Representative porkchop plot for the 2005 Mars launch opportunity.
"We have launch opportunities to Mars every 25 ½ months because
of the repetitive relative alignment of the planets," says Dan Johnston,
an engineer in JPL's Navigation and Mission Design Section and mission design
manager for the 2005 Mars Reconnaissance Orbiter. Opportunities to Mars
this decade are in 2003, 2005, 2007 and 2009. "These are the
opportunities where our launch vehicles have enough energy to send a
spacecraft to Mars."
A tremendous number of calculations considering multiple variables must
be performed to discover all the possible trajectories available and their
unique characteristics in a given launch opportunity. Porkchop plots are
visualizations that allow mission planners to view key parameters that must
be considered, says Johnston.
Based on porkchop plots developed several years ago for the 2005 Mars
launch opportunity, the Mars Reconnaissance Orbiter team developed a
"reference mission" on which to base its project planning. That
planning is now well-underway, and on October 3,
NASA selected contractor
Lockheed Martin Astronautics in Denver, Colo., to build the spacecraft and
provide mission operations support.
What a porkchop plot really represents, says Johnston, is a solution to some
orbital mechanics equations known as Lambert's theorem, which he sums up
thusly: "If I know where the Earth is and where Mars is on some given
day, and I know how long I would like to take to get to Mars, then I can
compute the departure conditions I need at Earth to be able to get to
Mars in the desired time."
"When you use Lambert's theorem to compute this, you come up
with a launch and arrival date pair that gives you a single, unique trajectory
solution for getting to Mars," says Johnston. "Some we call
'Type 1', which are short transfers of about seven months. Some are
'Type 2', which are longer duration transfers of about 10 months." The
shorter path to Mars is not necessarily the best, Johnston explained. Some
missions, such as orbiters, for example, may benefit from a longer trajectory
that delivers the spacecraft to the planet at a lower arrival velocity. That
way, less fuel is needed to brake the spacecraft when it arrives.