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.

Pluto and Charon

In 1930, Tombaugh discovered Pluto, the outermost known planet (Reaves, 1997; Marcialis, 1997). Several authors have derived the radius of Pluto with very small uncertainties; unfortunately, the derived values do not overlap. Consequently, only a broad range can be quoted (1145 to 1200 km) within which the true radius of Pluto may fall (Tholen & Buie, 1997). Pluto is by far the smallest planet of our Solar System; it is even smaller than many planetary satellites. Pluto's orbit is highly eccentric and inclined by more than 17° to the ecliptic plane (Malhotra & Williams, 1997). At perihelion (29.7 AU), Pluto is closer to the Sun than Neptune (30.1 AU), and at aphelion it reaches a heliocentric distance of almost 50 AU.