By Max Planck
Planck M., Jones R., Williams D.H. A Survey of actual conception (Dover, 1994)(ISBN 0486678679)(600dpi)(T)(126s)
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The size of the cometary nucleus can be inferred from the “classical” technique ﬁrst used by Dave Allen  in which simultaneous optical (scattered) and infrared (thermally emitted) ﬂux densities are compared. This method is so important to the study of small bodies that it is worth describing in more detail: in essence it is very simple. Photons from the Sun strike a body and are either reﬂected or absorbed. The fraction reﬂected is called the “Bond albedo,” A. Photons not reﬂected are absorbed, raising the temperature of the body and producing thermally emitted photons at longer wavelength.
23 deserve comment. (a) The nuclei of comets, both dead and alive, show a spread in color that matches that observed in the Trojans but which is distinct from the KBO color distribution. A few blue nuclei are known. We will argue below (Sect. 1) that these are most likely surfaces covered by rubble mantles. (b) The Trojans (which are often but incorrectly described as consisting of very red D-type asteroids) in fact show a wide range of surface colors, down to neutral (S = 0), and they are much less red than the majority of KBOs.
This reasoned position was drowned out by the assertion that Pluto must be the long-sought “Planet X,” predicted by Percival Lowell on the basis of a model of (what turned out to be unreal) deviations in the motion of Uranus. Still, everything is obvious in hindsight, and it is too easy to see what should have been done knowing what we know, and too diﬃcult to reconstruct the full state of confusion that reigned only a few decades ago. For example, Edgeworth in 1943  speculated about “clusters” in the trans-Plutonian region (clusters were his idea for the structure of comets) while Kuiper (for whom the belt is somewhat ironically named) in 1951  considered that this region should be empty, having been cleared of objects by strong perturbations from “massive” Pluto.
A survey of physical theory by Max Planck