The feverish activity of the 1970's in fluid critical phenomena slowed to a near standstill, partly because of gravitational limitations on the acquisition of further experimental data closer to the critical temperature. The current availability of low gravity on long-duration space missions has led to renewed experimental efforts.
Any pure fluid possesses a liquid-vapor critical point.
It is uniquely defined by a temperature, pressure, and density state in thermodynamics. At the critical point, it is unclear whether a fluid is liquid or vapor, and the fluid is highly compressible. Such compressibility is the root of the troubles caused by gravity, as illustrated in figure 1 . This leaves the other half below the critical density. Then, a thin portion between the two halves remains at the critical density.