The definition of the Kuiper belt appears somewhat confused. Some astronomers appear to use the terms "Kuiper belt object" (KBO) and "trans-Neptunian object" (TNO) to refer to the same class of objects. This is not correct. A trans-Neptunian object is any object having a semi-major axis higher than that of Neptune. A Kuiper belt object is an object that remains between 30 AU and 50 AU distant from the Sun. Thus the Kuiper belt is a subset of trans-Neptunian objects.
The conflation of the terms TNO and KBO persists, with some astronomers using the term "classic KBO" to refer to any object lying within the strict bounds of the Kuiper belt.
Technically the first object belonging to the Kuiper belt to be discovered was Pluto in 1930. Next to be discovered was Charon, the moon of Pluto, in 1978. However, the first discovery that caused the astronomical community to recognize the Kuiper belt as a real collection of bodies in the solar system was that of 1992 QB1 in August 1992.
The number of objects that the Kuiper belt contains is confused. Jewitt estimates more than 70,000, but independent corroboration of this number is singularly lacking. Parker counted 651 as of January 2003. Furthermore, Jewitt states that the 70,000 objects he estimates have diameters of 100 km or greater. Wilson calls these "super-comets" thus emphasizing that they are far larger than true comets.
Delsanti and Jewitt now estimate that the total mass of all objects in the Kuiper belt cannot be more than one-tenth the mass of the Earth. The primary basis of this new estimate is that Kuiper belt objects are now known to be much brighter than originally supposed, and brighter than the members of the Jupiter family of comets.
The Short-Comet Theory
The near-universal consensus among secular astronomers is that the Kuiper belt is the source of short-period comets. But the evidence from current observation poses many problems for this theory. First, as Delsanti and Jewitt have admitted, the total mass of the Kuiper belt is severely limited—to ten percent of the mass of the earth, including the masses of Pluto and its moon Charon. Second, the objects known to exist in the Kuiper belt (however many there may be) have diameters of 100 km or greater, far greater than the diameter of any comet head. Third, even the 70,000 objects that Jewitt estimates that the Kuiper belt contains is far fewer than the billion or so objects that would actually be required to supply short-period comets over the presumed 4.5-billion-year age of the solar system. Fourth, some of these objects, like Pluto, are in orbits that are severely inclined to the ecliptic, totally unlike the short-period comets to which they are supposed to give rise.
Champions of the short-comet theory contend that actual short-period comets result from fragmentation of the large KBOs or else are simply too dim for Earth-based telescopes to observe.
Observation and exploration
The only observation of the Kuiper belt or of the objects within it has been by Earth observatories and the Hubble Space Telescope. However, the New Horizons mission, launched on January 19, 2006, will arrive in the Belt in 2015, first to explore Pluto and its moons, and then to explore at least one other KBO. The Panoramic Survey Telescope and Rapid Response System (Pan-Starrs) survey project should be completed by then, and its findings might assist New Horizons mission planners in selecting their next target after Pluto.
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