Hi everyone. Currently, I'm working on a Sci-Fi novel for local community project. The story revolved around people of the earth who, one day, witnessed the sudden appearance of a spherical UFO the same size of the Moon. The UFO then managed to orbit the earth, almost in-sync with the Moon, so in evening people could witness the Moon, as usual, and in afternoon they could see the UFO rose and high in the sky. The problem is I want to make this story as realistic as possible, and in order to do so, I need help in answering these questions:

1. What effect would a second moon caused upon earth? More Tsunami, maybe? Earthquake?
2. Will there be climate change because of it?
3. From psychological side, what will be average persons' reactions upon seeing a UFO, a size of the Moon, suddenly appear in the afternoon sky?

That's it, for now. Any help will be appreciated.

 

What's the mass of the UFO relative to that of the moon? If the answer is "very tiny" then the answer to your first two bullets is "a negligible effect."

If the answer is anything else, then it has the potential to present a major perturbation to the system. The Moon and tidal system are major drivers of climate. Also, your UFO cannot set up a stable orbital system with the Earth-Moon system and have an orbital period comparable to that of the Moon's. You're likely to get a collision eventually: either between the UFO and Moon or else one of either the UFO or the Moon is propelled away from the Earth and the other is sent careening into the Earth.

 

I love your mind, Pers. That right there is an awesome way to end the story. The moon careens into Earth and the UFO takes a scattering of humanity off into the galaxy.

You can have that one for free, assholes!

 

Never watched Doctor Who before, but after reading those wiki, I'm thinking to start watching it soon. But what about the Physics effect of 'sudden appearance' of the UFO, related to people's reactions? Suppose it truly appears out of nowhere in the sky, what's going to happen to earth?

I planned to make the ratio between UFO's mass and the Moon 1:1. There'll be an explanation about it, but suppose that's the ratio, so it can't really have a stable orbit, right?

Actually, that's quite similar to what I planned to write.

Another questions, please bare with me:

1. What will happen to earth if the Moon destroyed, or thrown off its orbit? There's still the UFO in the sky, though, so what do you think will be happened?
2. If the human's first reaction is panic, how about the animals?

Thanks.

 

Reddit askscience:Pretend we have a second moon, basically identical to our current one, orbiting perfectly on the opposite side of the planet as our own.

The question is different, it's solely related to tidal differences with a second moon on the exact opposite side of the earth from our current moon. It could be helpful with how tides might change, though.

 

If it were a very light UFO, you could have it stably orbit at the Moon's period by locating at one of the Lagrange points, L4 or L5, which are located on the Moon's orbit about the Earth (to a good approximation). These are the two unique, "structurally stable" (meaning that perturbations to the orbit self-correct) orbit locations for bodies orbiting the Earth with the same period of orbit as the Moon.

With a UFO the same mass as the Moon, you couldn't passively locate it on an orbit with the same orbital period as the present Moon (the worst case of reinforcing resonance possible) or, for that matter, an orbital period that's an integral multiple of the Moon's period. What happens is that perturbations to the system (e.g., if the system isn't lined up just so, which is impossible anyway) will grow exponentially until catastrophe.

There ways around this, though, if you absolutely need synchronicity of orbits for your story. One would be to actively and continually "unperturb" the system--your UFO continually monitors the Earth-Moon orbits and adjusts its motion to try to undo any growing perturbations. This is effectively what we do as humans when we stand up, as that's also a structurally unstable situation and our proprioception allows us to continually adjust our balance. It's also what Larry Niven had to correct with his Ringworld series (hence, the plot of Ringworld Engineers). It does represent a pretty incredible level of technological savvy, though a theoretically feasible one.

Incidentally, simply introducing your UFO from afar into the Earth-Moon system is going to perturb the orbits inevitably. You probably will want to have the aliens attach some sort of propulsion system to the Moon as well to ensure that it stays where it should. Perhaps amateur astronomers notice this (possibly noting that the Moon's kinematics don't make sense--this is because the propulsion system fiddles slightly with the spin-orbit resonance between the Earth and the Moon, say). GPS and other satellites will also notice the perturbation to the system as you introduce another large mass.

The first would depend on how the Moon is gotten rid of. If propelled away, one would have all the same orbital problems as in bringing the UFO near in the first place. If its mass were compacted into a black hole somehow (which, incidentally, would have an event horizon of about 0.1 mm) and left in place, then you'd basically have the same system as before (with the same effects on tides, etc.)

Dunno about animals. That's not my forte.

 

Note that for a UFO to be approximately as large and as heavy as the moon, you have to match the average density, which is higher than solid aluminum. Compare that to your friendly neighborhood aircraft carrier, the density of which is less than a third of that of the moon. We won't even talk about planes or actual spacecraft.

So it probably makes the most sense for the UFO to be that civilization's planet or moon with the usual, solid core, as opposed to a Death Star-like, built-from-scratch ship. Of course if you're already assuming a propulsion system able to move a planet that size, exotic materials with compressive strength greater than flawless diamond isn't that much of a stretch.

Also note that you can make the UFO smaller and closer, and still have its gravity affect the Earth by the same amount. I'm too lazy to do the math on whether that would help the density issue or make it worse.

 

The gravitic force equation is Gm1m2/r^2. The relevant ratio is m/r^2, where m is the total mass of the ship/moon and r is the distance from the center of whatever it's effecting. If the total mass is halved, then the equation becomes m/2(x^2), where x is the new distance.

Solving for x gives r/sqrt(2), or ~0.71r

So yea, you can make it smaller and move it closer, just remember, if you're going for hard sci-fi, that the change in distance is exponential while the change in mass is linear. The more the mass changes, the (proportionally) less the distance will change (for comparison's sake, reducing the mass to m/4, means the distance is only r/2).

Edit: Now for the OP's questions:

1) This depends entirely on exactly how the orbit of the ship relates to Luna, and their relative gravitic strengths. If, for the sake of simplicity, they orbit diametrically opposite one another, then the tides would disappear almost entirely. If they orbit aligned with each other, the tides would approximately double in strength. I'm far from qualified to comment on the environmental ramifications of either scenario (or anything in between), but I'm reasonably confident both extremes would lead to bizarre and potentially very, very bad effects. I just don't know enough about tidal movements and what they effect to say more though.

I can say that earthquakes are almost certainly not going to result. Lunar gravity does not have a significant impact on tectonic movement, to the best of my knowledge. That's mostly governed by the currents of magma in the Earth's core, and that's driven more by convection than anything else.

2) Absolutely. The full extent of the effects I can't even begin to fathom, but there would be severe environmental effects of something like that. It wouldn't surprise me greatly if it was enough to cause another ice age, but I have only the most rudimentary scientific knowledge driving such a conclusion.

3) Poorly, most likely. Many societies, especially religious ones, are likely to outright collapse into anarchy over the proven existence of sentient extraterrestrial life. There's no easy way to predict what will happen, but the societal change will be massive, no matter what.

1) If Luna is completely destroyed, the debris somehow escapes Earth orbit, and the ship is still in place, then things would go back to how they were before the ship appeared. The only difference is that instead of the Moon providing tidal forces, the ship would be. If the debris stays in orbit though (most likely as a new planetary ring, though that really depends on how it's destroyed, large portions of it would probably crash into the Earth after all), the changes are harder to predict. Most likely it would mean that the ship provides the primary tidal forces (and brings back actual tides if its presence removed them), while the remains of Luna provides a stabilizing force to keep the tide above a certain minimum level.

That assumes Luna and the ship are on the same orbital plane though. If they're at angles to each other, there's be a really weird tidal force that would make the oceans 90* off from Luna's orbit much lower and the oceans along it much higher, while the ship acted much like Luna does now and shifts the tides globally. The model is hard for me to envision properly atm though, so take it all with a grain of salt.

2) Animals would "panic", insofar as they would notice the sudden and severe climate change and try to find environments better suited to them. There would probably be another wave of mass extinctions, and as a consequence lots of humans would starve, but it wouldn't be the end of the world.

 

Actually tides would increase in height. The water on both sizeds of the earth would be pulled away from the earth.

 

Well that was the easy part.

To make it appear the same size to observers on the Earth in your example above, you also have to reduce the radius of the UFO by about 1/2 (not exactly, since you have to take into account the radius of the Earth, but close enough for government work). So since volume is proportional to rUFO^3, the volume will be 1/8 that of the moon. Which means the average density will have to be twice as high, which makes it higher than Earth, which is the densest planet in the solar system, which means it's a bad idea.

Curiously (at least, it's not intuitive to me) the imaginary closer body would have about the same surface gravity as the moon.

So nevermind! The thought of a gravity well with a similar effect as the moon zipping around the Earth every 90 minutes in LEO seemed fun to think about, but even at that distance it would still have to be almost as heavy as the real life moon of Saturn that looks like the Death Star (but far denser).

 

Many, many thanks for the answers, guys. Those are some great inputs you gave. And Pers, as much as I like your ending, unfortunately the novel I'm writing will be targeted at younger audience, but I think I can improve it a bit, making it more suitable for children.

Heh, again, thanks.

 

A UFO could be in perfect sync with the moon's orbit. At first, scientists (like Pers) would be skeptical or confused as to how it's possible, but if it is a UFO with alien technology, who's to say it cannot manipulate gravity in some way? I think we can imagine that in ten thousand years time, if we're still around, we'll be a space-fairing species. In order to properly conduct such space-fairing, and still utilize the earth, we'll need many large bodies orbiting it. The 'death star' from Star Wars was pretty big, and I could see such a ridiculous structure as being useful as some kind of docking bay for ships wanting to send their items down to Earth/some planet. What I'm implying is, that any space-fairing species would need to have fairly advanced understanding and technology to synchronize the orbits of their large craft or structures, in order to not destroy the planets they want to utilize.

Would that be a reasonble explanation for why a moon-sized (or at least, moon-massed) UFO could remain in perfect orbit with our moon? Perhaps this UFO changes the mass of the moon in order to make its orbit possible?