The Voyager 2 spacecraft, which is rapidly approaching Saturn, is scheduled to measure the almost invisible gaps between the planet's enormous rings down to the length of a city block.

An instrument is to peer through the rings at a star named Delta Scorpii, the fourth brightest in the constellation Scorpius, when Voyager flies past the rings less than 5,000 miles away. The star will be on the opposite side of the rings and almost 1,000 light years distant.

Measuring the star's light as it flashes through the rings will give the best information ever on the number of rings, their thickness and widths, and the widths of the tiny gaps between them.

"We will see the star blink on and off as we move along the rings," Voyager Project Scientist Edward C. Stone said at a press conference yesterday at the National Aeronautics and Space Agency headquarters here. "We'll be measuring the width of the ring gaps you can barely see in a ring system 43,000 miles across on a scale no bigger than a city block."

Now moving at almost 35,000 mph, the 1,800-pound spacecraft yesterday was more than 947 million miles from Earth and less than 8 million miles from Saturn.

Launched from Earth almost four years ago, Voyager 2 will fly by Saturn on Aug. 25 no more than 60,000 miles from the top of the planet's brightly-colored cloud tops, the closest any spacecraft will have come to either of the two giant planets, Jupiter and Saturn.

Nine hours after passing Saturn, the spacecraft will gaze through the outermost ring at a star named Beta Taurri that is five times closer to Saturn than Delta Scorpii. The spacecraft will perform the same experiment, measuring the star's light as it flashes on and off behind the rings.

"Our resolution of the gaps between the ringlets on this pass will be five times better," Stone said.

Besides performing the most precise measurements of Saturn's brilliant rings, Voyager 2 will get the best closeup photographs of the planet and five of its icy moons -- Hyperion, Ieapetus, Phoebe, Tethys and Enceladus.

Not only will Voyager 2 be passing closer to the planet than Voyager 1, it will have the advantage of a better sun angle. Voyager 2 is approaching the planet's rings from the sunlit side, not the dark side as Voyager 1 did. The planet is also tilted more toward the sun this time, giving spacecraft cameras more reflected light for better exposures.

"The cameras are also working better; we're getting better pictures this time on the way in," Stone said. "After all, this is it. This is our last chance in the foreseeable future to get this close to Saturn."