"The porkchop plots for the 2005 Mars opportunity show that for
our launch vehicles, this is one of the most challenging opportunities to get
to Mars in this decade," says Johnston. The interplanetary doorway to
Mars open at that time requires the spacecraft to be delivered to a spot
above Earth's higher latitudes. This high-latitudinal orbit requires more
launch energy - a bigger rocket - than most launches.
In terms of physics, the price paid for poising a spacecraft at that
jumping-off point can be compared to that paid for a bigger, steeper roller
coaster ride, says Johnston: "The cars have to be pulled up a longer
hill to get to the highest point before the roller coaster's first plunge,"
he says. "In effect, we need a steeper plunge to get onto our flight
path in 2005. We've got to push ourselves up higher to get onto the Mars
trajectory for this opportunity."
Nearly half the Reconnaissance Orbiter's entire 1,800 kilogram mass
(3,968 pounds) is propellant that will be used to brake the spacecraft's
speed when it reaches Mars, allowing it to be captured into orbit. The
spacecraft also carries a heftier science package with about twice the mass
of the instruments carried by its recent predecessors. All these factors
mean that a rocket the size of an Atlas 3 or a Delta 3 will be required to
send the Reconnaissance Orbiter on its way. (By comparison, its sibling
spacecraft Mars Odyssey, at about half the mass, was lofted on its
voyage by a Delta 2.)