Explosion of Krypton

Figure 1.3: The unfortunate history of the planet Krypton.

As an illustration of the points raised in the previous discussion, let us consider the unfortunate history of the planet Krypton. As is well-known, Krypton—Superman's home planet—eventually exploded. Note, however, that before, during, and after this explosion, the net external force acting on Krypton, or the fragments of Krypton—namely, the gravitational attraction due to Krypton's sun—remained the same. In other words, the forces responsible for the explosion can be thought of as large, transitory, internal forces. We conclude that the motion of the center of mass of Krypton, or the fragments of Krypton, was unaffected by the explosion. This follows, from Equation (1.72), because the motion of the center of mass is independent of internal forces. Before the explosion, the planet Krypton presumably executed a standard Keplerian orbit around Krypton's sun. We conclude that, after the explosion, the fragments of Krypton (or, to be more exact, the center of mass of these fragments) continued to execute exactly the same orbit. See Figure 1.3.