An island nation has a lot of coastal territory and has become gravely concerned about the threat of global warming. They've decided to fix this by launching reflective satellites into orbit -- similar to China's man-made moon plan, but instead of reflecting sunlight to Earth at night, they're reflecting sunlight away from Earth during the day.

Is this viable?

I assume it's not actually viable since real-life geoengineering efforts seem to be mostly interested in adding reflective material to the atmosphere. But what's the problem?

Does it take an unreasonable number of satellites? Are the satellites too expensive even when bulk-manufactured? Are the satellites too expensive to launch into orbit? Do the satellites get damaged too quickly by orbital debris? Is there some sort of weird thing where the satellites act as solar sails and the pressure pushes them out of orbit? Or is it just that the satellites fly over countries that would prefer not to suffer eclipse conditions most of the time?

Could we do this?

Yes, we could. It's been proposed before in a number of forms. Most calculations agree that this sunshade would need to reduce solar insolation by anywhere from 2-10%. If we take an optimistic figure - the lower bound of 2% - then we could achieve this by putting a shade 4.5 million square kilometers in area at the Sun-Earth Lagrange point, $L_1$. Alternatively, we could send 16 trillion tiny satellites there, or put a 1 million square kilometer lens at $L_1$.

What are the problems?

This might be technically feasible. However, there are some barriers to it.

• Given current launch costs, these plans would require perhaps hundreds of billions of dollars.


• 
  $L_1$ isn't stable; we'd need to continuously adjust the orbit of every object we place there. This costs money, too.


• You'd probably have to get the agreement of most countries before undertaking a project that would affect literally the entire planet.

Physics says that yes, this can work. But it would require orbital readjustment, a large amount of money, and international agreements.

Other notes

As jamesqf noted, this plan would not reverse effects of global warming like ocean acidification. These are directly related to the concentration of greenhouse gases such as carbon dioxide and methane, and we would need to reduce levels of those if we wanted to avoid certain catastrophic effects.

Additionally, it wouldn't be great to put the satellite(s) in orbit; they have to go to $L_1$. Why? Satellites spend half of their time on the daytime side of Earth and half of their time on the nighttime side of Earth. This means they would only be blocking sunlight for half of their orbits. Putting them at $L_1$ is much more effective.

As a final thing to keep in mind: Placing these objects at $L_1$ won't cause an eclipse or any similar effect. The umbra will not reach Earth. Instead, the satellites will simply decrease the overall amount of sunlight we get, rather than completely blocking any particular area.

Just to throw some numbers out, the space shuttle could deliver a 65,000 pound payload into space. To do this, it used 2.8 million pounds of fuel.

Sources:

• 
  NASA Space Shuttle FAQ: 1,607,185 pounds of fuel in the External Tank and internal.


• 
  Cool Cosmos: 1.1 million pounds in each rocket booster (two); 65,000 pound payload.

Space X's Falcon 9 produces an estimated 250 thousand kilograms of carbon dioxide per launch and has a payload of 13,000 kg. Using the energy content of kerosene, that's about four billion BTUs per launch.

So 13 Mg of reflectives would have to more than counteract four billion BTUs plus the greenhouse effects of 250 Mg of carbon dioxide. Plus of course the amortized carbon cost from building and operating the Falcon 9 (e.g. moving it from its landing area to its launch pad).

One proposal would use 16 Tg of reflectives. That's more than a million Falcon 9 launches.

This would be much more feasible if the reflectives were built in space and moved into position by solar sail.

Some will point out that we could instead burn hydrogen. Of course, that produces water vapor, another greenhouse gas. It's less of a greenhouse gas than carbon dioxide, but hydrogen is also less energy dense than kerosene.

Just use glitter!!

There are some real world proposals to use a cloud of glitter as a mirror

https://www.jpl.nasa.gov/news/news.php?feature=4553

In the proposed Orbiting Rainbows system, the small cloud of glitter-like grains would be trapped and manipulated with multiple laser beams. The trapping happens because of pressure from the laser light -- specifically, the momentum of photons translates into two forces: one that pushes particles away, and another that pushes the particles toward the axis of the light beam. The pressure of the laser light coming from different directions shapes the cloud and pushes the small grains to align in the same direction. In a space telescope, the tenuous cloud would be formed by millions of grains, each possibly as small as fractions of a millimeter in diameter

It only stands to reason if you can use it to capture images, you can also use it to reflect unwanted sunlight. Then it's just a matter of scale.

Another big advantage is, think if it gets hit with orbital debris or meteorites. So what it punches a hole in a cloud of glitter. Adjust the lasers a bit, no hole.

A Bonus Idea

As a bonus idea, it would be worth while to convert the sunlight into electricity. In this example it would take more advanced "glitter" but the biggest stumbling block would be transmitting the power, not actually generating it.

But this way, not only are you "shading" the planet and reducing the temperature, your also supplying a large amount of (essentially free) solar electricity. Which would also be beneficial to any climate change scenario as it would cut down on the use of power generation on the planet (fossil fuels, nuclear waste etc..)

Cheers!

**Satellite Design **

If you wanted to use satellites to decrease climate change, then you would either need lots of very large, reflective, and heat absorbent satellites. Another way that you could use satellites is by having them use some kind of filter or collection device within the atmosphere that would gather up pollutants like carbon dioxide for the satellite to then retrieve and find some way to dispose of.