Frontiers of Science:

New Rings for Uranus

By Dr. Sherwood B. Idso

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    Which planet of the solar system has rings around it? If you think the answer is Saturn, you are right—but only half right—for on March 10, 1977, scientists discovered that the planet Uranus is also encircled by a group of these amazing heavenly halos.

    Astronomers have been aware of the rings of Saturn for a long time. In fact, Saturn’s rings were discovered 177 years before Uranus itself was detected in the heavens. Galileo was the first person we know of to see the rings. Using a telescope that did not give him a very clear view, however, he incorrectly identified them as two moons! It was not until 45 years later, in 1655, that a brilliant Dutch observer named Christian Huygens determined their true nature.

    The exciting discovery of the rings of Uranus, barely two years ago, was made with much more modern equipment. Imagine a flying observatory—a special jet airplane equipped with a telescope and other scientific instruments. On the night of March 10, 1977, the craft was airborne at 41,000 feet over the Indian Ocean. Its telescope was pointed at a special star, code-named SAO 158687. Astronomers had calculated that on this evening Uranus would pass in front of SAO 158687, and that light from the star would be blocked from falling on portions of the earth for several minutes. Such “star eclipses” are called occultations, and they are used to help measure the size of any planet that passes between the star and the earth.

    Much to the surprise of the scientists aboard the flying observatory, their instruments showed several decreases in starlight before Uranus itself moved in front of the star and made it disappear from view. Then after the star reappeared from behind Uranus, there were several more reductions in the light from SAO 158687. In checking the 40-foot long data chart the next day, the astronomers found that both sets of starlight reductions showed the same pattern. To produce equally spaced shadows on both sides of the star eclipse, Uranus had to be encircled about by a group of narrow rings!

    Five separate rings were discovered on that historic night. The names given them were the first five letters of the Greek alphabet—alpha, beta, gamma, delta, and epsilon—starting from the innermost ring and moving outward. With respect to size, the inner four rings were found to be only about six miles wide, while the outer one measured close to sixty miles. By way of comparison, Saturn’s outer ring is about 10,000 miles wide, and its inner ring extends for nearly 15,000 miles. The rings of both planets, however, are very thin. Their thickness may perhaps be measured only in feet.

    Much remains to be learned about the new rings of Uranus. For instance, what are they made of, and how did they get there? Scientists believe that the rings of Saturn are composed mostly of ice. What composes the rings of Uranus is still a mystery, and these and other questions may well have to remain unanswered until we can send a spacecraft to the planet. Perhaps some clues will come from the Pioneer XI flight that will encounter Saturn in 1979 and the Mariner flight that will arrive a year later. As we learn more about the rings of Saturn, we will also undoubtedly learn more about the rings of Uranus. And as our knowledge of them expands, so will our appreciation for these wondrous works of God, these heavenly halos of the solar system.

    An artist’s rendition of how Uranus and its system of rings might appear to an approaching Voyager spacecraft in 1986. The rings are much exaggerated for clarity. (Rick Sternbach, courtesy Sky and Telescope magazine.)

    The planet Saturn with its famous rings. (Courtesy University of Arizona’s Lunar and Planetary Laboratory.)

    The Voyager II spacecraft that successfully photographed Jupiter and its “Big Red Eye” is shown here just following its encounter with Saturn. It will now begin a new leg of its journey that will take it to a rendezvous with Uranus in early 1986. (NASA artwork.)

    The planet Uranus with its five newly discovered rings: alpha, beta, gamma, delta, and epsilon. The spacings of the rings about the planet are to correct scale, but their widths have been somewhat exaggerated.