The Origin and Evolution of the Solar System

32 The structure of the Solar SystemSaturn’s outer retrograde satellite with an orbital radius of just under 13 millionkilometres.Several tens of other bodies have been detected, with estimated diametersin the range 150–360 km, which are close to, or further out than, the orbit ofNeptune. These are Kuiper-belt objects and they are usually considered to belinked to comets rather than asteroids. There is uncertainty about the relationshipbetween asteroids and comets—whether they are two manifestations of somecommon source of material or represent two different types of material with completelydifferent origins. All the objects described as asteroids are in direct orbitsand mostly have moderate inclinations and eccentricities. By contrast comets arefrequently in retrograde orbits, have a wide range of inclinations and eccentricitiesand also have a considerable content of volatile material. They are also associatedwith regions well outside that occupied by the planets. Ideas about the origin ofvarious types of object in the Solar System are heavily linked with ideas of theorigin of the system itself.1.5.2 The distribution of asteroid orbits: Kirkwood gapsA diagram giving the frequency of asteroid periods, such as figure 1.17, indicatesthat the distribution has prominent gaps. These were first explained by theAmerican astronomer, Daniel Kirkwood, in 1866. He pointed out that the twovery prominent gaps, marked A and B, correspond to one-third and one-half theperiod of Jupiter, and that these gaps were a manifestation of some resonancephenomenon. For example, an asteroid with one-half the period of Jupiter willmake two complete orbits while Jupiter is making one. Thus the two bodies willalways be closest in the same region of the asteroid’s orbit so that the perturbationby Jupiter at closest approach will always be modifying the asteroid orbitin the same direction. The asteroid’s period will steadily change in one directionuntil the asteroid and Jupiter are sufficiently out of resonance for the nearestapproaches, and hence maximum perturbations, to occur all round the asteroid’sorbit with much diminished effect. Similarly, for the one-third resonance, the asteroidis perturbed at two points on opposite sides of its orbit. Other Kirkwoodgaps at two-fifths and three-sevenths of Jupiter’s period are also evident in figure1.17. However, to illustrate the complexity of the resonance process there isa small concentration of asteroid orbits corresponding to two-thirds of Jupiter’speriod. The Kirkwood gap phenomenon is dynamically related to the formationof the gaps in Saturn’s rings due to perturbation by the inner satellites Mimas andEnceladus.1.5.3 The compositions of asteroidsSpacecraft observations of asteroids have given new information concerning theirstructure and composition. A near passage of the asteroid Gaspra by the Galileospacecraft gave the very detailed photograph shown in figure 1.18. It has dimen-