|Fig. 1: Schematic for a Dyson-Harrop satellite. |
The sun is the largest source of energy on Earth. As such, it is not unexpected that scientists searching for alternative energies often turn to solar power. This solar power can be harvested with photovoltatics stationed on the surface of the Earth or, conceivably, mounted on satellites in orbit. Alternatively, scientist are looking to utilize a more exotic feature of our sun, the solar wind.
Solar wind is a stream of charged particles that originates in the upper atmosphere of the sun.  These protons and electrons are ejected from the corona at approximately 400 km/s due to the extreme temperatures at the surface of the sun. [1,2] As the force of gravity decreases at further distance from the sun, it is no longer strong enough to contain the highly conductive hot gases at the outer layers of the corona.  The resulting stream of particles consists primarily of fast and slow streams. The fast solar wind travels at approximately 750 km/s and has a composition similar to the sun's photosphere.  Alternatively, the slow solar wind travels at approximately 400 km/s and has a composition more similar to the corona. The temperatures of these two streams are 8 × 105 K and 1.4 - 1.6 × 106 K for the fast and slow winds respectively. In interacting regions where the high speed wind catches up to the slow, magnetic clouds form. While the complete details of precisely how and where the coronal gases are accelerated to supersonic speeds, scientists still postulate potential uses of these charged particle streams such as solar sails and, more recently, solar wind power.
Brooks L. Harrop and Dirk Schulze-Makuch of Washington State University recently proposed a solar wind power satellite to function as an alternative to the futuristic Dyson sphere.  Their 8,400 km wide solar sail would theoretically generate 1 billion billion gigawatts of power, magnitudes higher than the energy needs of the Earth. The suggested device consists of a copper wire pointed toward the sun; the wire is charged to generate a cylindrical magnetic field which would collect electrons found in the solar wind (Fig. 1).  The electrons are then funneled into a metal spherical receiver to produce current. Part of this collected current is fed back into the copper wire to generate the magnetic field. Thus, the system is self-sustaining. Any power that is not require to maintain this magnetic field would be available for transport back to Earth. Once the current has been drained of its electrical energy, the electrons can fall onto a ring-shaped sail, where sunlight can re-energise them enough to maintain the satellite's orbit around the sun. Theoretically, a device consisting of 300 meters of copper wire and a 10 meter wide sail could generate enough power for 1,000 homes. However, these 1,000 homes are a great distance away from the satellite generating power. Herein lies the main technical deterrent to Harrop and Schulze-Makuch's proposal; how do you get the power back to Earth?
Harrop and Schulze-Makuch propose the use of an infrared laser trained on satellite dishes back on Earth to transport generated power.  An infrared beam was selected because it has the ability to penetrate the Earth's atmosphere. However, the laser beam has a stupendous distance to travel before it even comes close to Earth's atmosphere. The solar wind satellite would need to be in orbit millions of kilometers from the Earth. The Advanced Composition Explorer (ACE) satellite was launched in 1997 with the purpose of detecting solar wind.  It was placed at the first Lagrange point (L1), one of the locations where the gravitational pulls from Earth and the sun are opposite but equal; L1 is approximately 1.5 million kilometers from Earth. The distance to any location where these solar wind satellites could orbit would be on the same order of magnitude as the distance to L1. Over this great of a distance, even the most focused laser beam would spread out drastically and lose enormous amounts of power before reaching Earth. The power of the spread beam when it reaches Earth would "less than moonlight."  Thus, to make solar wind power satellites remotely feasible, a far superior laser consisting of an enormous, nearly perfect lens would need to be developed.
© Katie Sokolowsky. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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