In this lesson, students will: Use algebraic computations to determine the relative positions of Earth and Mars during which an optimal (low-energy) transfer of a spacecraft can occur. Combine this information with planetary-position data to determine the next launch opportunity to Mars. Materials Graph paper, quadrille ruled (one piece per student) 8.5-by-11-inch or larger piece of thick cardboard (per student) Two push-pins (per student) String, approximately 30 cm (per student) Calculator Pl