Speaking at the at the British Science Festivalin Guildford, Professor Colin McInnes of Strathclyde University explained his work at the leading edge of deep space propulsion technology. He described how solar forces can be divided into two types: solar wind, which is produced by protons and electrons, and radiation, produced by photons. The pressures produced by radiation are three orders of magnitude greater than those produced by solar wind.
This potential to harness the pressure of the sun’s radiation allows physicists to predict the feasibility of solar sailing since deflection produces twice the energy of absorption.
According to Friedrich Zander this thrust effect could be used as a fuel-less form of space propulsion. With large, light-reflective sails, which would need to be about the size of a football field, this thrust could be used to propel a space craft. Changing the angle of the sails to the stream of radiation from the sun could allow the craft to manoeuvre, orbiting closer in to, or further out from the light source, or changing its plane relative to a sun or planet.
The forces of solar pressure are small and decrease rapidly the further the sails are from the source of light, but still could build up acceleration over time and make it possible to travel through space relatively quickly and cheaply without needing to carry large amounts of fuel. In the vacuum of space where there is no resistance to overcome, these craft could travel at up to 45 kilometres per second.
This would make it possible to travel to other planets within the solar system.
Using solar pressure, satellites will be able to adjust their orbits around the earth which were previously fixed by the combined gravitational pull of Earth, sun and moon. A satellite would be able to take up a position observing one spot on earth rather than hanging in stationary orbit waiting for the Earth to revolve beneath it. This would make data collection much cheaper and improve weather forecasting.
Small auxiliary vanes have been suggested to give greater manoeuvrability and the gravitational pull of nearby planetary bodies can also be used. Zone plates and masers have been suggested to give greater manoeuvring capacity.
Many attempts have been made to put these elegant theories to good use.
Since the pressure exerted by the sun’s rays is barely perceptible, the sails need to be of minimum mass. Reflective Mylar sails, a hundred times thinner than a human hair, have been designed to unfurl automatically in space. But even the thinnest sails of useful size would take up a cubic metre of space when folded ready to be deployed once orbit is reached. Both strut-based and parachute-based sails have been unsuccessful as they have been too fragile to survive deployment.
Radiation pressure is so small it does not work in the same way as aerodynamic pressure, so hot spots can form leading to bagging or wrinkling, which damage the sail and make its performance unpredictable. Nano tube mesh weaves with spaces of less than half the wavelength of light have been proposed but so far can only be produced in laboratory conditions,
Solar wind has also been proposed as a propulsive force using magnetic or electrically charged fields around conducting wire tethers, but this would limit manoeuvrability severely. NASA has tested these theories in a vacuum chamber, but it costs hundreds of millions of pounds to put even one sail in space.
Tests have been attempted unsuccessfully for over a decade, and more are continuing, including NASA’s FASTSAT, launched in Alaska in November, 2010.
Meanwhile in May 2010, IKAROS , the world’s first spacecraft designed to use solar sailing propulsion, was successfully launched by Japan and is proceeding towards Venus.
http://www.britishscienceassociation.org/web/BritishScienceFestival/
http://www.strath.ac.uk/mecheng/?id=97
http://www.strath.ac.uk/press/newsreleases/2007archive/headline_78369_en.html
http://www.guardian.co.uk/science/blog/2009/sep/10/solar-sail-space-exploration
http://www.guardian.co.uk/science/satellites
http://www.jspec.jaxa.jp/e/activity/ikaros.html
http://www.nasa.gov/mission_pages/smallsats/fastsat/10-156.html
http://www.space.com/8416-japanese-solar-sail-headed-venus.html
http://www.rsc.org/chemistryworld/Issues/2009/July/HoistingTheSolarSail.asp
