Electric Sailing under Observed Solar Wind Conditions

Abstract

In this paper, sailing and navigation in the solar wind with a spacecraft powered by an electric sail is addressed. The electric sail is a novel propellantless spacecraft propulsion concept based on positively charged tethers that are centrifugally uncoiled and stabilised to extract the solar wind momentum by repelling the solar wind protons. Steering of such a sail ship is realised either by changing the tether voltage or the sail spin plane. To model the solar wind, we use spacecraft observations for the density and wind speed at 1 AU and assume that the speed is constant and density decreases in square of the distance from the Sun. Using the electric sail thrust formula, we describe the sail response to the solar wind variations, especially, the self-reefing effect leading to a smooth spacecraft acceleration even during periods of large densities or fast winds. As a result, the variations of the acceleration are statistically small relative to the density and wind speed variations. Considering the navigation, we adopt an optimal transfer orbit to Mars originally obtained for constant solar wind speed and density. The orbit and associated sail operations including a coasting phase are then used as the navigation plan to Mars. We show that passive navigation based only on the statistical results is far too inaccurate for planetary missions and active navigation is required. We assume a simple active navigation system that monitors only the actual orbital speed with an onboard accelerometer and matches it with the optimal orbital speed by altering the tether voltage independently from the future solar wind conditions. We launch 100 test spacecraft with a random launch date and show that with the active navigation 85% (100%) of the spacecraft reach a distance relative to Mars less than about 10 (70) Mars radii with a residual speed less than 20 m/s (80 m/s). As a conclusion, the electric sail is highly navigable and it suits for targeting planets and asteroids, in addition to broad targets such as the heliopause.

Electric solar wind sail mass budget model

“…The maximum voltage for which the hardware is designed should be larger, perhaps 40 kV, because otherwise the thrust would be decreased when the solar wind density is lower than its average value (Toivanen and Janhunen, 2009). Trading off the hardware voltage limit against other design parameters is outside the scope of this paper, although we will explore the effect of the value of V 0 at the end of this section.…”

“…Even though the solar wind density n exhibits large natural variations, a simple strategy of varying the tether voltage V away from the nominal V 0 , such that P eg is constant, is quite effective for maintaining constant the daily, weekly or monthly averaged thrust at a given solar distance r (Toivanen and Janhunen, 2009). Two plasma physical effects are responsible for this, at first, surprising behavior.…”

“…Actually, the solar wind properties vary widely along, basically, all relevant timescales. However, due to certain plasma physical effects such as Debye length scaling, the E-sail propulsive thrust tends to vary much less than the solar wind dynamic pressure when a simple constant power strategy is applied to adjust the tether voltage in response to solar wind density variations (Toivanen and Janhunen, 2009).…”

Electric sails are potentially more effective than light sails near most stars

“…The force per unit length (dF/dz) generated by an electric sail has been estimated by Ref. [58] and [59] to be…”

E-sail test payload of the ESTCube-1 nanosatellite

“…The sail is kept open with the help of centrifugal force, the spacecraft must therefore be kept in rotating motion. Once operational, E-sail technology is expected to revolutionize the space travel within our solar system [2][3][4][5][6][7][8][9].…”