The shortest distance between two planets is not a straight line.
At least that's the case in the business of spaceflight. The shortest
distance is a curved line that takes into account a number of factors,
including the orbits of the two planets and the position of each planet
within its orbit.
The MER mission takes advantage of a rare close alignment of Earth and
Mars in 2003. On June 10, 2003, the first Mars Exploration Rover (MER)
spacecraft was launched on a Delta II rocket from
Cape Canaveral, Florida.