Dev Tracker

Quote #7613768


LORE (Story, Background, Character, etc.)
2017-04-11 16:28:07

For my own education, if the asteroids are in an orbit around the star, wouldn't decreasing the mass change their orbital path (cause them to drift out)?


Yes, removing mass will weaken the gravitational force between asteroid and thing it orbits. The orbit will be affected.

Although changing the mass of the asteroid will decrease the force of attraction between the two bodies, the orbital path of the asteroid will not change as the mass of the asteroid has decreased proportionally.

The force of attraction due to gravity is given as

F = GMm/R^2

where F is force, G is the gravitational constant, M is the mass of the larger object (star in this case), m the mass of the smaller object (asteroid in this case) and R is the distance between the two bodies. The only two properties that will change here are m and F, which will both decrease proportionally.

As acceleration is equal to force divided by mass, which have both decreased proportionally there will be no change in the radius of the orbit as all other factors remain constant.

Another way of looking at it is comparing the above equation to acceleration due to centripetal force, which is given as

Fc = (m*v^2)/R

where Fc is centripetal force, m is the mass of the smaller object, v is the velocity of the smaller object and R is the orbital radius (same as R in the previous equation). In a stable orbit the force of attraction due to gravity will cancel the centripetal force:

F = Fc


GMm/R^2 = (m*v^2)/R

m and R cancel

v^2 = (GM)/R

So the above equation shows that the only factors that have an affect on the asteroids orbit are the velocity of the asteroid, the mass of the star and the distance the asteroid orbits at. This is the reason that satellites of greatly different size can all be put into geostationary orbit around the earth at a constant distance.

Recently DiscoLando commented on the cables between mining colonies and asteroids.


Can this be clarified as it seems to have sparked debates as to why it may be scientifically incorrect to use cables and they may actually be rigid rods.

Hi @The-Grizz,

I haven't seen any of the debates, but the idea was that the tethers were used to keep the asteroids in place while/after they'd been mined. Since the mass was changing, they might start drifting which could potentially lead to some kind of catastrophe.



Thank you for your response. I believe that would only work if the cables were stiff or otherwise had ends that had propulsion to move the asteroid away from the station to keep tension on the line. Imagine being in zero G and having a baseball attached to a string. Unless you are spinning and creating tension that way, any force on the ball towards you will move the ball that direction (such as ship impact, another asteroid, mining, etc). The string will not keep the ball from floating closer to you.

So unless the station is spinning (which it doesn't seem to be) or the cables are stiff, it makes no sense to have them there. Rule of cool may be the reason, which is fine but seems a bit odd given the degree of realism CIG is going for. If it was stated that the cables are stiff and act more like rods, it would make far more sense.

Hi @Foulwin42,

I'm not sure if the cables are meant to be rigid or not. I'd have to ask Design about their intent for that.

For my own education, if the asteroids are in an orbit around the star, wouldn't decreasing the mass change their orbital path (cause them to drift out)?


No, not unless that mass was ejected i.e there was a force applied(and thus an equal and opposite one on the asteroid). Just decreasing mass will not effect an orbit around something else unless they were both very close in mass, the only thing that would change it is if the mass of the object something is orbiting changes. So say it was a VERY large asteroid and it had satellites then say removing 20% of the mass of the asteroid would cause a significant perturbation in the stable orbits of the satellites, but wouldn't actually effect the orbit of the asteroid itself in it's own orbit presumably around a sun or even larger body.

Really I suppose if you were working on an asteroid say with a beam if you were cutting sections off the outgassing would impart a force and so that could be a reason for tethering. However that only controls one direction on one axis. You could say have them spinning so it's under tension, but that doesn't make much sense as it would be dangerous to work on and mass would naturally go the wrong direction if freed i.e. if you cut a piece of asteroid off it would move AWAY from the central point.

What would make slightly more sense is if the asteroid itself was netted and the cable was tethered to the net not the asteroid(as shown in the expanse and concepts for real world asteroid mining). So miners would be able to travel up the tether and then working inside the net mining out the asteroid, when it's depleted the net could be removed and the remnants ejected and a new asteroid towed into place and the process repeated. That would actually make a reasonable amount of sense from a fixed platform if it was quite labour or time intensive to mine asteroids or a large amount of processing is required in a very mineral rich field and so moving asteroids to the miners makes more sense than the other way around.

Hi @Khaide and @warriorscot

Just wanted to preserve your interesting comments on the ongoing asteroid mining conversation. Cool stuff.

(Also, I want to start using the term outgassing more in my everyday life.)



Source - Quote #7613768