The path pursued by a spacecraft in moving from one orbit to another, e.g. from the orbit of the Earth to the orbit of Mars. Generally speaking, such an orbit will be an ellipse which intersects the orbit of the target plane; if the spacecraft is to enter orbit round the target planet, or effect a landing, then the motors must be fired to achieve the correct trajectory. The transfer orbit requiring the minimum expenditure of energy is an ellipse which just touches the orbits of the earth and the target planet (see Figure 45) and is known as the Hohmann orbit; such an orbit involves long flight times. A fast transfer orbit, requiring much greater expenditure of fuel, would be hyperbolic.
