More Stellar Ring Occultations

Luck was on the side of the Villanova team. Another Neptune stellar occultation was to occur only two weeks later (on May 24,1981). It would only be a grazing event, but the star was very stable and well-behaved. The team, led by Harold J. Reitsema, could potentially confirm the Villanova team's discovery. Once the observations were made, measurements made by Reitsema's team did show a brief drop in brightness, but the data didn't look like what one would expect for a ring occultation. The interpretation was more consistent with the unlikely discovery of a new Neptune satellite, one with a diameter of approximately 180 km [6]. The only way to substantiate this discovery was with additional measurements. Unfortunately, the small size of the satellite, the poor knowledge of its orbit, and the paucity of occupation events made confirmation unlikely. The Villanova team would have to wait eight more years for the Voyager 2 Neptune encounter to have a chance at confirmation of their potential discovery.

Meanwhile, the two stellar occultations and their atypical results served to increase interest and speculation about the cause of these brightness decreases. The next observed stellar occultation event was on June 15,1983, and W. B. Hubbard was ready. Hubbard and his colleagues observed the event from six different locations. The multiple viewing angles allowed Hubbard to search as close as 0.03 radii of the planet's surface for rings. Unfortunately, no indications of rings were found [7]. Over the next six years many observations were made. Table 5.1 lists these attempts and their varied results. These inconsistencies vividly demonstrated how difficult these measurements are and how difficult it is to determine from Earth whether or not Neptune had rings.

With only a third of the occultation events showing possible brightness variations, the ring system, if it existed, was not typical. It was the job of the theoreticians to combine all the observed data and piece together a model that could explain these observations. However, no single model would fit all the findings. If Neptune's rings existed, the truth about them would contain some combination of the following hypotheses:

(1) Two shepherding satellites, each between 100 to 200 km in diameter could confine the ring (they would be too small to be seen from Earth) [9].

(2) The ring system was either incomplete or at least highly azimuthally variable [10].

(3) Newly discovered satellites might produce some of the observations.

(4) Incomplete rings could consist of a series of short arcs having a width of 100 km that center on co-rotation resonances of a single satellite in an inclined orbit [11].

Ring scientists were clearly puzzled. One of them, Philip D. Nicholson, stated that no more than three of all the stellar occultation events were probably real. However, based on this meager data set, he postulated that if Neptune did have rings it would have at least three distinct ring arcs, the outer-most of which was approximately 72,500 km from the center of the planet. The other two could not be resolved into a single location. In 1984 the situation was summed up by Elliot and Kerr who said, "If Neptune has rings, they almost certainly will not be discovered from the ground'' [12].

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