Hard on the heels of the announcement last week of the discovery of the 200th potentially hazardous asteroid, the announcement was made today of the discovery of the 200th member of the Transneptunian Belt. Also known as the Kuiper Belt or the Edgeworth-Kuiper Belt, the Transneptunian Belt is a collection of bodies orbiting the sun generally at distances somewhat larger than that of Neptune.
As with the PHA discoveries, the rate of TNO discoveries has increased very dramatically recently. Fully half of the TNOs have been found during just the last 12 months, with the first discovery having been in 1992 or 1930, according as to whether one does not or does choose to consider Pluto to be a member. Whether one does or does not include Pluto does not affect today's milestone, because the seven new objects being announced take the count well over 200.
While the rate of new TNO discoveries is gratifying, this greatly increases the problem of obtaining enough follow-up observations to ensure a reliable prediction for the next opposition--and then ensuring that recovery observations are then made. Some 61 percent of the TNOs with an opportunity so far for recovery have in fact been observed at a second opposition. Such success is actually quite encouraging, given that the observations at the discovery opposition have often been extremely meager, and that the orbit solutions are almost invariably complete guesswork. Although a second opposition is necessary for a reliable orbit determination, it is hardly sufficient, and continued occasional monitoring is very much in order. The recent recovery announcement of 1998 UU43 consisted of data on two consecutive nights last week of an object observed last year on one night in October and another in December. At least one of the presumed multiple-opposition TNOs, 1995 YY3, now appears to be lost.
It has been usual to separate the TNOs into two principal groups, namely, what are called the "classical Kuiper Belt objects", or "cubewanos" (this name arising from the designation 1992 QB1 of their prototype), with orbits of rather low eccentricity (though with inclinations up to 30 degrees or so) and mean distances between 42 and 47 AU from the sun (Neptune being at a distance of 30 AU); and the "plutinos" (Pluto itself being the prototype), their mean distances of 39 AU giving them orbits having revolution periods that are three-halves that of Neptune, this resonance in fact preventing close approaches to Neptune, even though orbital eccentricities up to more than 0.3 mean that these objects can cross Neptune's orbit. Almost 50 percent of the objects with good orbit determinations are cubewanos, and almost 40 percent are plutinos. It also seems that 5 percent or so have revolution periods that are twice that of Neptune, so they have mean distances of 48 AU, the rather large orbital eccentricities again allowing these objects to approach Neptune's orbit--but not Neptune itself. There are also a couple of resonant objects with revolution periods that are four-thirds and five-thirds that of Neptune.
The remaining well-observed TNO, 1996 TL66, ranges in distance between 35 AU and 135 AU from the sun. There are certainly other objects of this type, sometimes called "scattered-disk objects", although only four of the single-opposition objects, all of them discoveries in February 1999, have officially been assigned scattered-disk orbits. It is highly probable that several of the lost TNOs are actually in this category--which would help explain why they are lost, because scattered-disk status would very considerably augment the amount of sky needed to be searched to guarantee their recovery.
One can argue that the count of PHAs is arbitary because the rules defining a PHA are also arbitrary. But we can at least be sure that the accepted PHAs meet those rules. Given that only 34 percent of the currently known TNOs have been observed at more than one opposition, we cannot really provide a satisfactory definition for a TNO that we can guarantee will be met by the majority of the objects that have been classed as TNOs. Certainly, we seem to be on reasonably firm ground when it comes to the established cubewanos and plutinos (and also perhaps the other resonant objects), but beyond that there is a problem.
Part of the problem is that there is at some level really no dynamical distinction between a scattered-disk object and a centaur. A centaur is an object that in some way moves in the general range of the giant planets. Although Chiron, which in 1977 was the first such discovery, currently moves rather neatly between the orbits of Saturn and Uranus, close approaches to these planets can change this. But half of the 16 objects classified as centaurs actually have their farthest points from the sun beyond the orbit of Neptune--i.e., into the domain of the TNOs. One of these objects, 1995 SN55, is currently well beyond Neptune, at 39 AU from the sun. Its classification as a centaur is quite arbitrary, and it could equally well be classified as a TNO: it is probably not a plutino or other "regular" TNO, but it could easily have been classified as a scattered-disk TNO. So if we are going to consider scattered-disk objects as part of the TNO population, we really should also include at least part of the centaur population.
The combined population therefore has well over 200 members--more than 220 if all the centaurs are included. Then there is the recent 1999 TD10, which we know to be currently just beyond the orbit of Saturn, well inside the "centaur region", but that at its farthest from the sun is quite akin to 1996 TL66 and the other scattered-disk objects. It is "both" a centaur and a TNO, but it is currently being classified as neither.
Finally, there is the matter of the comets. We know that Chiron shows cometary attributes, and it is classified both as a centaur, with the asteroid number (2060), and a comet, with the designation 95P/Chiron. It is widely believed that the centaurs and TNOs generally are protocomets. There are other comets, such as 29P/Schwassmann-Wachmann 1 and 39P/Oterma, with current orbital characteristics that could also allow them clearly to be classified as centaurs. Furthermore, less than half a century ago, the orbit of 39P/Oterma, then inside the orbit of Jupiter, was much like those of many of the other short-period comets, notably, D/1993 F2 (Shoemaker-Levy 9), the string of objects that crashed into Jupiter in 1994.
So we can indeed celebrate and claim that today we acquired our 200th known TNO. But we don't know what that means.
Brian G. Marsden
1999 November 18
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