You might think that a single impulse from the launch pad would suffice to get a rocket to the Moon, Mars, or elsewhere. After all, an object in motion remains in motion unless acted upon by an outside force; and since there isn’t any air resistance in outer space, one might guess that a rocket could coast to its destination.|
At first glance, that argument sounds reasonable. Nevertheless, a space-faring rocket needs to be able to push itself forward, backward, or sideways at many points in its journey. For starters, Earth’s gravity extends beyond the atmosphere, and any spacecraft headed to Mars, Jupiter, or Saturn has to fight Earth’s gravity as well as the Sun’s. Furthermore, spacecraft need to maneuver to go into orbit around—or land on—distant moons, planets, or asteroids.