Trans-Neptunian objects and asteroids

All planets from Mercury to Neptune and most of their satellites have been observed from Earth-based telescopes and at least once, some repeatedly, from spacecraft. Therefore, sufficient information was available to emphasize the physical principles in the discussions in Chapter 6. Trans-Neptunian objects and asteroids have been explored to a much lesser degree. Their small sizes, for many their large heliocentric distances, and their low surface temperatures prevented detailed exploration. Until recently, only a few samples of an enormous amount of objects have been investigated. Therefore, the treatment of these objects, grouped in this chapter, is primarily a summary of presently known properties. Section 7.1 discusses Pluto and its satellite Charon Section 7.2 is devoted to comets and Section 7.3 to asteroids. 

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The restricted three-body problem Two bodies of finite masses, called primaries, revolve around their common center of mass in circular orbits and a third body with negligible mass moves under their gravitational attraction, but does not affect the orbits of the two primaries. In most astronomical applications the second primary has a small mass compared to the first primary, and consequently the motion of the third, massless, body is a perturbed Keplerian orbit. This is a model for the study of an asteroid (Jupiter being the second primary), a trans-Neptunian object (Neptune being the second primary) or an Earth-like planet in an extrasolar planetary system. 

Asteroids, comets and smaller particles are also grouped into SSSBs, small solar system bodies. All categories of objects described above appear at specific locations in the solar system. The inner solar system contains the terrestrial planets and the Main Belt of asteroids. In the middle region there are the giant planets with their satellites and the centaurs. The outer solar system comprises the Trans-Neptunian objects including the Kuiper Belt, the Oort cloud, and the vast region in between. 

Low-albedo objects are diverse in opposition behavior. Actually, asteroids of the primitive P and F types, located mainly in the outer asteroid belt, show linear phase curves down to phase angles of 2° [49,50], The first tentative phase curves for other dark and distant Solar system objects, like Centaurs and trans-Neptunian (Kuiper Belt) objects, also show linear phase dependence at small phase angles. However their phase curve slope is at least twice as large as that of low-albedo asteroids [51], For both types of dark objects there is an indication of a veiy narrow opposition surge at sub-degree phase angles. 

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