Barycentric Suspension

[Euracil]

I was thinking about a sci-fi setting in a binary planet system. Both planets have independent and intelligent life that want to come together to build a space station in between the two worlds. Then it hit me. (Ouch! Not literally!) What if they put the space station right at the barycenter. Both planets would pull on it equally and the station would be "suspended" by gravity. Is this at all remotely possible, cause if it is, then that'd be something fun to do in KSP should they ever add anything binary.

[SuperFastJellyfish]

In the Star Wars Corellian Trilogy, there were two worlds that orbited around Centerpoint Station. Talus and Tralus were their names if my memory serves me correctly. I thought that was a pretty cool idea when I read those books as a kid.

Edit: Also, this idea is totally possible. It would be like having a satellite or station at a La Grange point. It would need minimal station keeping to hold it's position.

Edit2: KSP only uses single body physics; meaning that at any one time there is only one source of gravitational forces being imparted on a ship/station/satellite. AFAIK, they have no plans on changing that anytime soon so it's impossible to do in KSP.

Edited June 18, 2013 by SuperFastJellyfish

[K^2]

Equilibrium point between two planets is unstable. There might be a stable halo orbit, but you definitely can't just put a station in the middle.

[ZingidyZongxxx]

Equilibrium point between two planets is unstable. There might be a stable halo orbit, but you definitely can't just put a station in the middle.

Assuming that there is enough distance between both planets the station could probably use small but efficient thrusters to stay right at the center. Not sure about this though.

It would probably be pretty cheap to send stuff to the station, considering that there would be very low gravity, even on the surface of the planet, in between the two planets. Of course, the opposite would be true for the other side of the planet, with gravity being nearly twice as high, assuming both planets are the same size. I'm not sure if both planets could hold a stable atmosphere, because the other planet might sort of "suck up" the atmosphere from one side of the other planet. It would be awesome if there was an atmosphere in between both planets, but I doubt that could happen.

[K^2]

Assuming that there is enough distance between both planets the station could probably use small but efficient thrusters to stay right at the center. Not sure about this though.

Entirely possible. I never tried to estimate the degree of instability. All I know is that some sort of correction would be required. It might be a very small amount, making it entirely sustainable.

[Brotoro]

If the two planets are very close together, they could even share a common atmosphere (see Robert Forward's Rocheworld series, starting with The Flight of the Dragonfly). And then you could just use aircraft to fly up to the station.

[K^2]

Are you sure? What about the Roche Limit? I mean, I'm sure it works a little different when two bodies are almost equally massive, but it's hard to imagine the tidal effects not to be catastrophic.

[Brotoro]

Are you sure? What about the Roche Limit? I mean, I'm sure it works a little different when two bodies are almost equally massive, but it's hard to imagine the tidal effects not to be catastrophic.

I'm sure that's what Robert Forward had in his books. And becuase his Ph.D. in Physics involved work in gravitation, I always assumed he knew what he was talking about.

[Brotoro]

Another cool concept Forward had was using long conductive tethers in orbit to drain the high energy particles out of Earth's radiation belt (the electric field around the tethers deflects the particles into the atmosphere, as I recall).

[tavert]

I'm sure that's what Robert Forward had in his books. And becuase his Ph.D. in Physics involved work in gravitation, I always assumed he knew what he was talking about.

My copies of all of Forward's books are at my dad's house so I haven't read them in years, but they do each have a fairly detailed technical appendix. The first book's appendix probably goes through the stability of the shared atmosphere.

[Themohawkninja]

Correct me if I'm wrong, but this would basically be a type of LaGrange point. You'd probably need an ion drive for course corrections, but it seems perfectly feasible.

[Themohawkninja]

Are you sure? What about the Roche Limit? I mean, I'm sure it works a little different when two bodies are almost equally massive, but it's hard to imagine the tidal effects not to be catastrophic.

It would depend on where the bodies are, and their mass. From what I can see (I just looked it up on Wikipedia), if you had two bodies of near equal mass, far enough apart that their Roche limits weren't inside each other, than there shouldn't be a problem.

So, from the requirements that the OP has for the planets, you'd need two Earth sized planets (given what we know about where life can flourish, let's not start a massive debate in THIS thread at least) that would need to be greater than:

2.44*6378(5.54/5.54)^1/3

2.44*6378(1)^1/3

2.44*18.54

45.23

45.23 km apart... okay that's WAAY to close to be the right answer. Anyone else want to try the math?

[Sathurn]

Look like you did the math right, didn't check you though.

Looking up the Roche limit to learn what it is. Done

This is the closest theoretical possible, for a liquid.

[andrewas]

Exponents first, remember. You took the cube root of the radius * the density ratio, it should be the radius times the cube root of the density ratio.

[Themohawkninja]

Exponents first, remember. You took the cube root of the radius * the density ratio, it should be the radius times the cube root of the density ratio.

So, if I understand you correctly, it should be 6378*(1^1/3)=6378?

That does sound a lot more accurate.

[andrewas]

Yup. Then 2.44*6378 = 15562 kilometers as a minimum. Tweak the orbit so the period matches the day you desire for your pair of planets (if they aren't tidally locked there will be huge tidal effects) and you have your twin Earth's.

[Themohawkninja]

Yup. Then 2.44*6378 = 15562 kilometers as a minimum. Tweak the orbit so the period matches the day you desire for your pair of planets (if they aren't tidally locked there will be huge tidal effects) and you have your twin Earth's.

Well, the Moon is 240,000 miles out and it's tidally locked, so you'd have to be pretty far out. That's not to say that I feel it's impossible though.

Maintaining such a station might be difficult, assuming this race is just as advanced, and has the same bureaucratic problems that we do.

[K^2]

Correct me if I'm wrong, but this would basically be a type of LaGrange point. You'd probably need an ion drive for course corrections, but it seems perfectly feasible.

Yes. It's the L1 point.

[Themohawkninja]

Yes. It's the L1 point.

Would it count as L1? Does one count as the satellite, and the other the main body?

[K^2]

L1 doesn't depend on which is which. L2 and L3 are swapped and their stability can change. L4 and L5 are likewise swapped, and their stability depends on the mass ratio. But otherwise, yeah, you basically have these 5 points defined for a pair of bodies, and it doesn't matter if they are similar in mass or not.