Neighborly Rendezvous
Traveling to other planets is just like rendezvousing with another spacecraft in a different orbit, because that is exactly what you are doing. The interplanetary spacecraft is moving from Earth’s orbit to, say, Mars’ orbit, to rendezvous with the red planet. In traveling to Mars, the spacecraft is moving to a higher orbit. Remember from Orbital Mechanics part 1, to raise the apogee of an orbit, we fire the engines at perigee. On a Mars mission, Earth orbit is the perigee—but since we are leaving the home planet and moving into solar space, it is now called perihelion. Mars’ orbit is aphelion. A Hohmann Transfer Orbit trajectory does the job nicely, with the engines fired in the direction of Earth’s movement in it’s’ orbit.