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First sharp look at Uranian ring.

This unusual picture of Uranus and the outermost of its nine known rings was made by computer-adding, or integrating, a series of six images taken by the Voyager 2 spacecraft on the way to its Jan. 24 encounter with the planet. The rings were discovered in 1977, when they caused blinks in the light of a star as Uranus moved in front of it, but this is the first picture to show even one of them in a way that was not extremely blurred by earth's atmosphere.

The ring shown, called the epsilon ring, is either eccentric or elliptical, and appears to range in width from about 20 to 100 kilometers. It is also believed to be extremely dark, reflecting about 1 to 2 percent of the sunlight that falls on it, suggesting that it may be composed of carbonaceous material like that covering some asteroids and the dark side of Saturn's moon Iapetus.

Making such a dark object visible, even with the capability of computer-enhancing Voyager's images, required not only the multiphoto sequence, so that the tiny brightness difference between the ring and the surrounding space in each frame could be combined, but also unusually long exposure times. The frames were taken with either 11- or 15-second exposures, producing a cumulative exposure time of 84.5 seconds. But that was not good enough for the narrow ring. The slightest motion of the spacecraft, such as the starting or stopping of its tape recorder, produces vibrations that trigger corrective firings of the craft's attitude-control system, so engineers at Jet Propulsion Laboratory in Pasadena, Calif., had to reprogram the system to minimize the corrective effects as well.

Even with such techniques, however, the other rings, which are narrower still, have remained invisible to the Voyager cameras. The six frames in this picture were all taken on Nov. 28, with the craft about 72.3 million km (44.9 million miles) from the planet. Uranus itself is highly overexposed, due to the long exposure times necessary for the rings. And a number of artifacts -- not parts of the real scene -- are present due to the extreme computer-processing, such as the dark region just above the planet, the bright region below it and the small, bright projections on its upper left.

The ring appears less prominent in the lower left portion of the image, and more prominent in the upper right, which is consistent with where researchers have expected to find its narrow and wide portions.
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Author:Eberhart, Jonathan
Publication:Science News
Date:Dec 14, 1985
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