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This also makes for a somewhat spooky dystopian science fiction. Society engineers a gigantic sunshade. Something goes wrong and they lose contact with the sha

This also makes for a somewhat spooky dystopian science fiction. Society engineers a gigantic sunshade. Something goes wrong and they lose contact with the shade unable to adjust it. Leading to crop failures, ecosystem collapse, and a global ice age. Frantically, the world governments race to launch a mission to repair or destroy the shade before life on earth is plunged into a permanent ice age from which a collapsed technological civilization could never hope to avert. That is, this thing should have a dead man's switch on it, and if it isn't renewed every few years, it will self destruct, alter course, stop station keeping, etc Of course, this is probably an implausible scenario but it would make for a great TV movie.

sbierwagen 11y ago
Radiation pressure will push it out of the L1 point pretty quickly (months?) without active stationkeeping. Tidal forces will want to rotate it edge-on to the Earth, and the tug of the Moon's gravity will want to pull it out of alignment between the Sun and the Earth. "Causing a ice age" is pretty rich, too. It can only block 1.7% of the light the Earth gets. If it gets any closer to the Earth, then it ends up in a non-geosynchronous orbit, (Well, not strictly geosynchronous. You know what I mean) and stops blocking the Sun. All the failure modes of a L1 sunshade are pretty benign.
- emiliobumachar 11y ago
  I believe gp was talking about a failure mode in which station-keeping goes on automatically when communication fails, as we learn that we massively overdone it because climate science is hard.
  - cromwellian 11y ago
    Right, I was talking about a sci-fi failure mode where the onboard program goes haywire and it ends up actively station keeping and maintaining the block for hundreds of years. Is 1.7% not something to worry about? IIRC, Solar variance of 0.1% has correlated with a 0.32F temperature variance. If that relation were linear, that would amount to almost a 5.5F variance. I'm not proposing this as a 'hard scifi' model for a story, but you could tweak the scenario to get suitably scary failures (multiple shades)
- tgb 11y ago
  Can you account for radiation pressure by modifying the orbit? It's a constant radial force. Searching for Lagrange points is searching for points where (in the rotating reference frame fixing the Earth) the combined gravitational forces of Earth and Sun are zero. Just add in the radiation pressure and you should get a Lagrange point slightly closer to the sun. Buut I haven't done the math so this is speculation.

sbierwagen 11y ago

Radiation pressure will push it out of the L1 point pretty quickly (months?) without active stationkeeping. Tidal forces will want to rotate it edge-on to the Earth, and the tug of the Moon's gravity will want to pull it out of alignment between the Sun and the Earth. "Causing a ice age" is pretty rich, too. It can only block 1.7% of the light the Earth gets. If it gets any closer to the Earth, then it ends up in a non-geosynchronous orbit, (Well, not strictly geosynchronous. You know what I mean) and stops blocking the Sun. All the failure modes of a L1 sunshade are pretty benign.

emiliobumachar 11y ago
I believe gp was talking about a failure mode in which station-keeping goes on automatically when communication fails, as we learn that we massively overdone it because climate science is hard.
- cromwellian 11y ago
  Right, I was talking about a sci-fi failure mode where the onboard program goes haywire and it ends up actively station keeping and maintaining the block for hundreds of years. Is 1.7% not something to worry about? IIRC, Solar variance of 0.1% has correlated with a 0.32F temperature variance. If that relation were linear, that would amount to almost a 5.5F variance. I'm not proposing this as a 'hard scifi' model for a story, but you could tweak the scenario to get suitably scary failures (multiple shades)
tgb 11y ago
Can you account for radiation pressure by modifying the orbit? It's a constant radial force. Searching for Lagrange points is searching for points where (in the rotating reference frame fixing the Earth) the combined gravitational forces of Earth and Sun are zero. Just add in the radiation pressure and you should get a Lagrange point slightly closer to the sun. Buut I haven't done the math so this is speculation.