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Chiron and Pholus: Heavenly Names
From: Cambridge University Press
| By:
John Davies |
EDITOR'S INTRODUCTION |
Searching the heavens for new and uncharted stars  can be a long and often frustrating experience for those who spend their lives behind the telescopes. But once you have verified a discovery, how do you decide what to name it? Some astronomers may opt for a straightforward numbering system, while others opt for something a little more imaginative. In this extract from his book Beyond Pluto, astronomer John Davies tells the story of the naming of two objects, Chiron and Pholus. |
 he tradition of asteroid naming is a long one. It began in the early 1800s, when the Italian astronomer Piazzi named the first asteroid Ceres. The next two hundred or so asteroids also received classical names, but with the introduction of photographic techniques for asteroid hunting in the 1890s, and a consequent surge in new discoveries, the naming protocol began to be relaxed. Soon a variety of people, places and organisations started to find a place in the heavens. In the case of obscure main belt asteroids the names chosen can sometimes be quite frivolous. Minor Planet 2309 Mr Spock, which is named after a cat, who was in turn named after the TV character, is a legendary example. Fortunately, Kowal, who describes himself as 'One of those old fashioned people who think that asteroids should be named carefully' and who perhaps suspected that there was more to 1977 UB than met the eye, was more circumspect. Since 1977 UB had been found early in his survey, and hopeful of finding some more objects later, Kowal looked for a group of mythological characters unrepresented amongst the asteroids. He found that the Centaurs, strange creatures that were half human and half horse fitted the bill. From dozens of Centaurs named in ancient literature, Kowal chose the name Chiron (pronounced Kai-ron), the most prominent Centaur and arguably the one with the best reputation. Mythologically speaking the Centaurs were a rowdy bunch given to drinking, rape and pillage, but Chiron was said to have devoted his efforts to astrology, medicine and the arts. He is described by some scholars as the King of the Centaurs and was known as a teacher as well as a healer. The choice was highly appropriate because the object was in an orbit between Saturn and Uranus and mythologically speaking, Chiron was the son of Saturn and grandson of Uranus. Although Kowal could not have known it at the time, it was an inspired choice. |
Interesting questions
The discovery of 2060 Chiron raised several interesting questions which occupied the popular media until the initial excitement died down. One question that was asked, and that we shall encounter again later in a different context, was whether Chiron was a new planet.
 Although the initial observations did not reveal anything about Chiron's surface, making it impossible to be sure if it was covered in reflective material like ice or darker material such as rock and dust, it was clear from its faintness that Chiron could not be more than a couple of hundred kilometres across. Since there are a number of main belt asteroids bigger than this, it was clear that Chiron was not a planet in the traditional sense of the word. However, classifying Chiron as a minor planet begged the question of whether it was unique. Was it just the brightest member of a new trans-Saturnian asteroid belt? Soon, further calculations based on the larger arcs obtained by using the precovery observations were available. These showed that the orbit of Chiron was not the circle originally estimated, but was an ellipse ranging from inside Saturn's orbit to a point just inside the orbit of Uranus. Such an orbit is not stable and it was clear that Chiron could only remain a denizen of the Saturn-Uranus region for, at most, a few million years. Although neither its precise past nor its eventual future could be calculated, it was easy to show that Chiron will eventually approach close to either Saturn or Uranus. When this happens gravity will drastically change its orbit, either moving it further into space or perhaps sending it closer to the Sun. Whatever its ultimate fate, Chiron is only a temporary resident of the outer solar system. |
Around the time of its discovery, it was variously suggested that Chiron could have been an escaped satellite of one of the outer planets, a rocky asteroid somehow ejected from the main asteroid belt or perhaps a giant comet. The comet theory drew a parallel with another unusual asteroid, 944 Hidalgo, which although rather smaller than Chiron was also in an unusually eccentric orbit. Hidalgo was thought by some astronomers to be a comet from which all the water ice had been removed and which as a consequence was no longer active. However, since in its present orbit Chiron does not approach the Sun closely enough to sublime any water ice on its surface, Marsden and co-workers suggested that Chiron was not so much a dead comet, but rather one which had never lived. If they were right and Chiron was a comet, it was a big one; its brightness suggested that it was about fifteen times bigger than the nucleus of Halley's comet. |
Further discoveries
A few more details of the nature of Chiron were revealed a decade later when David Tholen of the University of Hawaii, amongst others, showed that Chiron was brightening faster than expected as it approached the Sun. Like planets, asteroids do not shine by themselves, they merely reflect sunlight and their brightness at any given time depends on a number of factors. The main ones are the size of the object, how reflective it is and its distance from both the Sun and the Earth. Although it was not clear how reflective Chiron was, and therefore what its absolute brightness should be, it was fairly easy to calculate how its brightness should change as it moved around the Sun. Tholen and co-workers Dale Cruikshank and William Hartmann found that Chiron was not sticking to the rules; throughout the late 1980s it was getting too bright to be explained by just its steadily decreasing distance from the Sun. An airless object's reflectivity, or albedo, is determined by its surface composition and this is unlikely to change dramatically over a period of a few years, so the most obvious explanation was that the extra brightening was due to Chiron developing a comet-like coma of gas and dust. A coma would drastically increase the area of material reflecting sunlight and could cause the anomalous brightening. These speculations were confirmed in 1989 when Karen Meech and others took images which showed that Chiron had indeed developed a coma, and even had a comet-like tail of material blowing away from the Sun. |
At first, this cometary outburst was ascribed to Chiron warming up as it approached the Sun. It was speculated that solar heating somehow caused gases trapped below the surface to blow a hole in an insulating crust and allow a cloud of gas and dust to escape. Although water ice would be expected to remain frozen at the distance of Chiron, other volatile gases such as carbon monoxide, nitrogen and methane could be responsible. These gases can be trapped in amorphous (water) ice and can be released if the ice undergoes a change to the more regular, and familiar, crystalline form of ice. This transition from amorphous to crystalline ice can occur at quite low temperatures and has been proposed to explain the activity of various comets at great distances from the Sun. However, a careful study of old photographs showed that Chiron's activity was not restricted to periods when it was relatively close to the Sun. Outbursts were detected even when Chiron was near the most distant point of its orbit. To confuse things still further, Chiron's activity did not continue as it approached the Sun and actually diminished or even stopped during its perihelion passage in the 1990s. Whatever the source of the outbursts, it seemed that Chiron looked like an asteroid some of the time, but like a comet the rest of the time. Just like the mythical Centaurs, it was neither one thing nor the other. |
Charles Kowal continued his search for about eight years, finally finishing in February 1985. By then he had observed 160 fields totalling 6400 square degrees of sky. Although he found four comets and several Earth-approaching asteroids, Kowal never did find another Centaur. Chiron remained a lonely enigma for almost fifteen years. The next step forward came from a project initiated by Tom Gehrels, who had himself unknowingly recorded Chiron in 1977. The Spacewatch project was established by Gehrels to make a long-term, systematic search for new solar system objects using electronic detectors instead of photographic plates, and computers instead of blink comparators. From a telescope on Kitt Peak in Arizona, Spacewatch was scanning the skies for several nights a month, making repeated observations of the same area to find objects that moved noticeably in a few hours. Most of Spacewatch's discoveries had been of asteroids close to the Earth but, on 9th January 1992, David Rabinowitz was in the Spacewatch control room when the system's moving-object detection software drew his attention to an object apparently moving too slowly to be a normal asteroid. |
The moment of truth
Rabinowitz knew that false alarms resembling faint slow-moving objects were quite common. From time to time, the software linked together marginal detections of stars and electronic noise and mistook them for a single object moving across the images. However, Rabinowitz immediately realised that this source was much brighter than a typical false alarm. A quick examination of the images showed that the new object was pointlike, confirming its likely reality and suggesting that it could be a new, distant asteroid. Rabinowitz phoned Beatrice Mueller, who was working at the nearby 2m telescope and she immediately agreed to try and observe the new object. She made the observations the same night. Within a few days, further observations had been made by the Spacewatch telescope and additional sets of positions were being reported by other astronomers. |
One observation was from a pair of plates which had been taken on New Year's Eve by the comet-hunting team of Gene and Carolyn Shoemaker and their colleague David Levy. This group was really searching for fast-moving objects, but on this night their photographs also contained something that was moving rather slowly. They only had observations on a single night, not enough for the Minor Planet Center to do much more than file the positions away for future reference and certainly not sufficient for the object to be recorded as a possible new discovery. Because of this Carolyn Shoemaker admits she did not give measuring the object's position very high priority. At the time she thought it was probably a more-or-less ordinary asteroid, unlikely to be followed up by anyone else and so probably fated to be lost again. The observations were eventually reported to the Minor Planet Center on the 13th of January. They arrived about the same time as reports of another independent discovery made from photographic plates taken on 9th and 10th of January by Eleanor 'Glo' Helin. Well known for her work on asteroids passing close by the Earth, Helin was observing from Mt Palomar as part of a long-running search for fast-moving objects. As usual, while at the telescope she was concentrating on scanning her photographs as soon as possible so that any fast-moving objects which turned up could be reported and followed up immediately. Only once the observing run was over, and she was back in her office in Pasadena, California, did she have time to return to the photographs and search for anything moving slowly. Although the limited capability of the Palomar Schmidt telescope meant that very faint objects were likely to avoid detection, this time there was something recorded on the film. |
Although the Spacewatch team had been first to make a report of their detection, all the sets of positions were published together and the new object was given the temporary designation 1992 AD. From the preliminary orbit calculated using the January observations, it was possible to begin a search for earlier detections. Soon a candidate was found on a plate exposed a year earlier by the Shoemakers. Assuming this sighting was indeed 1992 AD, another detection was found by Beatrice Mueller on a 1989 image. Soon 1992 AD turned up on photographs taken in 1982 and 1977. From all these observations a definitive orbit could be calculated and the new object received the minor planet number 5145. Now it needed a name. David Rabinowitz was interested in moving away from the tradition of naming asteroids after characters from Roman or Greek mythology, especially since he felt that the Centaurs as a group were an unsavoury bunch. He favoured naming outer planet asteroids after creatures from the creation myths of a number of different cultures. He suggested that the new object be called Chaos. This would have been a very appropriate name for an object in a planet-crossing, and so probably unstable orbit. However, tradition prevailed and the object was eventually named 5145 Pholus, who was Chiron's brother. |
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