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0 pointsTuna-Fish7y ago0 comments

Earth-Sol L1 is probably a bad place for a solar shade. That's where a solar shade of any given size is the most efficient, but the cost of getting it there is much more than the cost of having more, less efficient ones.

A better choice is something akin to a semi-sun-synchronous Molniya-style-orbit. That is, orbit that is somewhat highly elliptical (so a satellite on it spends most of it's time near apogee, and zips through the perigee relatively quickly), and which precesses so that the apogee is always between the earth and the sun.

A satellite on such an orbit can easily spend >70% of it's time between the earth and the sun, and you could send many satellites there for a similar cost to putting just one to Earth-Sun L1.

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tgb7y ago

Is L1 the most efficient for a fixed size? The difference is ~6000 km for LEO versus 1.5 million km for L1 to the Earth's center and the r^2 law means you need to put a lot more area in L1 than in LEO, even after discounting for the fact that in LEO it'll be on the wrong side of the Earth half the time. Filling up appreciable fractions of LEO might have it's own problems.

Retric7y ago

r^2 is not that relevant. The moon is 384,400 km from the earth and can cause a total solar eclipse despite only being 3,474 km wide where the earth is 12,742 km wide.

The geometry works out more or less linearly with distance. So 1.5 / 0.384 = 1.065. Which means and object 1.065x as wide as the earth would also provide a total solar eclipse. Making sunshades more than 2x effective L1 vs LEO.

PS: Only 1/2 the time they are on the sun facing side, but they are not perpendicular with sunlight for that entire half. At a 45 degree angle a sunshade blocks ~70% as much sunlight vs a 0 degree angle.

Retric7y ago

LEO to L1 is not that bad using solar sails or ionic propulsion.

There are going to be real trade-offs to any orbit, but I think the minimum cost is going to be above X mass in L1 means X mass into LEO as you can't use less mass than that in any other orbit.

Tuna-FishOP7y ago

But then you have to put solar sails or ionic propulsion into the shade.

BFR could deploy a completely passive sail directly into such a sun-synchronous elliptical orbit, return to land, and launch again after a few hours.

Retric7y ago

Anything that blocks that much light is going to be a huge solar sail and need a lot of station keeping.

The only reason ionic propulsion might be considered is the they are already going to be solar panels. If you make a large chunk of them from solar panels you then have crazy energy for space based industry and plenty of energy for ionic propulsion.

Also, with that much mass in orbit you are going to want some way do move dammaged segments.

elihu7y ago

Supposing you can re-orient parts of the "huge solar sail" then wouldn't it become rather like the sail of a ship or the wing of a plane, and be usable as a control surface for maneuvering?

I'd imagine that in the neighborhood of L1 you would try to find some equilibrium point a little closer in to the sun so that the force of the solar wind matches gravitational pull to the sun. If you start drifting in to the sun, you expose more surface area, and if you're drifting towards the Earth, you re-orient the sail to absorb less solar wind.

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ninkendo7y ago

How are you going to use a solar sail to go towards L1 from LEO?

Retric7y ago

You don't have a keel, but you can use the suns gravity with a similar goal. The sun's gravity is much stronger than a sail's thrust so you can't hover, but you do end up in a higher orbit by aiming it at the sun. However, this averages over an orbit so you don't gain energy over time.

But, aim the sail off-center say 45 degrees from the sun and you can pick if that's adding or subtracting angular momentum from your orbit over time. Reduce angular momentum and gravity pulls you into a lower orbit and thus more orbits/year. Reverse that and your gain angular momentum and get fewer orbits per year. L1 is a special case as it's balanced between the sun and earth so you have the same number of orbits per year.

Now, rather than just orbiting the sun your also orbiting the earth. But, similar manovers work just fine.

PS: It's even more complected as you end up in elliptical orbits if you don't keep things balanced etc, but that's the basic idea.

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tgb7y ago

Retric7y ago

r^2 is not that relevant. The moon is 384,400 km from the earth and can cause a total solar eclipse despite only being 3,474 km wide where the earth is 12,742 km wide.

Retric7y ago

LEO to L1 is not that bad using solar sails or ionic propulsion.

There are going to be real trade-offs to any orbit, but I think the minimum cost is going to be above X mass in L1 means X mass into LEO as you can't use less mass than that in any other orbit.

Tuna-FishOP7y ago

But then you have to put solar sails or ionic propulsion into the shade.

BFR could deploy a completely passive sail directly into such a sun-synchronous elliptical orbit, return to land, and launch again after a few hours.

Retric7y ago

Anything that blocks that much light is going to be a huge solar sail and need a lot of station keeping.

Also, with that much mass in orbit you are going to want some way do move dammaged segments.

elihu7y ago

Supposing you can re-orient parts of the "huge solar sail" then wouldn't it become rather like the sail of a ship or the wing of a plane, and be usable as a control surface for maneuvering?

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ninkendo7y ago

How are you going to use a solar sail to go towards L1 from LEO?

Retric7y ago

Now, rather than just orbiting the sun your also orbiting the earth. But, similar manovers work just fine.

PS: It's even more complected as you end up in elliptical orbits if you don't keep things balanced etc, but that's the basic idea.

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