Beyond the Moon

One surprising thing about launching rockets across the solar system is that most of the rocket’s fuel is used just getting away from Earth. Science fiction author Robert A. Heinlein wrote: “Once you’re in Earth orbit, you’re halfway to anywhere.” This is not only because of Earth’s strong gravity but also because Earth is moving at 19 miles per second (30 km per second) in its orbit around the Sun. Any spacecraft going to another planet already has that much speed as soon as it leaves Earth. To get to Venus, second planet from the Sun, a spaceship actually has to slow down to 17.2 miles per second (27.5 km per second). The spaceship will then fall downward toward the Sun in a long loop, speeding up as it falls and meeting Venus in about five months. When the spaceship gets there, it will have to slow down a little more to match orbits with Venus.