Ancient cultures looked to the patterns of tea leaves or animal entrails
to divine the course of the future. At JPL, the course of a future Mars
mission can be found in a porkchop.
Porkchop plot, that is. In the sometimes peculiar vocabulary of JPL
mission designers, that nickname describes the porkchop-shaped,
computer-generated, contour plots that display the launch date and arrival
date characteristics of an interplanetary flight path for a given launch
opportunity to Mars or any other planet.
Developing a porkchop plot is the first thing on the menu when
mission designers are scoping out an interplanetary voyage. This is the
sort of task accomplished by engineers in JPL's Navigation and Mission
Design Section, whose unique, high-caliber expertise is signified by its
recognition as NASA Center of Excellence.
Sending a spacecraft to another planet has been compared to
throwing a dart at a moving target -- only the thrower is also on a
moving platform, the Earth. It is further complicated by the fact that the
Sun's gravity curves the trajectory of the dart. At launch, the spacecraft is
aimed to arrive at the point the planet will be months from now.
Getting to the planet Mars, rather than just to its orbit, requires that the spacecraft
be inserted into its interplanetary trajectory at the correct time so it will arrive at
the Martian orbit when Mars will be there. This task might be compared to
throwing a dart at a moving target. You have to lead the aim point by just the right
amount to hit the target. The opportunity to launch a spacecraft on a transfer orbit
to Mars occurs about every 26 months.
To be captured into a Martian orbit, the spacecraft must then decelerate relative
to Mars using a retrograde rocket burn or some other means. To land on Mars,
the spacecraft must decelerate even further using a retrograde burn to the extent
that the lowest point of its Martian orbit will intercept the surface of Mars. Since
Mars has an atmosphere, final deceleration may also be performed by
aerodynamic braking direct from the interplanetary trajectory, and/or a parachute,
and/or further retrograde burns.
From "The Basics of Spaceflight" http://www.jpl.nasa.gov/basics/bsf4-1.html