Recap - Space Probe

A $20\ \mathrm{kg}$ probe is fired straight up from a small airless moon (radius $50\,000\ \mathrm{m}$, surface gravity $0.02\ \mathrm{m/s^2}$) by a spring launcher with $k = 128\,000\ \mathrm{N/m}$ compressed by $0.5\ \mathrm{m}$. Find the distance $r$ from the moon's centre where the probe stops and turns back.