Lagrange points

That's wrong

i suggest the same, because an effect can only travel with maximally the speed of light.maybe in the end it will get a tiny bit wider but thats all.

how can it be wrong ? travveling with c, it has an infinite amount of energy.

A rock the size of a car would be more devastating than the bombs they dropped on Japan.

A rock the size of a house would probably plunge the world into a nuclear winter for a hundred years.

Anything bigger would absolutely end life on the planet.

> implying you think that noone would give a shit about your plot
Er, it's a /sci board, it's supposed to be about science problems, no? If I go into a dozen paragraphs of plot description, wouldn't that be off topic? ...That said, if anyone's really interested, then:

...then, the aliens in question are a post-singular race of mind uploads inhabiting a huge spaceship that goes around eating all the resources on planets it meets. Being basically a machine the size of a planetoid, they need a lot of energy and raw materials for maintenance. Which means they're extremely resource-efficient. They set up a base at L4 point because you don't need much fuel to maintain a stable position in there. Then, they send resource extraction units in direct line to whatever side of Earth is facing them at the moment. They're much more technologically advanced than humanity, so they think trying to fly around Earth to outmanuever what's basically monkeys with sticks is just wasting resources. Better to send a nigh unstoppable unit in direct line, wasting least amount of fuel, let it plow through paperboard human defenses, and collect all the good stuff.

> Its exact movement relative to Earth surface is fairly complicated, but you could expect being above given longitude approximately every 25 hours
So basically, yey, for the observer on Earth, they move! And object travelling from a Lagrange point towards Earth in a straight line will land in different area every 25 hours, right?

> if you want an object in space always overlooking the exact same area, you want a geostationary orbit
Nop! I actually want it to move so that different countries get to deal with nasty aliens.

you dont understand relativity.
it is travveling with c
it has mass
> it has infinite energy

...

also desu its impossible that it travels at the speed of light.

>They set up a base at L4 point because you don't need much fuel to maintain a stable position in there. Then, they send resource extraction units in direct line to whatever side of Earth is facing them at the moment
God I hate science fiction writers, they never know what the hell they're talking about. You can't just drop something from the Lagrange point and have it fall to earth.

>pretending i bait
>only to avoid a discussion he already lost

...

Liu Cixin did it better.

where is your counterargument then ?

unrelated poster, but I think you sound gay

ok, lets say 90% c

and i think you are retarded.

now thats what im talking about.

> You can't just drop something from the Lagrange point and have it fall to earth.
Er, I know?.. Not drop, of course. But send an actual ship with actual propulsion from it towards Earth? As in, what wastes more fuel: launching a ship from a base at L4 then manuevering in zigzags to reach the side of planet that it's not currently facing, or launch a ship from a base at L4, face its nose towards Earth, and then just accelerate in straight line until it reaches the planet?

>will land in different area every 25 hours
What do you mean by this? Every 25 hours the same side of the Earth will face the station. Look, Earth's surface angular velocity is 360°/day, Moon's angular velocity is ~13.2°/day. Moon's orbit is prograde.
[math]360t=13.2t+13.2[/math]
Solving this tells us that it takes Earth ~0.038 days or ~0.9 hours to "catch up" with the Moon after a full rotation. If Moon's orbit had no inclination to Earth's equatorial plane, it would mean that every 25 hours Moon would be exactly over the same point on Earth. But it has, and then there is nodal precession and apsidal precession of this orbit and because of all this Moon sweeps a large area around Earth's equator. It takes decades for the system to cycle to the initial state and it is virtually impossible to shoot the same point on Earth from the Moon twice without in-flight corrections. L4 and L5 are practically second moons and everything above applies to them just as much.

To drop from any orbit into earth you'd want to accelerate in the direction opposite your motion in orbit and 'drop'

Oh, I see... So every 25 hours the Moon - or L4 and L5 points - face the same spot on Earth. If I get it right, which I may not because I'm an idiot.
...Does that mean that for an observer on Earth, L4 and L5 points are moving with the same speed and the same direction as the Moon?

Sorry, I'm not a physicist or a mathematician, I'm a writer, and a shitty one at that, so I'm doing my research while being a huge dumbass about it.

Yep, I know.

You just suggest pointing the nose Terrawards and hitting the gas - much less fuel efficient than just killing your tangential velocity. That's why it'll take longer but waste less fuel to drop to the sun from Neptune than from earth

Thanks for the correction! And that is why you (as in me) should do your research before writing anything. I think I should read some articles on spacecraft. And play some Kerbal Space Program.

KSP is a fun intro into orbital mechanics, no arguments there. Its Wiki isn't plain tutorials but full on formulae and shit

Yes, kind of. For an observer at the Earth-Moon system's barycenter (it's close to Earth's center) L4, Moon, and L5 move with the same speed and the same direction (again, not exactly but close to it, due to moon's orbit being slightly elliptical). For an observer on Earth's surface, not really the same. Because of Earth's rotation they would appear to be on weird distinct loopy orbits with common center (Earth), but these orbits are similar to each other, and can be perfectly overlaid one on another by rotating them around their center.

Thanks! That is exactly what I wanted to know when I made this thread. Exact hard math notwithstanding, it means than in my shitty novel the alien menace (tm) will be able to land anywhere on Earth by waiting for a moment when it faces their lair at L4 with right side, then launching from there in whatever is the most fuel-efficient trajectory the dude from was suggesting. Works for me. Now, to read some more references on orbits and stuff so that I don't look any more of an idiot than I already am.

And, for a social sciences graduate with unhealthy obsession with sci-fi like myself, this stuff is simply fascinating!

So basically what I suggested (point the nose at Earth and JAM IT IN) is like that, right?

there's no point to make a "Base" at a lagrange point though

Something like low lunar orbit would make more sense
Or landing on the moon for rapid deployment of mining equipment to build whatever munitions/drones they need for invading Earth

Since you already have tangential velocity (right on your pic) you'll veer right around the earth, or if you are speedy enough, hit the earth fast but extremely inefficiently fuel wise.
What you do is check the direction you are spinning along the orbit, put your ass there, and burn till you kill your tangential velocity. Then you start falling towards earth directly under you. If fuel is of no issue then at that point feel free to go jam wise.

> there's no point to make a "Base" at a lagrange point though. Something like low lunar orbit would make more sense
Perhaps... But what I had in mind is that, as I said in , the main body of the enemy is a machine the size of a planetoid itself, so... "That's no moon" (c) Star Wars. They're too big to make a landing with the main body, they need to park it somewhere in direct view of Earth, even better if they don't have to spend any fuel to correct their position - hence L4 or L5 because those are stable and the object that nests in there can stay there forever, or so I understand.

Like that?

Yeah except once you burn all your tangential velocity away you drop to earth directly. Like a dozen or so of minutes burning asswards and then 3-4 days of freefall directly at earth.

So after tangential velocity is 0, you can point the thrusters to Earth and just plunge down, controlling the fall with small bursts if necessary? Also, considering the kind of propulsion the ship has, it probably can take less than 3-4 days, no? They're highly advanced (but very resource-greedy) aliens, they don't have like, chemical rockets.

Once tangential is at 0 the ship starts freefalling. A freefall from a lunar orbit to earth is about 3 4 days. Since at that point you have no tangential velocity you can burn straight down to make that trip shorter. Just make sure to slow down for atmospheric entry or you're fucked. Alien tech can probably accelerate enough for a shorter trip AND manage to slow down in time.

I see. Thanks! That pretty much answers all my questions. You, good sir, are a marvel! Have a snek for gratitude.

Aw shit I love sneks. Thanks man

Point is, getting up to relativistic speeds is going to be very energy intensive, so it would very much make sense for the aliens to need to restock at a moon/asteroid to go from a dense/compact "interstellar" core to their full invasion force.

All orbits are stable, you don't need to spend fuel to maintain any orbit unless you are either in Low Earth Orbit due to drag.

en.wikipedia.org/wiki/Lagrangian_point#Stability

It's actually the opposite, trying to maintain position in a lagrange point is whats not stable.

You should watch videos of Kerbal Space Program for this stuff, lets you visualize orbits easily enough.

Point of order, KSP does no multiple body somulations so L points are nonexistant in it.
Children of a Dead Earth is a nice resource for that.

> it would very much make sense for the aliens to need to restock at a moon/asteroid to go from a dense/compact "interstellar" core to their full invasion force
Well, what I had in mind is that there's no invasion force to speak of. Their huge planet-sized mothership hangs in some stable spot with engines powered down (I thought L4 would be best spot, but maybe not after everything I learned from this thread). Then, from this mothership, several small (like, 500 meters or so) extraction ships are launched towards Earth. They land without any opposition, march through human cities unopposed, crack open our mines and gobble on uranium or whatever. Then they bring that shit back to the mothership, and once Earth is empty, they leave. But... It's 2017 and we're cavemen to them. So, no need to unwrap the interstellar core into an invasion force as there's no need for "invasion" or "war". They're like a kid poking an anthill with a stick, they don't give two shits what humans throw at them because it won't work. The novel is a story of futility.

> You should watch videos of Kerbal Space Program for this stuff, lets you visualize orbits easily enough.
Yep! And play it too, it sounds cool.

Thanks for suggestion!

> they're like a kid poking an anthill with a stick
Actually no, make it a dude in full SWAT armor excavating the anthill with a diesel shovel. Basically a 500 meters tall ship stand in the middle of a mine, human jets fly around shooting missiles at it, and it doesn't even shoot the jets down because it gives no crap.

Well, first off I don't think you understand how truly MASSIVE a "planet sized mothership" actually is
Go look up the masses of asteroids

en.wikipedia.org/wiki/List_of_Solar_System_objects_by_size

50 km diameter is still a 10^18 kg object

Mining involves moving through vastly more dirt & rock than output, so the Earth would be choking on dust from their giant open pit mines.

So what makes sense would be a low earth orbit, to minimize communication delays & travel time to/from earth.

> Well, first off I don't think you understand how truly MASSIVE a "planet sized mothership" actually is
...I probably don't understand, yeah. My sense of scale is a bit off I guess. Speaking of which, won't having what is basically a second moon on orbit send Earth's tides outta whack?

> so the Earth would be choking on dust from their giant open pit mines
Like they care about our survival.

> So what makes sense would be a low earth orbit
I see... Thanks for suggestion!

> So what makes sense would be a low earth orbit
...Wait, but low Earth orbit means they'll be subject to Earth gravity. If that mothership is that huge - even if it's not planet-sized but smaller - won't they need to constantly burn so that something that massive doesn't fall down?

Do you not understand what an "orbit" is?

Spinning around the celestial body, no? Momentum would keep throwing you sideways and not down, yeah, but Earth would still pull you down a bit? If I go to Wiki, it says,

> A low Earth orbit is an orbit around Earth with an altitude between 160 kilometers and 2,000 kilometers. Objects below approximately 160 kilometers (99 mi) will experience very rapid orbital decay and altitude loss.

...So it will still kinda fall? (I am very stupid, sorry!)

Youre not stupid and this time that guy fucked up. The term Low Orbit does often mean atmosphetic orbits that decay, often used for re entry. What he probably means is a low altitude orbit that is out of the atmosphere still.

objects below 160 km are running into Earths atmosphere, thats why they slow down and eventually fall to earth

the ISS is at 400 km orbit, still needs a bit of boosting every year due to atmospheric drag.

It's not about up or down, but about horizontal velocity.

> Youre not stupid
I am a sociology major. Anyone who would willingly choose to major in sociology is got to be quite stupid.

> It's not about up or down, but about horizontal velocity.
As in, if you go sideways fast enough, you won't fall down even as Earth tries to pull you down?

Bingorino. Now you're getting it.

youtube.com/watch?v=5Lz5u_AjAI4

skip to 3:30 so you can visualize what orbit actually is
Maybe mute it, dnno why there are so many britfags making kerbal videos.

When your trajectory no longer hits the ground anywhere, then you are in orbit

Actually L1 and L2 are unstable and a calculated perturbation of an orbit could send a body into a trajectory on the interior manifold.

Thanks! Now watch me making some other stupid question and ruining the impression of being a person of at least average intelligence.

...So I guess, I should place my slightly-smaller-than-a-planetoid mothership on low Earth orbit, flying sideways fast enough so that Earth won't pull it down. While ploping down extractor ships that dig huge holes in the crust sending tons of dust in the air. And all the oceanic tides are freaking out because there's a second Moon in orbit?

That's no moon...

Okay, maybe oceans are not freaking out but we're still very fucked.

probably not tides, but the weather is gonna be fucked and earth quakes being triggered by the quantities of debris moving around.

Oh jesus christ the colors on the DVD release are awful aren't they?

Also tbqhfam I am now curious to read your story to see how humanity X COMs these ayy lmaos out of here.

> Also tbqhfam I am now curious to read your story
...I'll spoil it for you and tell you that we won't. We can't destroy them, and nuking them just means clearing out the way for them because they'll shrug it off, but civilians beneath won't. They take the very resources we can or cannot build anything to oppose them from. Our infrastructure is destroyed on their touchdown because it's thousands of tons of metal dropping from orbit. And anything that survives is destroyed as they stomp their way through population centers towards the nearest ore mine. Millions dead indirectly because of fires and looting and lack of infrastructure. If there's really so much dust being released during mining, then there'll be worldwide pollution as they just dump all the waste into air. Which also means planetwide greenhouse, the skies are not conducive to light so plant matter withers. tl;dr - humanity gets raped and is left to bleed out and cry by the roadside while aliens get in the car and move on.

Yo are you snek guy from /kspg/?

No, that's actually third or fourth time I ever post on Veeky Forums, more of a lurker. Definitely haven't posted any sneks before, but I do love them.

This is really easy: each and every L point's location is determined by the position of the Moon and Earth together. The L points revolve around the Earth with the same period as the Moon: about 29 days. The Earth takes one day to rotate.

How about this snek

Love sneks they were my favorite animal as a kid. They have so much mysticism and science attacted to them. One of my favorites has got to be Garter Snakes

>rock with infinate energy in a "vacuum"

no chance that will happen because USA will shoot it out of the sky. they will detect them from their satellite, unless they can fly at the speed of light.

has got

> has got
Yeah, that. English's not my first language.

> no chance that will happen because USA will shoot it out of the sky
> Love sneks
And snek loves you!