What is meant by the terms “conjunction-class” and “opposition-class” missions?
There are two basic possibilities for a practical Earth-Mars trajectory: opposition- and conjunction-class trajectories. The two are so named because they are based on the relative positions of Earth and Mars at launch: opposition occurs when Mars is closest to Earth, and conjunction occurs when Mars is furthest from Earth. These positions can be visualized best when thinking of the Sun, Earth, and Mars lying across a straight line. At opposition, the Sun is in the middle, Earth is to the right, and Mars is further right. At conjunction, Mars is on the far left, the Sun is in the middle, and Earth is on the right.
