Figure 8-33Transformation and Evolution of a Long-Period Comet (a) The gravitational force of a giant planet can change a comet’s orbit. Comets initially on highly elliptical orbits are sometimes deflected into more circular paths that keep them in the inner solar system. (b–d) These figures show the evolution of a comet into gas, dust, and rubble, and why debris from some of these comets strikes Earth (see also Figure 8-40).