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, comprising a tubular body having a longitudinal axis and internally defining a separation chamber within which the gas phase of the fluid is separable, in use, from the liquid phase of the fluid; an inlet opening through which the fluid is injectable, in use, 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, in use, the separation chamber; the injection axis is inclined towards the liquid phase outlet opening so as to define a non-zero fluid injection angle with a direction orthogonal to the longitudinal axis.