The orbital mechanics math is fairly well understood so we know when planets will be in specific locations so it works out to be a more complex version of an or

The orbital mechanics math is fairly well understood so we know when planets will be in specific locations so it works out to be a more complex version of an orbital transfer. http://www.bogan.ca/orbits/transfer/Earth2Mars.html http://www.bogan.ca/orbits/transfer/Earth2Mars.html The key things is that they use gravity assists from planets to cause the probe to speed up or slow down. JPL has a good primer, webcast, and simulator on the topic. http://www2.jpl.nasa.gov/basics/grav/primer.php http://www2.jpl.nasa.gov/basics/grav/primer.php The gravity assists are the tricky part. In JPL's "Basics of Space Flight", there's a discussion of the Voyager 2 trajectory, http://www2.jpl.nasa.gov/basics/bsf4-1.php http://www2.jpl.nasa.gov/basics/bsf4-1.php