A rocket in space can travel without engine power at a constant speed in the same direction. This condition is best explained by the concept of

inertia. Newton's law one say to the effect "It takes a net force to change the momentum of inertial mass." p = mv is the momentum of a mass (e. g., rocket) traveling at velocity v. As there is no net force (discounting gravity as a possible force) p = constant = mv; so v = constant.