If two planets were travelling at .99c away from a planet in the center. What would be the speed difference between them? If you were on one of those travelling planets and launched a rocket in the direction you were already travelling, you could get it to go .99c away from you. If you related that rocket to the planet in the middle it's now travelling at 1.98c correct? You may say thats not possible, but If you didnt compare yourself to the planet in the middle, you wouldnt even know how fast your travelling. Similarly how do we know which direction and how fast Earth is actually moving? We could send a rocket .99c relative to us in any direction we want, that could be travelling faster than light relative to another celestial body. Its true that you cannot travel past the speed of light, because light always travels light speed away from you, but it seems can you travel faster than light, relative to another stationary body.