Electric sail
An electric sail (also known as an electric solar wind sail or an E-sail) is a proposed form of spacecraft propulsion using the dynamic pressure of the solar wind as a source of thrust. It creates a "virtual" sail by using small wires to form an electric field that deflects solar wind protons and extracts their momentum. The idea was first conceptualised by Pekka Janhunen in 2006 at the Finnish Meteorological Institute.[1]
Not to be confused with magnetic sail or photon sail.Development history[edit]
Academy of Finland has been funding electric sail development since 2007.[8]
To test the technology, a new European Union-backed electric sail study project was announced by the FMI in December 2010.[9] The EU funding contribution was 1.7 million euros. Its goal was to build laboratory prototypes of the key components, it involved five European countries and ended in November 2013.[10] In the EU evaluation, the project got the highest marks in its category.[11][12] An attempt was made to test the working principles of the electric sail in low Earth orbit in the Estonian nanosatellite ESTCube-1 (2013-2015), but there was a technical failure and the attempt was unsuccessful. The piezoelectric motor used to unfurl the sail failed to turn the reel. In subsequent ground-based testing, a likely reason for the failure was found in a slipring contact which was likely physically damaged by launch vibration.
An international research team that includes Janhunen received funding through a 2015 NIAC Phase II solicitation for further development at NASA's Marshall Space Flight Center.[2][13] Their research project is called 'Heliopause Electrostatic Rapid Transit System' (HERTS).[2][14] The Heliopause Electrostatic Rapid Transit System (HERTS) concept is currently being tested. For HERTS, it might take only 10 to 15 years to make the trip of over 100 astronomical units (15 billion kilometers). In the HERTS concept, multiple, 20 kilometer or so long, 1 millimeter thin, positively charged wires would be extended from a rotating spacecraft.
A new satellite launched in June 2017,[15][16] the Finnish Aalto-1 nanosatellite, currently in orbit, will test the electric sail for deorbiting in 2019.[17][18][19][20][21]
In 2017, Academy of Finland granted Centre of Excellence funding for 2018–2025 to a team that includes Janhunen and members from universities, to establish a Finnish Centre of Excellence in Research of Sustainable Space.[22][23]
Intrinsic limitations[edit]
Almost all Earth-orbiting satellites are inside Earth's magnetosphere. However, the electric sail cannot be used inside planetary magnetospheres because the solar wind does not penetrate them, allowing only slower plasma flows and magnetic fields.[24] Instead, inside a planetary magnetosphere, the electric sail may function as a brake, allowing deorbiting of satellites.[25]
Like for other solar sail technologies, while modest variation of the thrust direction can be achieved by inclining the sail, the thrust vector always points more or less radially outward from the Sun. It has been estimated that maximum operational inclination would be 60°, resulting in a thrusting angle of 30° from the outward radial direction. However, like with the sails of a ship, tacking could be used for changing the trajectory. Interstellar ships approaching a sun might use solar wind flow for braking.[25]