A spherical planet called "Glob" has a mass of 7.80 × 1018 kilogram and a radius of 6.30 × 104 meter. An astronaut on the surface of Glob throws a rock straight up. The rock reaches a maximum height of 1.20 × 103 meter above the surface of the planet before it falls back down. Find the initial speed of the rock as it left the astronaut hand? (Glob has no atmosphere, subsequently no energy is lost to air friction).