An inertial actuator for stabilising and/or aiming a platform along a predetermined direction comprising: a fluid ring (3; 306), of a torus shape, including a central ring hole (16; 332) crossed by a central axis (18; 322), having an internal size equal to the diameter of the ring hole (16; 332), filled with a conductive liquid and having at least one air gap area (34, 36, 38; 366), capable of being crossed by magnetic field(s) and having at least one effective area (24, 26, 28) for setting the liquid into motion, and at least one magnetohydrodynamic pump (4, 6, 8; 308) for setting the conductive liquid into motion around the central axis (18; 322) of the fluid ring (3; 306), each magnetohydrodynamic pump (4, 6, 8; 308) having a single and different air gap (84, 86, 88; 360), the air gap (84, 86, 88; 360) being associated with a single and different air gap area (34, 36, 38; 366) and with effective areas for setting into motion (24, 26, 28) included in the air gap area (34, 36, 38; 366), each magnetohydrodynamic pump (4, 6, 8; 308) including a single and different means (54, 56, 58; 355) for generating magnetic field(s) crossing the associated air gap (54, 56, 58; 360), and including at least one single and different magnetic conduction element (64, 66, 68; 354, 356), configured for conducting and channelling the magnetic field(s) generated in the effective area(s) for setting into motion (24, 26, 28), each magnetic conduction element (64, 66, 68; 354, 356) being separated as close as possible to the central axis (18; 322) of the whole of the ring (16; 332) by a set back distance of the magnetic circuit which is specific to it, characterised in that the ratio of the set back distance (d1, d2, d3) of any magnetic conduction element (64, 66, 68; 354, 356) of any magnetohydrodynamic pump over half of the internal size (Ti) of the fluid ring (3; 306) is greater than or equal to 0.5, preferably greater than or equal to 0.7, and in that the fluidic ring (3; 306) comprises at least two distinct effective areas (24, 26), for setting the conductive liquid into motion, each centered around one of a respective radial middle half-plane and spaced apart angularly around the central axis according to a spacing pitch that is substantially equal to 360 divided by the number P of effective areas. a fluid ring (3; 306), of a torus shape, including a central ring hole (16; 332) crossed by a central axis (18; 322), having an internal size equal to the diameter of the ring hole (16; 332), filled with a conductive liquid and having at least one air gap area (34, 36, 38; 366), capable of being crossed by magnetic field(s) and having at least one effective area (24, 26, 28) for setting the liquid into motion, and at least one magnetohydrodynamic pump (4, 6, 8; 308) for setting the conductive liquid into motion around the central axis (18; 322) of the fluid ring (3; 306), each magnetohydrodynamic pump (4, 6, 8; 308) having a single and different air gap (84, 86, 88; 360), the air gap (84, 86, 88; 360) being associated with a single and different air gap area (34, 36, 38; 366) and with effective areas for setting into motion (24, 26, 28) included in the air gap area (34, 36, 38; 366), each magnetohydrodynamic pump (4, 6, 8; 308) including a single and different means (54, 56, 58; 355) for generating magnetic field(s) crossing the associated air gap (54, 56, 58; 360), and including at least one single and different magnetic conduction element (64, 66, 68; 354, 356), configured for conducting and channelling the magnetic field(s) generated in the effective area(s) for setting into motion (24, 26, 28), each magnetic conduction element (64, 66, 68; 354, 356) being separated as close as possible to the central axis (18; 322) of the whole of the ring (16; 332) by a set back distance of the magnetic circuit which is specific to it, characterised in that the ratio of the set back distance (d1, d2, d3) of any magnetic conduction element (64, 66, 68; 354, 356) of any magnetohydrodynamic pump over half of the internal size (Ti) of the fluid ring (3; 306) is greater than or equal to 0.5, preferably greater than or equal to 0.7, and in that the fluidic ring (3; 306) comprises at least two distinct effective areas (24, 26), for setting the conductive liquid into motion, each centered around one of a respective radial middle half-plane and spaced apart angularly around the central axis according to a spacing pitch that is substantially equal to 360 divided by the number P of effective areas.