The goal of sending probes anywhere in the Solar System in reasonable time has remained elusive. Finnish scientists have invented a new propulsion method which utilises the solar wind, promises high speed for small payloads and may be technically possible to build in the near future.
The solar wind is a very tenuous but high speed (300-800 km/s) plasma stream blowing radially outward from the Sun. The solar wind powers the aurora and governs space weather. The average dynamic pressure (force per unit area) of the solar wind is 2 nanopascal, corresponding to 0.2 grams weight per square kilometre.
Using such a weak dynamic pressure for pushing a spacecraft requires a very large area sail, much larger than what can be provided by a solid surface. In the electric sail, the sail is formed by an electric field existing around a thin, charged tether whose voltage is maintained by an onboard solar-powered electron gun. A 20-km long tether made of wire which is thinner than human hair fits in a small reel, but gives a square kilometre effective area when stretched out in space and charged. In the paper published today in Annales Geophysicae, two-dimensional first-principles plasma simulations run on a supercomputer were used to compute the thrust per unit tether length in different solar wind conditions and tether voltages to check the feasibility of the method. Theoretical analysis and one-dimensional simulations were used to validate the results.
The results indicate that ~50 nN/m force per unit length of the tether can be achieved in average solar wind, which could enable final speeds in the range 50-100 km/s (10-20 AU/year) for a lightweight spacecraft. At such high speed one could reach e.g. Pluto in less than four years and fly out of the heliosphere into interstellar space in less than 15 years. Because the electric sail needs no propellant or other consumables, it might also provide cheap transportation of e.g. raw materials such as water mined from asteroids and used for in-situ fuel making at high Earth orbit.
Janhunen, P. and A. Sandroos, Simulation study of solar wind push on a charged wire: solar wind electric sail propulsion, Annales Geophysicae, 25, 755-767, 2007
Kumpula space centre website on electric sailing
Pekka Janhunen and Arto Sandroos, Finnish Meteorological Institute/Space Research,
email: pekka.janhunen at fmi.fi