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B. G. Marsden (CfA), W. B. Offutt (W & B Obs.)
The appreciation that there are a large number of sizable objects in the outer solar system is arguably the most important solar-system discovery from groundbased observations during the twentieth century. However, most of these objects are at transneptunian distances and very faint, and the difficulty of obtaining the necessary time for follow-up observations on professional telescopes has made it necessary to involve amateur astronomers in this work, if there is to be much hope of learning anything about the objects discovered. The first two transneptunian discoveries, Pluto and 1992 QB1, are generally considered to represent the principal components of the population, namely, the plutinos, in 2:3 mean-motion resonance with Neptune, and the cubewanos, in low-eccentricity orbits with mean distances from 41 to 47 AU. Since only one object out of three has been observed at more than one opposition, and since only one out of three of the present single-opposition discoveries has been followed up for more than a month and a half, the relative number of members in these categories is still far from clear, and, even more significantly, there is next to no information on the fraction of the bodies that are in more eccentric, less stable orbits, characterized by objects as diverse as (2060) Chiron, (5145) Pholus, 1996 TL66 and 1999 TD10.
The recent example of 1998 XY95 was one where observations by an amateur astronomer were crucial. Professional observations of this 22nd-magnitude object at its discovery opposition were confined to two consecutive nights, and the motion was consistent with cubewano status. Using it as a test case for his own transneptunian search, though not without trials and tribulations, the second author observed the object on several occasions, and although the first author suspected the ``scattered-disk'' nature of the object when it had been under observation for two months, full confirmation was not possible until the December 1999 opposition. Our ``pro-am'' collaboration was key to the discovery of 1998 XY95 as only the second multiple-opposition scattered-disk object known, and it played a significant role in cementing 1996 TL66 as the first.