  XMM-Newton EPIC observations of the planetary nebula (PN) NGC 7009, the
Saturn Nebula, have detected extended X-ray emission in its central cavity. The
diffuse X-ray emission must originate in the shocked fast stellar wind. The
spectra show that the temperature of the hot gas is 1.8*10^6 K. The rms density
derived from the volume emission measure is a few tens H-atom/cm^3. The hot gas
does not appear over-pressurized with respect to the nebular shell. NGC 7009
represents an evolutionary stage at which the influence of the hot gas in the
PN interior starts to decline due to the diminishing strength of the fast
stellar wind and the expansion of the central cavity.
