# Could a satellite make the gravity of a planet increase?

Let's say that planet X with a gravity similar to Earth increases its gravity by a certain amount. The increase is great enough to make living conditions difficult or impossible: for instance, the G-force could make buildings structures crack.

Now let's say this happens every 2 years (or whenever) for a specific period (2 weeks or a month), and that after this time it goes back to its regular "Earth gravity". Could this happen because of a satellite orbiting planet X was closer during that period? Is this possible to some extent or am I just talking nonsense?

• While it doesn't have to do with gravity, the book Deepness in the Sky by Vernor Vinge has a great, recurring climate event on the main planet and discusses the engineering the society has to do around that. – Nathaniel Ford Jul 13 '16 at 21:20
• You're stating that a satellite has its gravity change, then asking if planets can have its gravity change? This has no answer. It's an example of cheese. – JDługosz Jul 13 '16 at 21:45
• If I correctly understand, she is asking if a satellite orbiting a planet would increase the planet's G force by getting closer to the planet. It doesn't seem she is asserting anything about the satellite, gravity or changes in it. – Luís Henrique Jul 14 '16 at 0:24
• Hi JDlugosz I'm not stating anything on the satellite gravity. What I'm asking is what @Luís Henrique is saying above, if that helps. Thanks – SusannahDean Jul 14 '16 at 11:31

Mostly you're 'talking nonsense'.

Gravity derives from and is proportional to the mass of a planet. A secondary body that moves closer to a primary body will not increase either the mass or the gravity of the primary body.

For the gravity to increase, the planet would have to increase its overall mass (and either size or density) for a short period. It is hard to conceive of a reason (in understood physics) why this would be the case.

Far more plausible is that the secondary body's gravity is high enough that the tidal forces of the planet become very extreme, as oceans and other 'loose mass' are affected by the passing secondary body. This might make things difficult or even 'unliveable', especially in coastal areas which could, in theory, be pounded by tsunami-level waves.

• I understand what you meant there and I think you are correct. But I am also thinking of an hypothetical situation: suppose you are standing on Pluto. Would a scale measure you lighter on the side that faces Charon and heavier on the side in which Charon would not be visible? I mean, do the gravitational pull of both bodies "cancel out" when you are between them, and add up otherwise? – Renan Jul 13 '16 at 21:09
• @Renan The difficulty is that gravitational strength is inversely proportional to the square of the distance. Thus, on the surface of Pluto you are ~740mi from the gravitational center of Pluto and ~12200mi from from the center of Charon. Assuming equal mass, Charon would exert (1/740^2)/(1/12200^2) or a third of a percent the same force as Pluto: barely noticeable. – Nathaniel Ford Jul 13 '16 at 21:14
• Awesome! I had never got to those calculations. With these numbers your answer is irrefutable, +1. – Renan Jul 13 '16 at 21:25
• @NathanielFord thanks for clarifying. I'm basically trying to come up with a theory that explains why the conditions in a planet would change dramatically every X years, so humans could not live there or would find it very difficult. The premise is the following: a human colony lands on a planet which seems suitable to terraforming. After some years the planet conditions change for no reason, making life difficult. These should occur periodically as if it was part of the planet cycle. Suggestions? Thanks – SusannahDean Jul 24 '16 at 18:25
• I do highly recommend reading "A Deepness In The Sky". – Nathaniel Ford Jul 24 '16 at 18:29

What is Gravity?

Gravity is probably the least-well understood of the Four Fundamental Forces, so it might be possible that science will discover a way to do this, but for now it is not physically possible given our current understanding of physics.

Gravity is a direct result of mass. All mass creates a gravitational field, no matter how small. Large collections of mass, such as planets or stars, will produce very large gravitational fields. The only way to increase or decrease the gravity of a planet is to add or remove mass proportional to the increase of gravity.

For example, if you wanted to increase the gravity of the Earth from 1g to 2g, you would need to add at least 100% more mass to the Earth, effectively doubling the Earth's mass. This is wildly fantastical.

Exotic Solutions?

That being said, there is a theoretical type of matter called Dark Matter. Very little about Dark Matter is understood. In fact, it has never been "seen" in the traditional sense because it does not interact with electromagnetic waves. It must exist, though, because galaxies could not form without it, and there does exist evidence of its existence in the form of Gravitational observations.

If your planet were somehow being affected by Dark Matter (which DOES seem to have some sort of mass, or at least produce gravity) it might be possible to increase the effective gravity of a planet somehow. I'm hesitant to say it's possible, though, because Dark Matter is very weird, and might not interact with regular matter in predictable ways. I'm also confident no satellite in orbit could manipulate Dark Matter in this way; it would have to be a natural phenomenon if you're actually willing to use it at all.

Less Exotic Solutions

If I were writing a story where a planet was rendered uninhabitable due to periodic disasters, I would focus on Geological Activity. Earthquakes on the Earth are relatively timid compared to what is possible. A moon (or other orbital body with high enough mass) could cause dramatic Earthquakes and Volcanic Eruptions. This would be especially true of the moon were in a highly elliptical orbit. Every time it passed close to the planet it would cause huge issues.

Yes. Here's an example of a satellite that does exactly that:

Any sufficiently large body is going to have strong gravitational effects on the planet it orbits. We generally call these tides. As to the question: "Can these gravitational forces be strong enough to destroy buildings?" Well, it depends on how strong the buildings are.

Any gravitational effect on a planet is going to be capped by the gravitational force of the planet you're standing on. Why? Because more gravity than that and the planet would break apart. In general, the distance that two bodies can exist at, relative to one another, and be structurally sound due to hydrostatic equilibrium is defined by the Roche limit, which you can use to figure out how close together your planet and its satellite can be. Roughly defined, the Roche limit is the distance at which the difference between the cetripital force required to keep the planet orbiting and the force of gravity from the more distant body is equal to the force of gravity from the body you are standing on.

If you're standing on the larger body, you can get gravity ranging from around zero G to around 1.1G, if the satellite is close in size to the primary and orbiting very closely. Gravitational effects need not always be that high, however. If the satellite has a highly elliptic orbit around the primary. In that case, gravitational effects would range from this upper bound, when the satellite was at periapsis, to relatively negligible, when the satellite was at apoapsis.

For higher gravitational forces (i.e. above 1.1G), your best bet is to be standing on a relatively small body orbiting a relatively massive one at a great distance. If the front and back of your planet are about the same distance from the primary, relative to the distance of your planet to the primary, they'd experience about the same gravitational force from the primary. Again, since tidal forces can't be greater than the gravity of the planet you're standing on, apparent gravity will vary from a bit more than 0G to a bit less than 2G when your planet (which is now the sattelite) is at periapsis in its orbit around the primary.

Most buildings, if constructed appropriately, can probably withstand 2G of acceleration. The rampant volcanism caused by the intense tidal heating your planet/satellite will experience from the tidal forces in question will probably destroy quite a few, though.