SPACE-BASED SUNSHADES

Over the past decade, global warming has slowly become recognized as an inevitability. However, debate is still ongoing as to its exact eventual impact, especially how severe its consequences will be for the global environment and human civilization as a whole. To combat the worst of the effects, a number of radical strategies have been put forward, but few are as ambitious as plans to block a significant portion of Earth’s sunlight from space.

Further in the future, space-based sunshades could have practical applications beyond just cooling off our home planet. This technology could also be used to reduce the average temperature of hothouse planets such as Venus in anticipation of their eventually terraforming, or in shading large collections of artificial habitats or other space structures from the sun’s radiation.

Looking to the possibility of a worst-case scenario, where the Earth is headed irrevocably towards a catastrophic ecological collapse in a period of ten to twenty years, Professor Roger Angel of the Steward Observatory Mirror Laboratory and the Center for Astronomical Adaptive Optics has a radical proposal.

Angel’s plan is to launch billions of small optical glass disks to the L1 Lagrange Point between Earth and the Sun, located about a million miles or so inward toward the star. Two feet in diameter, 1/5000^th of an inch thick, and weighing less than an ounce, these mirrors would divert up to two percent of incoming sunlight away from Earth if employed in large enough numbers. They would be attached to miniature solar sail panels, which can be tilted and tacked to maintain their position using radiation pressure alone.

This maneuverability is important, as the L1 point is only metastable, which means that even a tiny outside force--like the solar wind--would send an object drifting away. The swarm would also be widely dispersed, forming a very rough corridor some 100,000 miles long between Earth and the sun, making drift very likely in a large portion of the mirrors. Also, the disks would need to maintain proper alignment to intercept the incoming sunlight effectively.

The glass disks would not outrightly block the sunlight, but rather refract it like a lens a few degrees so it avoids hitting Earth. Diverting just 2% of incoming sunlight would reverse the current trend of rising global temperatures, reducing the ambient warmth in the atmosphere within a year back to the average global temperatures found in the twentieth century.

The exact number of disks needed is estimated at 16 trillion, with the total shade massing as much as 20 million tons. With current launch costs hovering around $10,000/pound this could prove prohibitively expensive and difficult to engineer.

Angel proposes using electromagnetic launch guns on mountaintops to cut the cost as much as possible, with vehicles carrying a stack of one million glass flyers launching every few minutes. Even so, Angel estimates the project would cost over $4 trillion dollars (about 1/3 of the money generated annually by the US economy) and take 30 years to complete even with full international cooperation and dozens of launch guns working around the clock. However, compared to the hundreds of trillions of dollars in damage and the 150 million-plus refugees predicted by the most pessimistic projections of global warming, creating the sunshade may be a bargain. Convincing politicians and corporations of that would be another matter.

Angel has emphasized that even if the his sunshade concept was ever actually employed, it would only be a temporary solution, and would fully degrade within 50 years after the first launch if not periodically replenished. He has repeatedly emphasized that the only real solution to global warming is renewable energy sources and reducing greenhouse gas emissions.

A heliogyro is a specialized type of solar sail. Rectangular vanes dozens or hundreds of miles long radiate out from a central hub, making the vessel resemble an enormous space-borne pinwheel. The vanes are kept rigid by the craft’s rotation, and sections of them can be individually canted and tilted to catch the incoming sunlight at different angles, allowing it much greater maneuverability than other types of solar sails.

Because of their enormous size (the surface area of their sails would usually measure hundreds or thousands of square kilometers,) a fleet of several dozen to several hundred such craft parked at the L1 point would do much the same work as trillions of Robert Angel’s glass disks. By tacking against the light pressure from the sun, they can maintain their position for long periods of time without the need for onboard fuel-consuming engines.

However, creating such vehicles in quantity will not be possible without some kind of established orbital manufacturing infrastructure. Most of the raw materials would either have to be lifted from the Moon or nearby asteroids; lifting the materials from Earth wold be even more prohibitively expensive than the Glass Disk Sunshade.

A variant of this idea was used in the novel Mother of Storms by John Barnes.

A comet, when it nears the sun, undergoes extensive outgassing, creating the celebrated "tail" of dust and ionized particles behind it. This tail is often thousands of miles wide and can reach millions of miles in length.

This unusual proposal postulates taking advantage of this phenomenon by parking a number of small comets between the Earth and the Sun at the L1 point. The comet tails would "blow" back away from the sun and toward Earth, creating a "shade" of particles a million miles thick. Though the gas and particulate in comet tails is extremely tenuous, the cumulative effect of several dozen in close proximity could block out a small portion of sunlight intercepting the planet.

The main hurdle here is diverting the comets and maintaining their position at the metastable Lagrange point. Even small objects of this class tend to mass millions of tons and are irregularly shaped, making precision maneuvering difficult to say the least. Using the impulse from a nuclear detonation can send them on their way, and carefully planned trajectories could deliver them to L1, but maintaining them at that position for a long period of time could prove very problematic. Large arrays of long-endurance ion engines would have to be mounted on each one, and the refueling and maintenance of them would have to be ongoing.

One of the main disadvantages of this scheme, aside from the inherent difficulties of playing cosmic billiards with million-ton snowballs, is that all the outgassing from the comets would create a tenuous cloud of comet tail particles that become trapped by Earth’s gravitational field. The longer the sunshade would be needed, the thicker this cloud would become. The ionized particles especially, suspended in Earth’s magnetic field, could remain aloft indefinitely. It would eventually start to interfere with satellite and radio communication, and if the sunshade is left in place long enough (decades or more), it could even add an eerie glow of scattered light to the night sky.

Even if proven feasible, this scheme could not be implemented until comet and asteroid diversion on a large scale has been proven as a viable technique.