The transneptunian region of the solar system continues to hold surprises. Hard on the heels of the large and unusual 1996 TL66, the subject of a letter in NATURE on June 5, what may be an even larger object in an even more inclined orbit was announced yesterday evening on Minor Planet Electronic Circular 1997-M05.
The new object, known as 1996 TO66, brings to 45 the number of known or suspected members of the so-called Kuiper Belt at and a little beyond the orbit of Neptune. With a diameter of 2300 km, Pluto is the largest member of the Belt, and Pluto's satellite Charon, with a diameter of 1200 km, seems to be the second largest. At perhaps 600 km, 1996 TO66 now becomes the third largest known member, followed by 1996 TL66 at 500 km. But whereas TL66 has an extremely eccentric orbit that takes that object out to more than four times the distance of Neptune, the distance range of TO66 seems to be more typical of the Kuiper Belt.
TO66 was originally discovered in October 1996 by the same observing team and on the same observing run that produced TL66. The team, led by Jane Luu, of the Harvard-Smithsonian Center for Astrophysics, and David Jewitt, of the University of Hawaii, and including also student software-specialist Chad Trujillo, found both objects with the help of a wide-field electronic-imaging device on the 2.2-meter telescope operated by the University of Hawaii on Mauna Kea. In each case, immediate orbital computations by Brian Marsden, also of the Harvard-Smithsonian Center, showed that the objects were of more than usual interest as regards size and orbital orientation, although knowledge of the actual character of the orbits required the acquisition of many more data during subsequent months.
The first follow-up observations of both TL66 and TO66 were obtained in November by Harvard student Warren Brown and University of Arizona student Carl Hergenrother with the 1.2-meter telescope the Smithsonian maintains in southern Arizona. Hergenrother also made a tentative single detection of TO66 in mid-December. Since no time was available on either the 2.2-meter or the 1.2-meter telescope in January for confirming that December point, the help of Warren Offutt, a retired engineer and active amateur astronomer, was enlisted. Offutt operates a 0.6-meter telescope in the mountains of New Mexico and is the only amateur astronomer who has so far succeeded in detecting, with amateur equipment, any of the Kuiper Belt members apart from Pluto. By January, however, the much faster motion of the earth around the sun meant that TO66 was observable for a rather short time after sunset, and then with it low in the sky, making observations particularly difficult, even for much larger telescopes.
Although thwarted by bad weather and poor conditions generally, Offutt did succeed in obtaining critical observations of TO66 on three separate nights in mid-January, and these confirmed the validity of Hergenrother's December observation. Hergenrother made use of some unexpected "free" time on the Smithsonian telescope to record TO66 as soon as darkness fell on the last night of January, and Offutt was able to follow the object a few nights longer. Despite the long arc of observation, the orbital nature of TO66 was still far from clear, and for the next four months the object would be essentially behind the sun and completely unobservable.
As June arrived, Hergenrother was poised to pick up TO66 as it emerged from the sun's rays into the morning sky. By prearrangement, the 1.2-meter telescope was scheduled for this purpose on the mornings of June 11 and 12, after the bright evening moon had set. Although he was armed with Marsden's prediction, based on the 115-day arc of earlier observations, there would necessarily be some uncertainty in the expected position, and the object's motion during the limited time available on a single morning would be too small to permit him to identify the object among the stars. Hergenrother secured a single frame of the field on the morning of the 11th. On the 12th, clouds marched over the region when he was only part way into the exposure; he decided to close down, and the dome hadn't been closed for more than two minutes before it started raining!
So it was again Offutt to the rescue, his telescope being scheduled for his use on any clear night. Offutt obtained a single frame of the field as it just barely cleared the treetops some 40 minutes before the onset of dawn on the 14th. The next two nights were cloudy. On the 17th, Offutt got three good frames, each exposed for 22.5 minutes. Although an initial cursory inspection showed nothing of interest, he did, later in the day, find something showing very weakly on all three exposures some 12 arcseconds from the prediction. It was not obviously moving, and to make a measurement it was necessary to add the three frames together. But there was nothing at the same position on the 14th! On estimating now how much TO66 would have been shifted from the prediction on the 14th, Offutt succeeded in finding a weak image there, but reliable measurement was impossible because it was almost merged with a field star.
Computing away in Massachusetts, Marsden agreed that Offutt's June 17 candidate was promising, but any attempt to use the limited June 14 data was inconclusive. At this stage the choice was between comparing notes with Hergenrother and having Offutt try the field again on the 18th, which would be the last morning he could do so for two weeks because of interference from the moon--after which time the summer rains could be expected to set in in earnest. Despite some moon and haze, Offutt obtained three more frames on the 18th, and on comparing them with those from the morning before, he quickly found and measured the elusive image. Marsden agreed that the shift relative to his computation was completely consistent, and he isolated a small region for Hergenrother to examine on the 1.2-meter June 11 frame. Within 25 minutes of e-mailing this refined calculation to Hergenrother, he had received back from Hergenrother an accurate measurement of the correct June 11 image!
Completing the orbit solution was now a straightforward matter, although the eccentricity of the orbit is still not well determined. This is mainly because the object is evidently in the outer part of its orbit, half as far out again as Neptune is. Only one other well-observed Kuiper Belt object is currently out farther than this. Nevertheless, the eccentricity, perhaps about 0.11, does seem to be within the range of eccentricities of the "main" part of the Kuiper Belt, or the "cubewanos". The average distance of TO66 from the sun, 43 astronomical units (i.e., roughly 43 times the earth's distance from ths sun), is also inside the rather confined cubewano range of 42-46 astronomical units.
Particularly interesting about TO66 is the fact that the angle between the orbit planes of TO66 and the major planets is four times that of its nearest cubewano competitor. At 27 degrees--to the earth's orbit--this inclination exceeds the 20-degree maximum of Pluto and the other well-established "plutinos", a group of bodies located at an average of 39 astronomical units from the sun and that in many cases actually cross Neptune's orbit. The inclination is also greater than the 23 degrees for the unique TL66, althugh it is less than the 32-degree figure for 1996 RQ20, another body that orbits the sun somewhat beyond the cubewanos but is more comparable to them in size. If the 0.11 eccentricity is confirmed, TO66 will be 38.8 astronomical units from the sun at its closest point, noticeably closer than the minimum of 40 for the other cubewanos. But its orbital orientation still prevents an approach between TO66 and Neptune (which is 30 AU from the sun) of less than 10 AU--as is also the case for the low-inclination cubewanos. The plutinos are also prevented from currently passing within at least 10 AU of Neptune because their orbits resonate with Neptune's.
With its early recovery in the dawn and the resulting orbit improvement, 1996 TO66 should be well observable for the remainder of 1997, inviting careful study of its physical characteristics. It will be best observable, on the meridian at midnight, around the time of the September equinox, shining then as a star of twenty-first magnitude.
1997 June 19
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