Flight Dynamics (spacecraft) - Interplanetary Flight

Interplanetary Flight

In order to completely leave one planet's gravitational field to reach another, a hyperbolic trajectory relative to the departure planet is necessary, with excess velocity added to (or subracted from) the departure planet's orbital velocity around the Sun. The desired heliocentric transfer orbit to an outer planet will have its perihelion at the departure planet, requiring the hyperbolic excess velocity to be applied in the posigrade direction, when the spacecraft is away from the Sun. To an inner destination planet, aphelion will be at the departure planet, and the excess velocity is applied in the retrograde direction when the spacecraft is toward the Sun. Since interplanetary spacecraft spend a large period of time in the heliocentric orbit between the planets, which are at relatively large distances away, the patched-conic approximation is much more accurate for interplanetary trajectories than for translunar trajectories. The patch point between the hyperbolic trajectory relative to the departure planet and the heliocentric transfer orbit can be assumed to occur at the planet's sphere of influence radius relative to the Sun, as defined above in Orbital flight.