Lets suppose there is an observer at a certain distance from a planet that is moving at 0.9c as seen by the observer. We take as a reference, a frame where the observer is at rest. A spaceship leaves the planet and begins accelerating relative to the planet in the oposite direction. The observer should see the space ship decelerating and at some point having zero velocity, then the ship will apear to acceletate further in the opposite direction. Relative to the observer I suppose it is possible in principle can reach 0.9c. But when the observer checks the relative velocity between the planet and the spaceship it will conclude the spaceship managed to change its velocity bu 1.8c. Is this correct ?