Hiding an orbital colony 'behind' a planet from a flare star

[Melbourne Guy]

So, I think it's possible to place an object in an orbit such that it remains perpetually in the planet's shadow, like in my awesome graphical representation. But is it?

 

[Rive]

That's a really delicate question. Although orbits with several day period should exists (in match with the usual period for the planet orbiting its red dwarf sun), I think these would be with really high radius and I'm not sure that at that distance the planet would be able to provide significant protection against charged particles of those flares. Especially since the magnetic field of a (habitable?) planet is supposed to give those particles some twist.

 

[Melbourne Guy]

Thanks, @Rive, I wondered if it would highly situational, with attributes such as planet size, distance to star, how active the star is, etc., making any answer a strong, "maybe, but..."

I might use the concept incidentally, but won't make it a common method of protecting habitats from flare stars.

 

[Algr]

Well the planet of such a small star would be tidally locked. So why not put the colony on the planet's night side surface? (Of course you have drawn a jovian there, so that might answer my question.)

 

[Orodruin]

So, I think it's possible to place an object in an orbit such that it remains perpetually in the planet's shadow, like in my awesome graphical representation. But is it?

View attachment 300378

Yes, this is possible if you place the object in the L2 Lagrange point. However, depending on the masses and sizes of the involved bodies, L2 may lie in the umbra or antumbra. If it is the latter, then it will not be perfectly shadowed as, seen from the Lagrange point, the star will cover a larger part of the sky than the planet. For the Sun-Earth system, it is a borderline case if I recall correctly.

This is where the James Webb telescope is placed for precisely this reason.

Also note that the L2 Lagrange point is unstable. In order to maintain the orbit you will need to regularly correct it.

 

[Melbourne Guy]

Well the planet of such a small star would be tidally locked. So why not put the colony on the planet's night side surface? (Of course you have drawn a jovian there, so that might answer my question.)

Not all the colonies in my novel are planet-based, @Algr, and especially the early ones, which were just inflated habitats to lay claim to the star system. For instance, in late 2204, Proxima Development Corp. inflated a hab around Proxima Centauri and shuttle staff back and forth to substantiate their claim to the system. Minter and Hogan did the same around Gliese 1061 d, with 200 staff rotated on a six-monthly basis.

The legality of a single company / state owning an entire star system based on merely sending a probe was struck down by the Planet and System Agreement in late 2206, which established that ownership is per-discrete object (planet, moon, etc) not an entire system.

Also note that the L2 Lagrange point is unstable. In order to maintain the orbit you will need to regularly correct it.

Thanks @Orodruin, that could lead to some spectacular mishaps, esp. in the early years of colonisation, I'll have some fun with that.

 

[Algr]

Yes, this is possible if you place the object in the L2 Lagrange point.

It sounded like Melbourne Guy was describing something different then a Lagrange point. This idea can only work for planets that orbit very close to their stars. If a planet orbits its star in five days, and the station orbits the planet in five days, then the orbit could be timed such that the planet was always between the sun and the station. (Or indeed at any other angle desired.)

 

[Orodruin]

It sounded like Melbourne Guy was describing something different then a Lagrange point. This idea can only work for planets that orbit very close to their stars. If a planet orbits its star in five days, and the station orbits the planet in five days, then the orbit could be timed such that the planet was always between the sun and the station. (Or indeed at any other angle desired.)

No, by definition of the description in the OP, what is required is a point that is stationary in the corotating frame of the star-planet system and that by definition is a Lagrange point.

 

[Orodruin]

Thanks @Orodruin, that could lead to some spectacular mishaps, esp. in the early years of colonisation, I'll have some fun with that.

Any civilization advanced enough to know how to construct an orbital colony will likely be aware that L2 is unstable. The plot twist could likely only be some sort of technical failure of the propulsion system intended to keep the station at L2.

 

[Melbourne Guy]

The plot twist could likely only be some sort of technical failure of the propulsion system intended to keep the station at L2.

That's exactly the type of thing that sprung to mind

And in this case, it will be sabotage...but whether it's the protagonist seeking to justify the flames of war or the antagonist seeking to cripple their opponent will be the twist!

 

[Melbourne Guy]

It sounded like @Melbourne Guy was describing something different then a Lagrange point.

No, by definition of the description in the OP, what is required is a point that is stationary in the corotating frame of the star-planet system and that by definition is a .

I'm going to have to work out a Lagrange point and see if it fits, because my initial understanding is not enough to confirm (or deny) @Algr's observation. Certainly, I want the station hidden from the star at all times, but as an orbit, not having to continually fire the thrusters.

 

[Office_Shredder]

The Lagrange point didn't require continuously firing your thrusters. You just have to occasionally reposition yourself, because if you are too close to the planet you fall towards the planet, and if you are not close enough to the planet you will drift further out into space. The actual net gravitational pull is 0 if you exaaaactly nail the point, but for actual practical placement, is very very close to 0. The Webb telescope for example, in its final thruster firing for insertion, turned its thrusters on for five minutes.The telescope also hasa disadvantage, which is that it only has thrusters on one side, so has to intentionally stay closer to the planet. If you have thrusters on both sides you can stay closer to the middle and use less fuel. If you're putting up a habitat that people are permanently living in, it doesn't seem like a stretch that they could have enough fuel to maintain position for a hundred years without refueling (the Webb telescope has enough fuel for twenty years based on how efficient its initial insertion was)

 

[Algr]

Since the planet would be tidally locked, the orbit Melbourne Guy is describing would also be a geosynchronous orbit.

 

[Orodruin]

I'm going to have to work out a Lagrange point and see if it fits, because my initial understanding is not enough to confirm (or deny) @Algr's observation.

There is nothing to confirm. For any two body system in circular orbit, there are exactly five stationary points for a test particle in the corotating frame, which your setup would require it to be. These stationary points are the Lagrange points L1-L5 and out of those only L2 satisfies your requirement of having the lighter of the two bodies in between the larger and the Lagrange point. There simply are no other such solutions.

 

[Algr]

There simply are no other such solutions.

Why do PF forum people do this? We just described a solution and you just ignore it and declare that there isn't one. If it's wrong, at least make some effort to say why.

What we are describing is a special case that would not work on Earth, do you understand that part?

 

[Orodruin]

Why do PF forum people do this? We just described a solution and you just ignore it and declare that there isn't one. If it's wrong, at least make some effort to say why.

What we are describing is a special case that would not work on Earth, do you understand that part?

I did say why:

For any two body system in circular orbit, there are exactly five stationary points for a test particle in the corotating frame, which your setup would require it to be. These stationary points are the Lagrange points L1-L5 and out of those only L2 satisfies your requirement of having the lighter of the two bodies in between the larger and the Lagrange point.

This has nothing to do with the Earth-Sun system, but with basic classical mechanics and the motion of an smaller mass object in the corotating frame of two larger bodies in circular orbit. The setup required by the OP asks for the orbital station to be stationary in the corotating frame. This is the very definition of a Lagrange point. Any solution satisfying this condition is a Lagrange point of the two body system (the Earth and Sun system has nothing to do with this apart from having its own set of Lagrange points). There simply are no other such points.

Your basic mistake is that you assume you can have any period of orbit of the planet without the orbit being affected by the star. This is not the case. As long as you have an orbital period small enough to be quite close to the planet, this is a very good approximation because the gravity of the planet dominates. However, as you increase the period the distance to the planet increases and when the period is of the order of the orbital period of the two body system the gravity of both bodies is relevant.

There is no need to get snappy about it when I quite clearly stated the reason why no points other than the Lagrange points work: It follows directly from looking at the problem in the corotating system and searching for stationary points. The only such points are the Lagrange points. If there were more than five, those would also be Lagrange points, but the solution is well known to admit only five such points.

 

[hmmm27]

Find a tide-locked moon and dig a pit.

Phobos has a neat divot that always almost-directly faces Mars.

 

[Melbourne Guy]

Find a tide-locked moon and dig a pit.

Phobos has a neat divot that always almost-directly faces Mars.

That's definitely one option,@hmmm27, but not necessarily the easiest for the first interstellar colonies. Inflating a habitat in orbit is simpler than having to land - even on an airless planet - and the first FTL craft are rudimentary, so it takes a while before 'on the ground' (or in the ground, I guess) bases are built.