Reivindicaciones

1. A system for in-orbit propulsion via floating conductive tethers on board a spacecraft (1), characterised in that it comprises two sets of floating electrodynamic conductive tethers (2, 3) connected respectively to each of the two poles (4, 5) of an electrical power source (6), wherein each set is formed by at least one floating conductive tether, and wherein the floating tethers themselves are responsible for the capture of electrons and their emission through thermionic or photoelectric effect thereof by the floating tether itself. 2. The system, according to claim 1, wherein each set of conductive tethers formed by two or more conductive tethers has its conductors electrically connected to each other at one or more points along their length. 3. The system, according to any of claims 1, 2, wherein at least one of its electro-dynamic tethers has an end mass connected to the opposite end of the pole that connects to the electrical power source. 4. The system, according to claim 3, wherein the end mass comprises at least one of the following elements: mass of passive ballast, tether deployment system, another satellite, dock, satellite cover. 5. The system, according to any one of claims 1, 2, 3, 4, wherein one or more of its electrodynamic tethers has a substantially circular cross-section. 6. The system, according to any one of claims 1, 2, 3, 4, wherein one or more of its electrodynamic tethers has a substantially annular cross-section. 7. The system, according to any one of claims 1, 2, 3, 4, wherein one or more of its electrodynamic tethers is substantially tape shaped. 8. The system, according to any one of the preceding claims, wherein at least one of its electro-dynamic tethers is composed by a material that facilitates thermionic or photoelectric electron emission. 9. The system, according to claim 8, wherein the material that facilitates thermionic electron emission includes at least one of the following compounds: LaB<6> , CeB<6> , BaO-W, Ba-W, BaO and 12CaO-7Al<2> O<3> (C12A7:e<-> ). 10. The system, according to claim 8, wherein the material that facilitates photoelectric electron emission includes at least one alkaline metal or any of the compounds Ag-Cs<2> O-Cs, Ag-Cs<3> Sb and Na<2> KSb. 11. The system, according to any one of claims 1, 2, 3, 4, 5, 6, 7, wherein one or more of its electro-dynamic tethers is composed of a conductor substrate coated along the entire length or a part thereof with a material that facilitates thermionic or photoelectric electron emission. 12. The system, according to claim 11, wherein the material that facilitates thermionic emission includes at least one of the following: LaB<6> , CeB<6> , BaO-W, Ba-W, BaO and 12CaO-7Al<2> O<3> (C12A7:e<-> ). 13. The system, according to claim 11, wherein the material that facilitates photoelectric emission includes at least one alkaline metal or any of the compounds Ag-Cs<2> O-Cs, Ag-Cs<3> Sb and Na<2> KSb. 14. The system, according to any one of claims 1, 2, 3, 4, 5, 6, 7, wherein one or more of its electro-dynamic tethers is coated along the entire length or a part thereof with an insulating material. 15. The system, according to any one of claims 1, 2, 3, 4, 5, 6, 7, 11, 12, 13, 14, wherein at least one of its electro-dynamic tethers is substantially manufactured from materials that include at least one of the following: graphene, aluminium alloys, copper, beryllium-copper alloys. 16. The system, according to claim 15, wherein the surface of one or more of its tethers has been prepared to reach a high quotient of thermal absorptivity/emissivity in order to obtain high temperatures that favour the thermionic emission of the coating. 17. The system, according to any one of claims 1, 2, 3, 4, 5, 6, 7, wherein at least one of the tethers connected to one of the poles is coated with a material that facilitates photoelectric emission, and at least one of the tethers connected to the other pole has the segment nearest the pole electrically insulated from the plasma.