Editing Interplanetary spaceflight (section)

===Hohmann transfers===
[[File:Hohmann transfer orbit2.svg|frame|right|Hohmann Transfer Orbit: a spaceship leaves from point 2 in Earth's orbit and arrives at point 3 in Mars' (not to scale).]]
For many years economical interplanetary travel meant using the [[Hohmann transfer orbit]]. Hohmann demonstrated that the lowest energy route between any two orbits is an [[ellipse|elliptical]] "orbit" which forms a [[tangent]] to the starting and destination orbits. Once the spacecraft arrives, a second application of thrust will re-circularize the orbit at the new location. In the case of planetary transfers this means directing the spacecraft, originally in an orbit almost identical to Earth's, so that the [[aphelion]] of the transfer orbit is on the far side of the Sun near the orbit of the other planet. A spacecraft traveling from Earth to Mars via this method will arrive near Mars orbit in approximately 8.5 months, but because the orbital velocity is greater when closer to the center of mass (i.e. the Sun) and slower when farther from the center, the spacecraft will be traveling quite slowly and a small application of thrust is all that is needed to put it into a [[circular orbit]] around Mars. If the manoeuver is timed properly, Mars will be "arriving" under the spacecraft when this happens.

The Hohmann transfer applies to any two orbits, not just those with planets involved. For instance it is the most common way to transfer satellites into [[geostationary orbit]], after first being "parked" in [[low Earth orbit]]. However, the Hohmann transfer takes an amount of time similar to ½ of the orbital period of the outer orbit, so in the case of the outer planets this is many years – too long to wait. It is also based on the assumption that the points at both ends are massless, as in the case when transferring between two orbits around Earth for instance. With a planet at the destination end of the transfer, calculations become considerably more difficult.