Porkchop plotting of the mission trajectory options "starts the very
first day, before you start anywhere else" with a project, says
Johnston. "The porkchop plots let you determine the basic
requirements of the launch vehicle you'll need as well as a preliminary
estimate of the propellant load necessary for the spacecraft. If the
spacecraft cannot carry all the propellant it needs to reach its desired
science orbit, a mission option using
aerobraking may have to be employed."
After all, he says, "We don't want to use up our baggage allowance
just flying fuel to Mars; we want to fly instruments that send information
back to us."
The porkchop plot also provides information on the conditions of the
capture orbit when the spacecraft arrives, including the time of day it will be
on Mars when the northbound spacecraft crosses the equator in its first
orbits. This is particularly important for the 2005 Mars Reconnaissance
Orbiter, because the scientific instruments onboard require that the
spacecraft end up in a nearly-polar, Sun-synchronous 3 p.m. orbit. That
means that the territory beneath the orbiter as it crosses the equator of
Mars will always be illuminated by a 3 p.m. Sun, providing optimum
shadow and brightness characteristics on the surface for the cameras
and spectrometers onboard. The 200 by 400 kilometer-high orbit
(about 125 by 250 miles) will provide opportunities for extraordinary
close-ups of the surface.
When the Mars Reconnaissance Orbiter arrives at Mars, it will initially
enter an 8:30 p.m. orbit. But up to six months of aerobraking, followed by
small propulsive trimming of the orbit, will bring the spacecraft to the
desired, nearly circular, Sun synchronous 3 p.m. orbit.
Another factor that's important to mission planners is selecting an arrival
date that will require the smallest propulsive maneuver to brake the
spacecraft's speed to allow it to be captured into orbit around Mars.
"We try to minimize the overall delta-V (change in velocity) necessary
to get into orbit," says Johnston.
When traveling among the planets, it's a good idea to minimize
the propellant mass needed by your spacecraft and its launch vehicle.
That way, such a flight is possible with current launch capabilities,
and costs will not be prohibitive. The amount of propellant needed
depends largely on what route you choose. Trajectories that by
their nature need a minimum of propellant are therefore of great interest.
To launch a spacecraft from Earth to an outer planet such as Mars
using the least propellant possible, first consider that the spacecraft
is already in solar orbit as it sits on the launch pad. This existing
solar orbit must be adjusted to cause it to take the spacecraft to
Mars: The desired orbit's perihelion (closest approach to the Sun)
will be at the distance of Earth's orbit, and the aphelion (farthest
distance from the Sun) will be at the distance of Mars' orbit.
This is called a Hohmann Transfer orbit. The portion of the solar
orbit that takes the spacecraft from Earth to Mars is called its
trajectory. (From "The Basics of Spaceflight" http://www.jpl.nasa.gov/basics/bsf4-1.html )