There are three phases in the planet encounter process of a spacecraft: the launch phase, the cruise phase, and the encounter phase. The launch phase primarily involves preparing for the actual launch of the spacecraft, by choosing the appropriate location and time. The cruise phase consists mainly of experiments and slight modifications of the trajectory. The encounter phase includes any flyover or surface encounter, both of which are most often designed to conduct experiments and collect data. Before launching a spacecraft, extensive knowledge of gravitation, mechanics, interplanetary trajectories and planetary orbits is necessary. There are many different laws related to gravitation and mechanics that significantly influence spacecraft. There are many different types of interplanetary trajectories, or trajectories that operate outside of a planet's gravity or to escape a planet's gravity. The many types of planetary orbits are extremely important to the cruise and encounter phases of spaceflight. Space flight is a very complicated process that relies on a lot of knowledge in physics, space and mathematics, and requires a lot of preparation. PhysicsGravitation and MechanicsGravitation, a fundamental law of the universe, is the mutual attraction of all masses in the universe. The equation used to calculate gravity on a planet's surface is F = mg, where Fi is the gravitational force, m is the mass of the object, and g is the acceleration due to gravity. The principles set out in Principia, Sir Isaac Newton's book which describes his laws of motion, are the three ways of changing the momentum of a body: the law of inertia, the law of acceleration, and the law of action-reaction. Johannes Kepler described three laws of...... middle of document ......apter 14. Launch phase." Basics of Space Flight Section III. Space Flight Operations. NASA, nd Web. May 21, 2014. “Chapter 15. Encounter Phase.” Basics of Space Flight Section III. NASA, nd Web. Web. May 21, 2014 "Chapter 3. Space Flight Basics Section I. The Space Environment. NASA, May 21, 2014. "Chapter 4. NASA Interplanetary Trajectories. , nd Web, May 21, 2014. Freudenrich, Ph.D. Craig. “How Space Shuttles Work,” HowStuffWorks.com, January 19, 2001. Web, May 21, 2014. “How to Launch a Rocket into Space in 5 (). ...4...3...2...1) steps." Science Live. TechMediaNetwork, July 25, 2011. Web. May 21 2014.