1fa1dd55__Ion_Propulsion___Dawn_Mission__Grades_9_12
[Grades 5-8] An object that is not being subjected to a force will continue to move at a constant speed and in a straight line. [Dawn Instrumentation Module] The change in the asteroid’s gravity field will change the Dawn spacecraft’s velocity. This, in turn, will cause a Doppler shift in the radio signals from the spacecraft. [Grades 9-12] 
Objects change their motion only when a net force is applied. Gravitation is a universal force that each mass exerts on any other mass.
Education > Structure and Properties of Matter: Ion Propulsion | Dawn Mission
Grades 9-12