And why is Mercury so hard to get there in terms of energy required? It's because its orbital velocity is so fast and you need to match that. (You can get the

And why is Mercury so hard to get there in terms of energy required? It's because its orbital velocity is so fast and you need to match that. (You can get there without matching the orbital velocity, but that won't be useful, you're either doing a non-capturable flyby or a very hard impact.) Mercury's orbital velocity is 48 km/s. Earth's is 30. An object at infinity would be zero. Kinetic energy is proportional to velocity squared. Square those numbers and you see the energy differential between Earth's orbit and Mercury's is greater than going from Earth to infinity.