An astronomy team’s accidental discovery of a set of planetary rings fueled new speculation on ring characteristics. Prior to this discovery, most theories of planetary rings had tried to explain why only one planet, Saturn, has any rings.
In 1973, Gordon Taylor
This particular occultation, of a star known as SAO 158687, was the first ever predicted for Uranus. Because of its importance, Cornell University astronomer James Elliot persuaded the National Aeronautics and Space Administration (NASA) to let his team observe the event from the Gerald P. Kuiper Airborne Observatory
The KAO is a C-141A cargo jet converted for use in astronomical missions. Named for the pioneer astronomer Gerald P. Kuiper, the KAO carries a 0.9-meter-diameter telescope and its controlling computer. Because the KAO can reach altitudes of 12,300 meters, above the bulk of the atmosphere, it provides much clearer views than ground-based telescopes. In the case of the Uranian occultation, the KAO would provide another advantage: It could fly to the Indian Ocean, where the event would be visible for the longest time.
Rather than rely on human eyesight or cameras to view the event, the astronomers used a photometer, a very sensitive light detector having ten thousand times the resolution of photographic equipment. Affixed to the telescope, it would divide the faint light from the star and the planet into three wavelength ranges and record their intensity onto both magnetic tape and a paper strip. As the occultation progressed and the star vanished behind Uranus, the continuous photometer record would provide the most precise “image” of the event.
The KAO left Australia’s Perth International Airport, heading southwest, at 10:37
Several months prior to the flight, an error had been discovered in Taylor’s original prediction of the occultation. There was a chance now that the occultation would not occur at all. On board the KAO, Elliot was wary that the error would cause them to lose this opportunity, so he started the continuous data recording forty-one minutes before the forecast occultation time. Only six minutes after the recording began, the photometer recorded a sudden drop in the starlight which lasted for several seconds. This “dip” was so faint that it went unnoticed until Dunham saw it on the data recording almost a minute later. Elliot suspected that the dimming may have been caused by clouds in the upper atmosphere or a momentary failure in the tracking system, but no one on the mission had a definite explanation.
Voyager 2 image of Uranus’s ring system. The long exposure produced a nonuniform smear as well as streaks from trailed stars.
Four minutes after the first dip, a second dip occurred. This time, meteorologist Pete Kuhn, who monitored the water vapor in the air, verified that no clouds could have caused the dip (or “secondary occultation,” as it was properly called). The team laughed when Elliot joked that it may have been caused by the “D-ring” of Uranus (that is, the fourth ring from the planet), which, in fact, it was. This was not the first time that this ironic joke had been made. When the prediction error had first been discovered, the astronomers realized that the event they intended to view might not occur; yet, they were committed to the flight and continued their preparations. Joseph Veverka,
On the KAO, a third dip happened a minute after the second. Because of the brief duration of the dips, Elliot raised the possibility that “small bodies like thin rings” were occluding the star. It is important to remember that Saturn’s rings are tens of thousands of kilometers wide (measured radially). Despite the team’s jesting, any Uranian rings “should” have been very wide as well. Now, as they saw the starlight dimmed by some sort of structure much narrower than Saturn’s rings, Elliot pictured a zone, or “belt” containing many full-size moons, as opposed to a set of true rings, composed of small particles. As Elliot wrote later, the team sometimes spoke of “thin rings” when talking about this belt, but, in truth, a ring only about 10 kilometers wide seemed “far-fetched” to them.
To confirm the theory of the “belt of satellites,” they continued observing the star after it passed from behind Uranus itself, to see if the star would be occluded by more moons on the other side of Uranus. The confirmation came when the five new secondary occultations occurred as expected. This made a total of ten of the mysterious occultations: five to the “left” of the planet and five to the “right.”
When the KAO landed back in Australia at 8:43
Nevertheless, one fact might contradict the belt-of-satellites hypothesis. When the star was occluded by Uranus itself, its light was naturally blocked completely. Yet, during the mysterious “secondary occultations,” the starlight was only dimmed, not blocked. A moon of Uranus would have cut off the light, unless it only partially blocked the star’s image. Elliot considered the possibility of ten such “grazes” to be “highly improbable, [but] not impossible. In any case, it seemed more likely than narrow rings.” Elliot realized that there was a definite way to determine the answer.
From the telescope’s point of view, Uranus’s known satellites, as well as any new satellites or rings, would orbit the planet in what appeared to be a wide circle with the planet in their center, like a bull’s-eye. If this new discovery were a belt containing ten or more new satellites, each would appear at a different distance from Uranus, and each of the dips would indicate that. On the other hand, if these were rings, then the spacing between the dips to one side of the planet would correspond exactly to that on the other side.
Elliot examined the chart recording made by the photometer, intending to disprove finally the ring hypothesis, but, instead, found matching spacings that could be caused only by rings. Later, Mink and Dunham reexamined the record by computer, and found the spacings to be even closer than Elliot had first believed.
Elliot and his colleagues had found something completely unexpected: a new set of rings which, unlike those of Saturn, were narrow, sharp-edged, and so dark that they could be seen only with twentieth century equipment.
In later years, further occultation studies of Uranus revealed four more rings, and in 1986 the Voyager 2
Astronomers are curious about why these rings are so dark. Whereas Saturn’s rings are reflective water ice, these new rings are too dark to be seen with any telescope. The particles are blacker than coal dust. By the early years of the twenty-first century, scientists still had not determined what they are made of.
Scientists no longer consider rings to be uncommon around large planets such as Uranus and Neptune. The Voyager space probes, as expected, found a ring at Jupiter—but this ring is unlike those found previously in that it is made of gas, not particles.
Finally, the Uranian discovery proved that stellar occultations can be used to find planetary rings. Occultation studies of Neptune have found evidence of rings there as well.
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Corliss, William R., ed. The Moon and the Planets: A Catalog of Astronomical Anomalies. Glen Arm, Md.: Sourcebook Project, 1985. One in a projected twenty-five-volume series whose purpose is to “collect and categorize all phenomena that cannot be explained readily by prevailing scientific theories.” This edition calls attention to the unusual darkness and narrowness of the rings of Uranus, the “disarray” among Neptune’s moons, some anomalous wet areas on Mars, and so on. Each entry is referenced scrupulously to established journals and suggests possible explanations to some of its anomalies. -
Elliot, James L. The Ring Tape. Cambridge, Mass.: MIT Press, 1985. Transcription of the audiotape made aboard the Kuiper Airborne Observatory during the Uranian occultation mission. Excerpted in both Rings and Sky and Telescope, it provides a unique opportunity to listen to discovery in progress. -
Elliot, James L., Edward Dunham, and Robert L. Millis. “Discovering the Rings of Uranus.” Sky and Telescope 53 (June, 1977): 412-416, 430. Elliot’s first-person account of the Uranian discovery, including excerpts from the audiotape made in the KAO cabin during the mission. Sky and Telescope is aimed at amateur astronomers; therefore, the piece is accessible to most readers with a high school science background. -
Elliot, James L., and Richard Kerr. Rings: Discoveries from Galileo to Voyager. Cambridge, Mass.: MIT Press, 1984. Discusses all planetary rings known and hypothetical, as of 1984. Includes a more in-depth retelling by Elliot of the KAO mission. Some chapters written by Kerr, who has reported on Jovian and Saturnian discoveries for Science magazine. Clear and entertaining for general readership. -
Esposito, Larry W. Planetary Rings. New York: Cambridge University Press, 2006. Covers ring history and evolution. Includes index and extensive bibliography. -
Hunt, Garry, and Patrick Moore. Atlas of Uranus. New York: Cambridge University Press, 1988. One of the few books devoted to the planet, it was published late enough to include information from the Voyager 2 flyby. Contains close-up maps of all moons and information on all eleven rings. This is a “coffee-table” book: large-page format, easy to follow, and with many attractive pictures. -
Miller, Ron, and William K. Hartmann. The Grand Tour: A Traveler’s Guide to the Solar System. 3d ed. New York: Workman, 2005. A general-readership book with an unusual focus: It examines all bodies in the solar system at least 1,600 kilometers in diameter, in size order beginning with Jupiter and going down to the largest asteroids. The paintings are stunningly effective. This book contains rare views of the Uranian and Saturnian rings from underneath.
Voyagers 1 and 2 Explore the Outer Planets
Pluto’s Atmosphere Is Discovered
Astronomers Discover an Unusual Ring System of Planet Neptune
Cassini-Huygens Probe Is Launched