PRINCIPLES OF ASTRODYNAMICSThe few physical principles underlying the study of spacecraft motion are elementary in their
basic form. Sir Isaac Newton first formalized the physical laws that determine the motion of a spacecraft: (1) Any body continues in a state of rest, or of constant velocity in a straight line, unless acted upon by external forces. (2) The rate of change of linear momentum of a body is equal to the external force applied. For a body of constant mass, forces are balanced by accelerations. (3) To every action there is an equal and opposite reaction. To these laws Newton added the law of gravitational attraction: any two masses attract each other with a force
proportional to the product of the masses and inversely proportional to the square of their separation distance.Johannes Kepler provided the empirical laws that describe
planetary motion and allowed a test of Newton's laws. Kepler's laws are the following: (1) The orbit of each planet is an ellipse with the Sun at one focus. (2) The radius vector joining Sun to planet sweeps over equal areas in equal intervals of time. (3) The square of the planetary periods is proportional to the cube of the mean distance to the Sun.