Mars is not always on the same side of the Sun as the Earth. The two planets come close to each other about every 2 years.
Each planet orbits the Sun. The planets closer to the Sun orbit faster, and the ones further away orbit more slowly.
Earth is constantly moving, and so are the other planets. Its like trying to hit a moving target while standing on a moving platform. You have to predict where a planet will be when you want your rocketship to arrive. Both planet and spaceship must arrive at the same place at the same time. It’s quite complicated!
As for how long it takes to travel to Mars, it depends on the method used to get there. Typically, the method that saves the most fuel and is the most efficient in terms of time is called a Hohmann orbital transfer. In the image below, the spaceship travels along the reddish trajectory, launched from Earth into an elliptical orbit around the Sun. The placement of both Earth and Mars at launch time and arrival time are depicted below.
Dont let the complexity of the image below make it more complicated than it seems. A simpler animation will follow near the end of this post.
See the gif animation here
