Why is there variation in the distance to Mars?

Answer 1

The distance between earth and Mars, if that is what is asked, varies because the two planets don't rotate around the sun at the same rate. Mars is farther away from the sun, and it moves more slowly in its orbit than earth does. That means that at times, the earth could be on one side of the sun, and Mars could be on the other side! There are times when the two planest lie in a line drawn through them to the sun, and it is then that they are closest (taking into account the eccentricities of the orbits of both planets). Note that if we wish to get to Mars by the shortest route, we have to do some calculating. We can't launch something when the two planets are lined up on the same side of the sun. It takes a probe a long time to get to Mars from earth, and a "launch window" will have to be chosen so that the probe can be sent up and out to where Mars will be when the probe has made it across the distance between earth's orbit and that of the Red Planet. This path will desctibe a curve, and launch will occur when earth is "behind" Mars and "catching up" to it in orbit. The name Hohmann transfer orbit is applied to the orbital mechanics of the maneuver. Links can be found below.