Method of operating a passive cyclone separator to treat a fluid in a microgravity environment to separate a liquid phase of the fluid from a gas phase of the fluid, the cyclone separator comprising: - a tubular body having a longitudinal axis and internally defining a separation chamber within which the gas phase of said fluid is separable from the liquid phase of said fluid; - an inlet opening through which the fluid is injectable into the separation chamber along an injection axis; - a liquid phase outlet opening, through which the liquid phase separated from the gas phase exits, in use, the separation chamber; and - a gas phase outlet opening, through which the gas phase separated from the liquid phase exits the separation chamber; wherein the injection axis is inclined towards the liquid phase outlet opening so as to define a non-zero fluid injection angle (α) with a direction orthogonal to said longitudinal axis; wherein the liquid phase outlet opening is arranged at a first axial end portion of the tubular body and wherein the gas phase outlet opening is arranged at a second axial end portion of the tubular body opposite to the first axial end portion; wherein the inlet opening is axially interposed between the liquid phase outlet opening and the gas phase outlet opening; the method comprising the step of: - feeding the fluid through the inlet opening into the separation chamber with a determined fluid inlet flow rate; and wherein a fluid inlet flow rate, measured in l/h, to axial distance between the inlet opening and the liquid phase outlet opening, measured in mm, ratio is between 0.5 and 3, preferably between 1.3 and 2.3; and/or wherein a fluid inlet flow rate, measured in 1/h, to axial distance between the inlet opening and the gas phase outlet opening, measured in mm, ratio is between 0.7 and 4.4, preferably between 2 and 3.5. - a tubular body having a longitudinal axis and internally defining a separation chamber within which the gas phase of said fluid is separable from the liquid phase of said fluid; - an inlet opening through which the fluid is injectable into the separation chamber along an injection axis; - a liquid phase outlet opening, through which the liquid phase separated from the gas phase exits, in use, the separation chamber; and - a gas phase outlet opening, through which the gas phase separated from the liquid phase exits the separation chamber; wherein the injection axis is inclined towards the liquid phase outlet opening so as to define a non-zero fluid injection angle (α) with a direction orthogonal to said longitudinal axis; wherein the liquid phase outlet opening is arranged at a first axial end portion of the tubular body and wherein the gas phase outlet opening is arranged at a second axial end portion of the tubular body opposite to the first axial end portion; wherein the inlet opening is axially interposed between the liquid phase outlet opening and the gas phase outlet opening; the method comprising the step of: - feeding the fluid through the inlet opening into the separation chamber with a determined fluid inlet flow rate; and wherein a fluid inlet flow rate, measured in l/h, to axial distance between the inlet opening and the liquid phase outlet opening, measured in mm, ratio is between 0.5 and 3, preferably between 1.3 and 2.3; and/or wherein a fluid inlet flow rate, measured in 1/h, to axial distance between the inlet opening and the gas phase outlet opening, measured in mm, ratio is between 0.7 and 4.4, preferably between 2 and 3.5. wherein the injection axis is inclined towards the liquid phase outlet opening so as to define a non-zero fluid injection angle (α) with a direction orthogonal to said longitudinal axis; wherein the liquid phase outlet opening is arranged at a first axial end portion of the tubular body and wherein the gas phase outlet opening is arranged at a second axial end portion of the tubular body opposite to the first axial end portion; wherein the inlet opening is axially interposed between the liquid phase outlet opening and the gas phase outlet opening; the method comprising the step of: - feeding the fluid through the inlet opening into the separation chamber with a determined fluid inlet flow rate; - feeding the fluid through the inlet opening into the separation chamber with a determined fluid inlet flow rate; and wherein a fluid inlet flow rate, measured in l/h, to axial distance between the inlet opening and the liquid phase outlet opening, measured in mm, ratio is between 0.5 and 3, preferably between 1.3 and 2.3; and/or wherein a fluid inlet flow rate, measured in 1/h, to axial distance between the inlet opening and the gas phase outlet opening, measured in mm, ratio is between 0.7 and 4.4, preferably between 2 and 3.5.