# Could you fall off a planet if it was being accelerated by engines?

## Some background

In the movie The Wandering Earth, the rotation of Earth is slowed to a stop, and then 10000 engines are strapped to one side of the planet and used to accelerate it out of the solar system. The plot also involves slingshotting around Jupiter, but this doesn't really matter for the purposes of this question.

## Other research

I found a somewhat related question on Astronomy, however this deals with acceleration due to gravity. In essence it states that if a planet accelerates due to gravitational forces (e.g. an eccentric orbit around a star), everyone and everything on the planet is also being accelerated by the same source and thus the acceleration wouldn't seem to affect anything.

I considered asking this question on Astronomy, however the question asking guide there seems to indicate that any hypothetical scenarios should be asked here. Similarly the Movies SE site doesn't seem like an appropriate place to ask this type of question.

## My Question

If a planet was accelerating due to engines on its surface, people on the planet should (in theory) be able to feel that acceleration, since they aren't being pushed by the engines, only the planet they're on is. I would imagine that if the acceleration wasn't significant it wouldn't really be noticeable, however if this acceleration was high enough (e.g. more than 1G) then would the people on the rear side of the moving planet actually fall off? Would the atmosphere of the planet counteract some of these forces in any way or would that be stripped off the planet as well?

## Notes

This question isn't about the mechanics or realism of strapping engines to a planet and moving it. I looked up a few similar questions on the site (e.g. here and here), and although they do highlight some of the possible results of doing this to a planet, they don't seem to deal with the consequences of such acceleration on everything on the planet.

• We usually advise to wait at least 24 hours before accepting an answer, to increase the chances of having good answers. Unsolved questions attract more attention, and we have users all around the world, thus a rough 1/3 of the users pool is now sleeping. – L.Dutch - Reinstate Monica Jun 27 '19 at 9:05
• @L.Dutch So should I un-accept the existing answer and wait till tomorrow? – Steven Mills Jun 27 '19 at 9:16
• It's your call, if you want to have a wider set of candidate answers or not. – L.Dutch - Reinstate Monica Jun 27 '19 at 9:18
• The only problem is, Wandering Earth used some fast-and-loose stats and description of the engines. If it's going fast enough for a person to fall off, then so will anything loose. Like air, water, dirt, rocks, etc. And it's not clear the structure of the planet would hold together being pushed that hard even by a huge grid of engines. – puppetsock Jun 27 '19 at 17:40
• @puppetsock : also, at those scales, the Earth behaves as a liquid. Engines won't accelerate it, they will just dig themlselves into the ground. – vsz Jun 28 '19 at 4:07

# Yes

Under the equivalence principle anything on the "back" of the planet that wasn't strapped down would be left behind if you accelerated at >1g.

we ... assume the complete physical equivalence of a gravitational field and a corresponding acceleration of the reference system. — Einstein, 1907

That includes people, atmosphere, water, large chunks of the planet itself. Of course as soon as you start losing mass like that your acceleration increases as you don't need so much force to move the lower mass, gravity also decreases and you start losing ever more mass ever faster. You will quite literally tear the planet apart.

So don't do that, I quite like it here.

What your essentially doing is the same effect that allows you to pull a table cloth off quickly and leave everything behind. In this case, the table cloth is the Earth, and everything on it is us, water, mountains, everything.

But the effects of this are even worse than that.

Drag racing is the closest example I can think of. When a drag race car accelerates, the rear of the body flexes and attempts to bend underneath the front of the car. Since the metal frame is rigid and won't bend, the front lifts in response.

The Earth can't shed off the energy by lifting up. So your giant engine accelerates into the planet, digging in deep. The forward facing part of the planet turns into a mushroom shape, the edges of the mushroom falling off into space and breaking apart.

People on the forward side are crushed to the ground. They have a few minutes of air left and can crawl around and kiss their loved ones before it all comes to an end.

The people on the back side died very quickly. The huge Earth quake from half the Earth literally compressing and moving killed everyone, shortly afterwards, their dead bodies float into space and float inside a ball of atmosphere and a trail of rocks from the edges of the Earth.

The people on the sides got the most terrifying end. They were just standing there, minding their own business, when suddenly the wind grabbed them and ripped them off into space. The sheering force would rip them to shreds while they tumble through the air. It would almost feel like slow motion, people in the back would see people in front of them being torn to bits.

As the Earth breaks apart, the mass reduces and the planet moves faster and faster until only the engine itself remains. With nothing guiding it, the engine would probably orbit the sun or crash into another planet.

The remaining Earth rubble would likely come back together as another planet in another 100 million years.

• I kind of feel like the sides of the planet would split and huge volcanoes would spew death onto the nearby population before the Earth had the chance to accelerate fast enough for the wind to rip people off of the surface. – Muuski Jun 27 '19 at 20:35
• @Muuski If it is accelerating at the speed of gravity, that is 9.8 metres per second every second. The Earths crust barely has time to respond at that speed. So at that speed, the Earth leaves the air behind before it even has a chance for the ripple of destruction to reach everyone. Imagine you were an ant riding on a base ball, do you feel the bat hit the ball, or the ball flying out from under you? – Trevor Jun 27 '19 at 21:05
• The Earth isn't a baseball. It's a water balloon. It deforms before it accelerates. In fact, no object accelerates all at the same time, but flexible objects make this way more obvious. – Muuski Jun 27 '19 at 21:08
• @Muuski A baseball is actually pretty soft and flattens when hit. But my point is that the people on the sides are really far away from where everything is happening. Their first indication of problem will be the huge wind. People on the trailing edge will probably get blown up first. – Trevor Jun 27 '19 at 21:16
• Here's one way of thinking about it: Split the planet in two. One side is not moving and the other side is slowly but forcefully crashing into it. Since both sides are brittle and relatively soft compared to the forces at play. They crack and spill material along the lateral plane. The available volume is rapidly depleting so the liquid parts of the planet go the path of least resistance: Out. There's another word for this, volcanoes. The front side of the planet still isn't accelerating much at this point though, that may still take some time. The molten rock rains back to the surface. – Muuski Jun 27 '19 at 21:47

Yes, but you could mitigate this by having a large "towing" mass near the earth in the direction of the desired motion, and as the earth moves ("falls") toward the towing mass, accelerate the towing mass away, maintaining a constant distance.

If the distance was large enough, tidal forces would be minimized (ie the gravitational pull is "about the same" for all points of the earth). My completely unfounded guess would be about 0.1 AU or so. A "small" (perhaps jupiter mass sized), highly charged, black hole might work. Charged, so you could direct its motion with propusion systems that could withstand the extreme tidal forces required to propel the black hole away from the earth using electrostatic repulsion/attraction.

Keeping the moon using this approach is probably out of the question given the danse macabre that would ensue in the context of the three-body problem.

• Why not put the black hole inside a hollow sphere inside the moon? – nalply Jun 29 '19 at 9:18
• @nalply what could possibly go wrong with that idea!? – Bohemian Jun 29 '19 at 9:19
• A lot, but if they can handle black holes they should be able to save the moon this way, shouldn't they? – nalply Jun 29 '19 at 9:41
• @nalply if they can handle black holes like that then finding mass (a "few" astroids or such) and assembling a new moon at wherever they're going is probably no big deal either. – Daniel Jour Jun 29 '19 at 19:17

Ultimately the answer is going to depend on how the 'engines' work and how much acceleration they produce.

As the other answers (and many of the comments) have already pointed out, simply putting a bunch of rockets on one side of the planet is going to end in disaster. If they're strong enough to give even a small amount of acceleration - say 0.01G for instance, only about 3,000 times the tidal pull of the moon - then the following are likely effects:

• All of the water on the planet will try to flow away from the direction of acceleration.
• The atmosphere will try to do the same.
• The force of the rockets pushing down on the surface will likely break it.
• All structures not on the 'front' of the planet will now be braced in the wrong direction and will probably fall 'backwards'.
• All life that relies on air and/or water will have to relocate to the back, where the ground is all broken and the radiant energy from the engines will kill them.
• The spinning core of the planet might cause you some grief too.

And that's assuming relatively small acceleration levels. Past 1G (assuming you brace the whole rear hemisphere of the planet in a big cup to stop the whole thing falling apart) all of the gas and liquid - and a fairly large portion of the solids - on the planet will no longer be on the planet, they'll be rather a large receding smudge in space. Which is OK, because well before this point the Earth at this point would be a fairly large liquid ball itself, at least half of which is dribbling over the sides of the cup.

All of which is why we don't ever want to use reaction drives to move planets we're attached to. At least not quickly.

If you really want to move a planet without breaking it up you need a nice large gravity well that you can manoeuvre around or manufacture at will. Since you put the Reality Check tag on I assume you don't want to just have magical space bending technology in your story, especially since you then have to explain why they didn't use that technology for something a bit more realistic. That brings us to the idea of using one of the gas planets of our solar system as a (very very slowly-) mobile gravity well.

Larry Niven used this idea in A World Out of Time, where a lot of laser-induced fusion rockets motors suspended in Uranus' upper atmosphere were used to move Uranus around over several decades until its gravity well passed close enough to Earth at the right time to change its orbit. The literal mechanics of it are a bit hinky, but not quite as hinky as the orbital mechanics. Which is probably why it didn't turn out all that well even in SF.

But since you've put this under Reality Check there's not a lot of other options. Slap a Space Opera tag on it and we can give you plenty of options. Giant spindizzy fields, selective inertial neutralisation, direct acceleration fields, temporal displacement fields (check how fast the Sun is moving through space, wait for a good moment and then jump the whole planet 3 months forward in time)... all fun ideas, none of which seem to have the least possibility of ever being real.

• "selective inertial neutralisation" Ah, the Lensman approach. – Wenlocke Jun 28 '19 at 9:17
• @Wenlocke Well, technically it was the Medonians who figured it out on the planetary scale. But yeah :) – Corey Jun 28 '19 at 14:03
• Or use a warp drive and just don't accelerate the planet... – Garrett Motzner Jun 28 '19 at 17:13

If you can fall off, so can everything else: air, water, soil, cars, buildings, even mountains.

Gravity holds the planet together. The local effective gravity direction is always “down”. Things on Earth rely on that to make sure they stay in place. Change that to pointing up, away from the ground, and eventually everything will be moving.

In other words, if you can fall off, the planetary engines will shortly no longer pushing an entire planet.