Ok, let me ask a very straight and simple question. Imagine that I have a spacecraft with imaginable engine which is able to indefinitely provide thrust enough

Ok, let me ask a very straight and simple question. Imagine that I have a spacecraft with imaginable engine which is able to indefinitely provide thrust enough for this spacecraft to move straight up from the planet’s surface at the constant speed of 1 mph. Will this spacecraft leave the planet in indefinite amount of time or not? If not - what is going to stop it?