What is inertia? Specifically, what is inertia in relation the the gravity properties of astronautical planets.

Inertia is the property of movement (properly velocity).

Inertia is most commonly referenced in connection to Newton's laws of motion, in that an object in motion stays in motion. This (normally, anyway) isn't true in everyday life, as forces like friction, gravity, et cetera, fight back causing velocity to be converted to heat. Objects to which this doesn't apply, like space probes, keep moving forever.