If you have the STK SatPro or the STK Astrogator license, the Mass page will be included in the satellite's Basic properties. The options on the Mass page allow you to specify a satellite's moment of inertia tensor and its mass.
Use the Inertia Matrix to specify the satellite's moments of inertia. The rows and columns in this matrix are ordered to correspond to the X, Y and Z body-fixed axes. The inertial matrix is currently used only during numerical integration of the spacecraft attitude.
When the HPOP propagator is selected, any value entered for mass on this page is overridden by the value for mass entered in the Additional tab of HPOP Force Models window.