Porkchop plot

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Representative porkchop plot for the 2005 Mars launch opportunity. The horizontal axis is departure dates, and the vertical axis is arrival dates. A given contour represents a constant C₃ solution. The center of the porkchop is the optimal solution for the lowest C₃.

Porkchop plot (also pork-chop plot) is a chart that depicts orbital trajectories for spacecraft. It is named for the characteristically porkchop-shaped contours that display combinations of launch date and arrival date characteristics of an interplanetary flight path for a given launch opportunity to Mars or any other planet.^[1]

By examining the results of the porkchop plot, engineers can determine when launch opportunities exist to make use of special low-energy orbits needed to send payloads outwards using the lowest characteristic energy (C₃), which allows for lower fuel usage and/or larger payloads.^[2] A given contour, called a porkchop curve, represents constant C₃, and the center of the porkchop the optimal minimum C₃. The orbital elements of the solution, where the fixed values are the departure date, the arrival date, and the length of the flight, were first solved mathematically in 1761 by Johann Heinrich Lambert, and the equation is generally known as Lambert's problem (or theorem).^[1]

For the Voyager program, engineers at JPL plotted around 10,000 potential trajectories using porkchop plots, from which they selected around 100 that were optimal for the mission objectives. The plots allowed them to reduce or eliminate planetary encounters taking place over the Thanksgiving or Christmas holidays, and to plan the completion of the mission's primary goals before the end of the fiscal year 1981.^[3]