The tendency of a rocket to move in a straight line in the direction it is pointed at launch. A hobby rocket employs "passive" stability, in that it depends on its inherent stability (the relationship of its centers of pressure and gravity) and the counterbalancing pressure of the airflow on its fins to keep it moving in the correct attitude. Missiles and spacecraft usually employ "active" stability schemes, in which attitude jets or movable vanes help correct the rocket's course in flight. Stability involves a rocket's ability to continue to fly straight when the angle of attack is greater than zero. A stable rocket continues to fly in its intended direction, even if moderate wind or other influences try to change its course. A poorly-designed rocket can be either unstable, marginally stable [under stable] or excessively stable [over stable]. An unstable rocket (whose center of gravity is behind its center of pressure) will fly erratically when launched. A marginally stable rocket (whose center of gravity is less than one diameter ahead of its center of pressure) might be able to maintain a straight flight attitude if there is no side wind (angle of attack is zero.) However, if outside forces such as wind intervene (angle of attach greater than zero), the rocket can be thrown off its flight path and behave in an erratic manner. An excessively stable rocket (whose center of gravity is much more than one diameter ahead of its center of pressure) will tend to turn toward those outside forces rather than remain on the intended line of flight. See Weathercocking, Over Stability, Marginal or Under Stability