Planets don’t circle the sun, their paths follow elliptical orbits. Think of them as race cars on separate tracks traveling at different speeds – sometimes they’ll whiz past each other, other times they’re far apart. Planetary distance is measured from the Sun, orbital point closest to the Sun is perihelion, furthest away is aphelion. The closest point between Earth and Mars happens when Earth is at aphelion, Mars is at perihelion, and both planets are on the same side of the Sun – this is called opposition. In theory, at true opposition Earth would be 54.6 million kilometers from Mars. I say theory because it hasn’t happened in recorded history, 2003 marked the closest nod in 50,000 years at 56 million kilometers.
So how do you get to Mars? Roughly every two years Earth/Mars orbits are close enough to contemplate interception. With current capabilities it takes approximately 250 days, all science has to do is calculate where Mars will be when rovers come knocking. Take 6 minutes to ponder a clip from the 2006 IMAX documentary Roving Mars –