• @CalebHines I edited your post to use Mathjax. Let me know if there's a problem, or simply roll back the edit yourself. The edit is currently awaiting review.... Commented Oct 21, 2014 at 22:28
• As a point of curiosity: the moon is already on an escape vector. It's not going very quickly, but it's going. While it will still take a goodly chunk of energy to make it leave sooner than a billion years, any energy will speed its departure. Commented Dec 2, 2015 at 17:12

There aren't many options. Even if you completely shattered the moon then the remains would continue on a merry orbit and most likely turn into a ring.

The most likely scenario would be a rogue body (a reasonably large planet, neutron star, black hole, etc) passing through the solar system. If that passed close enough it could massively disrupt the orbits of any body it passes. Capturing the moon away from the earth, the earth away from the moon, or even just splitting them up and sending them both careening away across the solar system.

The problem with this though is that it would almost certainly have a huge effect on the earth as well. The close passage of the heavy body would at a minimum cause massive tides and interesting weather systems. The force that separates the earth and the moon could also easily send us into an orbit not very hospitable to life or even if the other body was massive enough break us out of orbit around the sun entirely.

It would also disrupt the orbit of all the other planets in the solar system to varying degrees, it would definitely make for interesting times!

Beyond that a hypothetical FTL drive attached to either the earth or the moon could cause it one to fly away from the other. Equally an incredibly powerful non-FTL drive could over time have the same effect.

• what about an impact with a reasonably sized comet? Commented Oct 10, 2014 at 9:25
• A comet wouldn't be big enough. You'd need something with a substantial proportion of the Moon's mass. All the craters on the Moon didn't cause it to fly away after all - and some of them are pretty big :) Commented Oct 10, 2014 at 9:27
• @guido if you fine-tune it (including size, as in, make it massive), maybe, but it's unlikely it'll be the same place after the impact. Commented Oct 10, 2014 at 9:27
• @draco18s actually, that's not how orbital dynamics works. You need to hit it from behind to make it move out. Commented Dec 2, 2015 at 18:06
• My mistake. Either way, some vectors are going to be more efficient than others, making the impact vector at least as important as the imparted energy. Commented Dec 2, 2015 at 21:15

Hit it with a rock. A big rock.

Something like Ceres might do, if you could somehow get it into an orbit that hits the moon with sufficient relative velocity. Alas, moving Ceres significantly from its current orbit is likely itself a non-trivial task.

A stray Kuiper belt object might be more practical, if only because there are more sufficiently large bodies out there, and also because the long fall from the Kuiper belt to the inner system would naturally give the impactor a highly eccentric orbit that could intersect the Moon at a sharp angle and high velocity difference.

You'd still have the problem of getting the object to the inner system in the first place, but I could buy a scenario where a collision (or a near-miss) with another KBO sends the would-be impactor on an unstable orbit leading to an eventual close encounter with Neptune, which, with some good (or bad, depending on how you look at it) luck, might send it towards the inner system and an eventual collision with the Moon.

Of course, you could go further afield and have the object come in from the Oort cloud, or even from interstellar space. Most solar system formation models predict a large number of small planetesimals getting scattered out of the system when it forms, so it stands to reason that there must be a sizeable population of stray planets out there in interstellar space, and that they'll occasionally make a near pass to a star such as the Sun.

Of course, such encounters are (fortunately) not that common, and most such bodies will just pass through the solar system without hitting anything anyway, but having one fall in and hit the Moon is still perfectly within the realm of possibility. As a bonus, a stray planetesimal could potentially fall in from any direction, even well away from the plane of the ecliptic, which could let you get some quite interesting orbital changes when it hits.

In any case, a body smaller than the Moon, passing the Earth at the Moon's distance, isn't going to directly disrupt the Earth to any significant extent (unless you count making a lot of astronomers soil their underwear when they first spot it). Any tidal effects will, by definition, be smaller or comparable to the lunar and solar tides the Earth already experiences, and any gravitational effects on the Earth's orbit should be negligible.

The bad news, however, is that anything massive hitting the Moon at high speed is going to scatter off lots of smaller rocks when it hits, some of which will likely hit the Earth. So Earth as a whole might be fine, but you'd likely be looking at some rather big meteor impacts as secondary effects, potentially disrupting the biosphere and any civilization down here. Worse yet, since the scattering from the lunar impact is likely to be rather chaotic and unpredictable, we won't be able to easily predict how many secondary impacts might hit the Earth, or when and where they would hit.

There is very little that could affect the orbit of the moon without also having a direct influence on the earth. A massive body with enough gravity to yank the moon away, would also have similar effects on the earth's orbit, not to mention those of us on it.

But let's say the moon can get pulled away by little green repo men... The moon is pretty damn important for life on earth and here are some of the effects we would see:

1. The moon is partially responsible for the ocean's tides. Without the moon's gravity pulling on it, the tides would change drastically.

2. The moon's gravity slows down the rotation of the earth. Without it, the earth would start spinning faster and our days would get shorter.

3. The moon stabilizes the earth's tilt. The earth's tilt varies by a small amount, and its changes are hardly noticeable in the short term. Without it, the earth "wobble" more with a tilt varying 10x more greatly than is current. Seasons would become more extreme.

The tougher part of this discussion is the fact that effects have countless other effects. Weather is the most chaotic and complicated system to put into the mix. All I can really say is that it would get really messed up.

• Thank you for this, but I am not interested in the effects of such event on the Earth; I am just interested in a feasible way to let the moon go, and the effects on the moon Commented Oct 10, 2014 at 14:23
• in your questions: "and the natural effects that would impact Earth as well." Commented Oct 10, 2014 at 14:25
• sorry it was a typo! Commented Oct 10, 2014 at 14:26
• it happens :) Glad I didn't bother to add what the effects would be on werewolves... Commented Oct 10, 2014 at 14:28
• Actually it wouldn't start spinning faster - it would just stop slowing down. Commented Oct 10, 2014 at 19:27

Turn the moon into a rocket.

There is a method of propelling a spacecraft called laser ablation. It involves vaporizing part of the spacecraft with a laser, the vaporized mass propels the spacecraft. A sufficiently energetic event that's strong enough to vaporize one side of the moon's crust would leave one side of the crust as a mass of plasma that gradually escapes, propelling the moon into a different orbit.

Since the moon is tidally locked, vaporizing the side of the moon opposite to its direction of movement would constantly propel the moon into a higher orbit, eventually escaping entirely. It would likely eventually settle into either an extremely elliptical orbit around the sun, or it would escape to deep space.

This would leave the moon mostly intact, however, it would likely contain a large crater (on the order of magnitude as the entire moon) that would gradually fill in, fracturing the surface of the moon.

• Heh, this seems like the XKCD link in the comments on the question Commented Oct 29, 2014 at 19:24
• @Twelfth that's kinda what I was thinking here, but wanted to put it into a post instead of just linking to something. Commented Oct 30, 2014 at 12:23
• I think XKCD is actually missing a component you have here...the energy from these lasers should vaporize part of the moon (or atleast warm it pretty quickly). The result there should be the 'rocket' effect you describe here to some degree Commented Oct 30, 2014 at 16:40

Well you could fire small black holes at it, or transform its mass into small black holes and use their decay as an engine to move it. http://www.einstein-online.info/elementary/quantum/evaporating_bh

This is intended as an addendum to the answer which stated that it would be exceedingly difficult.

Factoid, the Moon is in orbit around the Sun. The Moon and Earth happen to share this orbit. The Sun's gravitational attraction for the Moon is more than 2x the Earth's gravitational attraction for it. So you not only need to move the Moon away from the Earth, you actually need to change its solar orbit too.

This is one tall order.

About the only method I can think of that has a chance of doing this without disrupting the Moon would be by performing MANY gravitational slingshot maneuvers (momentum exchanges) with small asteroid bodies. On one end, you'll sling shot these bodies past Jupiter and the other will sling shot past the Moon. You'll essentially be exchanging momentum between Jupiter and the Moon.

Be prepared to wait a while. I'd expect this to take a minimum of many hundreds of years or perhaps thousands.

"Escape velocity" is the present orbital condition of the moon. Gravitational pull between the earth and the moon has caused a round orbit to stretch into an elliptical orbit. and this elliptical orbit stretches more and more as the relationship is prolonged. The moon on the apex of its attempted escape either has or has not the velocity to leave the earth's gravitational pull. If it has not then it is pulled back for another pass. This process will continue and as the moon gains velocity it also increases ellipse of its orbit around earth, and this process will also continue until on of four possible outcomes happen:

• the elliptical orbit of the moon stretches to a point that the moon will pass to closely thereby striking the earth;
• it reaches escape velocity thereby leaving orbit;
• it and earth both change orbits because of the force of gravity pulling; between them which forces both bodies into a changed orbit around the sun; or
• the moon collides with earth and both bodies fracture into pieces.
• the moon and the earth orbit each other in a sling of gravity, both orbiting the sun in this sling Commented Sep 27, 2015 at 10:45
• sorry to be a hog, thanks for having me. Commented Sep 27, 2015 at 10:46
• Note that the question specifically states "I know about the moon orbit slowly spiralling away from Earth, but I am asking about some more specific event in time." Commented Sep 27, 2015 at 11:27

Electromagnetic anomalies that weaken the gravitational pull between the earth and the moon to such a state that escape velocity is made possible at much lower speed and with less inertia then being necessary to be freed from the gravitational relationship that binds them.

These "anomalies" are presently being produced all over earth by artificial means and the definition could be expanded to include any man made device that produces an electric field or magnetic force, or the combined total effect of all these artificially produced electromagnetic fields or forces, and not limited to only man made devices, yet also to include the electromagnetic and gravitational forces of galactic origin exerting their combined and constantly changing effects on other celestial systems,including our own, these effects combined also with the sum total of all artificial sources produced on and around earth, some of which would include;

very strong production of electromagnetic fields involved in particle collision projects, and possibly even some particle manipulations themselves and the molecules, molecular effects and or changes to molecular processes that have been produced by them both intentionally and unintentionally.

the entire cumulative effect of energy production in every nation on earth to harness electrical power and transmission of such, including nuclear processes.

the total and cumulative effect of the transportation technologies on earth, both fueled, and electric, and possibly even the effect of movement of mass amounts of cars, trains, planes, etc. in many places and with constant motion.

the total, combined, and cumulative effect of the communication technologies on earth, along with all the devices that make communication possible and the transmission of communication by various energies, such as microwaves, lasers, and electrical impulses.

....the list of "anomalies" is ever expanding from many origins, on and off this planet.....

it's best summed up by this description....a rock, possibly a very big one, thrown in a pool of water makes ripples, these ripples when encountering objects in the water cause other ripples, and these ripples then influence the ripples around them until soon there are ripples accumulating in every direction changing all the othe ripples as well as themselves being changed by other ripples, and where some of these ripples meet there are momentary "anomalies" making a small splash instead of a ripple.

....these "anomalies" effect change of a previous state of existing.