## August 22, 2013

### Statites close to the Sun

Statites close to the sun could intercept large amounts of solar energy without having a large size. The concentrated solar energy could be directed to power various space applications. This might be done primarily with passive structures instead of converting from photons to electricity or microwaves for beaming. Very large lens and terawatt and petawatt laser beams have been proposed for pushing solar sails for interstellar travel. Achieving the capabilities of that level of beams and lens might be easier by going for statites that are close to the sun. Robert Forward had a 50,000 kilometer diameter lens in his proposed architecture for the starwisp. Geoffrey Landis did propose revised designs which would have far smaller lens and lower power requirements. Statites close to the sun could enable higher power levels as part of architecture of a more manageable size.

Bolonkin has a paper where he discusses the forces on a solar sail that passes very near the sun. He discusses the maximum speed for a solar sail based on those forces while surviving the heat. A Statite would be mirrored solar sail that balances the gravity and light pressure to stay in one position relative to the sun.

The solar radiation pressure is very small 6.7 Newtons per gigawatt. That equals 9.12 X 10^-6 N/m2 at Earth's orbit (1 AU - Astronomic Unit = 150 million km) and decreases by the square of the distance from the sun. However, the solar light pressure significantly increases near sun and not far above it can reach 0.2 - 0.35 (up o.4 on Solar surface) N/m2.

The pressure of light equals P = 2E/c (where E is energy of radiation, c is light speed (c = 3X10^8 m/s)). The solar light energy at Earth's orbit equals 1.4 kW/m2, but near a solar surface it reaches up to 64x10^3 kW/m2
(it increases 47 thousand time!). As the result the light pressure jumps up to 0.4 N/m2.

Statites near the sun
- materials need to be able withstand high temperature
- need to have mirror surface for the purpose of guiding the light that is not used for keeping position

From Wikipedia the surface area of the sun
•7.5*10^13 square km

The energy from the sun 3.83 X10^26 watts

Therefore at the surface of the sun 5.1 trillion watts per square kilometer

At 1.5 solar radii from the sun, it would take 16 square kilometers to intercept 5.1 trillion terawatts.

Relatively Near term materials and capabilities that should be useful for a close to the sun Statite

Aerographite

New aerographite material is worlds lightest. It is composed of 99.99% air, aerographite has an ultra low density of just 0.2 mg/cm³ and is said to demonstrate extraordinary electrical properties.

Researchers Professor Karl Schulte and Matthew Mecklenburg created it from a network of hollow carbon tubes grown at the nano and micro scales.

According to Prof Schulte, aerographite's sparse nature means it can be compressed by a factor of a thousand, with the ability to then spring back to its original size. The material is also capable of supporting 35 times more weight than the same mass of aerogel.

Undamaged Graphene

Columbia University ccientists can grow sheets of graphene as large as a television screen by using chemical vapor deposition (CVD), in which single layers of graphene are grown on copper substrates in a high-temperature furnace. One of the first applications of graphene may be as a conducting layer in flexible displays. The graphene has a strength of 95 gigapascals. It has 90% of the strength of perfect molecular graphene and is stronger than molecular carbon nanotubes.

There are new factories that will be produce hundreds of tons of graphene by next year and about one third of them use chemical vapor deposition. Some of those factories could be adapted to use the methods that do not damage and weaken the graphene.

Solar Probe Plus launching in 2018 and getting closest to the sun in 2025

Solar Probe Plus will use seven Venus flybys over nearly seven years, using the gravity of Earth's sister planet to gradually shrink its oval-shaped orbit around the Sun. The spacecraft will will eventually come about 3.7 million miles (5.9 million kilometers) from the Sun, well within the orbit of Mercury and about eight times closer than any spacecraft has come before.

Closest approach to the sun will be about 3.7 million miles.

Spider Fab project for on orbit construction of kilometer scale objects

There is a NASA funded project to develop (currently just \$100,000 for a design) a process for automated on-orbit construction of very large structures and multifunctional components. The foundation of this process is a novel additive manufacturing technique called ‘SpiderFab’, which combines the techniques of fused deposition modeling (FDM) with methods derived from automated composite layup to enable rapid construction of very large, very high-strength -per-mass, lattice-like structures combining both compressive and tensile elements.

Bolonkin suggests a thin film of millions of prisms

Bolonkin points out conventional mirrors or multilayer dielectric mirrors have a high reflectivity only in narrow range of solar spectrum and decrease the adsorbed solar energy up 2 -5%.

There would clearly be a lot of challenges to making Statites close to sun and making reflective surfaces that can survive the temperatures but it still could be easier than

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