I am doing an Astronomy GCSE. I have to work out the orbital period for a satellite. I have got quite far with it, but I am really stuck. I have spent hours at it! This is the question:- A space laboratory is in circular orbit around the Earth at a distance of 6000km from the Earth's centre; its period is 2hr 12min. An orbital telescope is to be placed into orbit at 12,000km. Calculate its expected period. I am at the point where I need to square root 4(pi)²r³/GM = T A few questions. Is r the radius of the Earth (from core to surface) or core to satellite? How do I calculate GM? I know that M is the mass of the Earth (5.972 × 10^24 kg) but how do I work out G (the Universal Gravitational Constant?). I really need help and it is very frustrating. Any help or advice would be very much appreciated. Thanks!