A satellite of mass \(m\) moves in an elliptical orbit around a planet of mass \(M\), where \(M \gg m\). The distance from the center of the planet to the satellite at periapsis is \(r_p\), and the distance at apoapsis is \(r_a = 4r_p\). What is the ratio \(\dfrac{v_p}{v_a}\) of the satellite's speed at periapsis to its speed at apoapsis?