A trajectory along which a spacecraft moves from one orbit to another with the minimum expenditure of energy, first calculated by the German engineer Walter Hohmann (1880–1945) in 1925. Such trajectories are used for changing the orbits of a satellite around the Earth, or for sending a probe from the Earth to another planet. A Hohmann orbit is elliptical, and just touches the orbits of origin and destination. It requires two firings of the spacecraft’s rocket motor, one to break out of the original orbit and one to enter the destination orbit. The main disadvantage of the Hohmann orbit is the long flight times involved. From Earth to Mars the transfer time is 260 days; from Earth to Saturn would take about 6 years. For this reason gravity assists are often used to boost the spacecraft’s speed and hence reduce journey times.