A metallic object floats along in space. Its path, measured relative to a nearby planet, follows the curve r$($t$)=15+4$t $3$t $4$ for $0$ t $4$ hours and r in thousands of miles. $($a$)$ Find all the times, t$,$ when the object is stationary relative to the planet. $($b$)$ For what intervals is the object moving away from the planet? Toward the planet?