A scientific research drone, hovering at a fixed altitude, releases a small atmospheric probe from rest.

Point $Q$ is located 10 m vertically below the release point $P$.

As the probe passes through point $Q$, its downward momentum is measured to be 4.2 kg m s^-1.

Assuming there is no air resistance, calculate the mass of the probe.

(Take the acceleration due to gravity, $g$, to be 9.8 m s^-2.)