# Project Overview

This introduction is adapted from the Discovery Channel web site.

## Space Sunshield Dummies' Guide

Astronomer and professor Roger Angel thinks he can diffract the power of the sun by placing trillions of 2-foot lenses in space. He wants to create a 100,000-square-mile sunshade using electromagnetic propulsion to get the lenses into space.

Angel has produced a diffraction pattern that will be etched onto each lens. The pattern will cause the sun's rays to change direction, lessening the amount of harmful rays hitting the Earth's atmosphere.

Test 1:  Visual Light Refraction
Show light refraction ability of etchings using larger lenses, lights, smoke and lasers.

Test 2: Drop Tests
Drop a cradle holding lenses from a tall building to see whether the lenses survive.

Test 3: MEMs Demo
Demonstrate MEMs technology at NASA's microdevices laboratory.

Test 4: Centrifuge
Experiment in a centrifuge to see whether the lenses will withstand high g-forces.

Test 5: Coil Gun
Launch lenses from a coil gun, a form of magnetic propulsion, to test the viability of using this method to put the lenses into space.

Test 6: Rocket Test
The team will launch a 43-foot-high rocket to an altitude of around 11,000 feet. The payload will be released from the escape tower and the cradle will return as a lander vehicle on a parachute. The lander will contain a cradle that will hold the lenses. The aim is to keep the lenses intact.

### Experiment Assumptions

* Why not use mirrors instead of lenses? In theory, a mirror would do exactly the same job as the lenses. However, sunlight (solar radiation) exerts a small force, and as a mirror blocks/reflects light back, the mirrors will be pushed out of position. A lens (i.e., something that will alter the light but allow it to pass through) is more suitable as the sun's energy will not push them out of position.

* As light passes through the lens, a special microscopic pattern made of up of thousands of holes would diffract light waves away from dead center. The plan would be for the lenses to be placed directly  between the sun and Earth, causing the lenses to diffract the sunlight away from Earth.

* The lenses should be placed at a position known as a Lagrangian or L1-point, 1 million miles away from Earth.  An L-point is a position in space where there is no gravitational influence from any planetary or solar bodies— in this case the sun and Earth — so an object will not be drawn toward either through the effect of gravity. In other words, it will stay in position.

* This means that the lenses — as well as not being pushed by solar radiation — can "hang" in space for many years without being driven out of their important climate-saving position.

* The lenses will require coordination to get into position, and to make sure they don't overlap or bunch up together. Three tiny devices called MEMS (micro-electro mechanical systems) attached to the circumference of each lens using solar power will "drive" each unit. Additional spacecraft will offer GPS coordination to allow the lenses to adjust their position.

* Once delivered into space, the lenses will form a "cloud" 62,000 miles long and a height roughly equal to Earth's (8,000 miles). Sixteen trillion lenses with a diameter of around 2 feet each will be required. The cloud will cover a million square miles.

* The lens cloud will only be partially covered by the lenses — only 1.8 percent. Therefore, 98.2 percent of sunlight will pass through the cloud unhindered, while 1.8 percent will be diffracted. This is the predicted amount needed to mitigate climate change.

* In order to keep costs down, the lenses will need to be as light as possible. They will therefore have to weigh around .04 ounces (1 gram).

* Space is a tough environment; the lenses need to be able to survive the 24/7 heat of the sunlight without changing its properties (becoming opaque) or breaking. The material needs to be light, inorganic and crystalline in structure so it will be tough enough for the task.

* The most easily sourced material that has these properties is silicon nitride, which can be manufactured as part of the process in the silicon wafer industries (i.e., microchips and transistors).

* The lenses (or membranes, as they are called) are manufactured by placing a photo-resistant mask on top of a very thin layer of silicon substrate. Roger Angel's pattern is illuminated onto this mask, and then the pattern etched onto the lenses by means of an acidic solution. Timing is crucial — if the lenses are left too long in the alkaline solution they will be destroyed; not long enough, and the optical properties will be wrong.

* So far, the only way humans have escaped Earth's atmosphere is by using rockets and fossil fuels. The approximate cost of getting any payload to a position like the L1 point is currently \$20,000 per kilogram (2.2 pounds). This would make the space sunshade economically unfeasible. Angel thinks a coil gun is the answer.

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