The nonlinear interaction between intense ordinary mode (O-mode) radio waves and modified magnetoacoustic waves in partially ionized space dusty plasmas is considered, including the combined action of the radio wave pressure and the electron Joule heating caused by the O-mode electric field. A set of equations containing the wave equation for the radio waves and the electron density and temperature fluctuations associated with the modified magnetoacoustic waves is derived. The set is then used to investigate decay and modulational instabilities of a constant amplitude O-mode radio wave in the Earth's ionosphere and mesosphere.