[dentstudent (OP)]

Contributed by Roger Angel, September 18, 2006


If it were to become apparent that dangerous changes in global climate were inevitable, despite greenhouse gas controls, active methods to cool the Earth on an emergency basis might be desirable. The concept considered here is to block 1.8% of the solar flux with a space sunshade orbited near the inner Lagrange point (L1), in-line between the Earth and sun. Following the work of J. Early [Early, JT (1989) J Br Interplanet Soc 42:567–569], transparent material would be used to deflect the sunlight, rather than to absorb it, to minimize the shift in balance out from L1 caused by radiation pressure. Three advances aimed at practical implementation are presented. First is an optical design for a very thin refractive screen with low reflectivity, leading to a total sunshade mass of 20 million tons. Second is a concept aimed at reducing transportation cost to $50/kg by using electromagnetic acceleration to escape Earth's gravity, followed by ion propulsion. Third is an implementation of the sunshade as a cloud of many spacecraft, autonomously stabilized by modulating solar radiation pressure. These meter-sized "flyers" would be assembled completely before launch, avoiding any need for construction or unfolding in space. They would weigh a gram each, be launched in stacks of 800,000, and remain for a projected lifetime of 50 years within a 100,000-km-long cloud. The concept builds on existing technologies. It seems feasible that it could be developed and deployed in 25 years at a cost of a few trillion dollars, <0.5% of world gross domestic product (GDP) over that time.

The upshot here is that he proposes something like 16 trillion satellites to be launched in a million launches over a period of 10 years..........

[Karen W.]

I find this fascinating! He really believes this to be a plausible step in protecting us for the future?

What do you think Stuart?

[dentstudent (OP)]

I'm sure the science behind it is sound. I'm not sure the practicality of sending a payload into space at the rate of 1 every 10 minutes for ten years is. But, we need this kind of thing to help push the boundaries of what we can achieve. If we don't have people with this kind of visualisation, then I think we would be poorer for it.

[another_someone]

From what I can gather, the Lagrange L1 for the Earth/Sun is about 930,000 miles away from us.  It is not an impossible distance, since we already have the SOHO satellite parked there to study the Sun.

Given the distance, I can well understand why he thinks he needs 16 trillion satellites there to make a noticeable impression on the incoming light.

The first question is, since the Lagrange point is only a single pont, if you have satellites close to, but not on, the point; you will need to design an orbital path for them, since they will not be sitting right on the point itself.

In terms of reducing costs, it would seem most reasonable to launch from the Moon rather than the surface of the Earth.

The other problem with such a large, but low density object in space is apart from the pressure of solar radiation, you also have to deal with the solar wind.

I am also not quite sure how he intends to stabilise the craft by using solar radiation pressure?  I can understand how solar radiation pressure (or the solar wind) can be used to drive the craft away from the Sun, or even perpendicular to it, but I am not sure how it could be used to take the craft closer to the Sun.  And if he intends to do extensive tacking, then the craft will need significant amount of space separating them (both to avoid collision, but even to avoid one craft becoming stuck in the lee of another craft).

Also, I would have thought that any body, or system, that is large enough to cover a substantial portion of the sky from 930,000 miles distance, would inevitably be a large target for meteorites, comets, and other bodies roaming the solar system.