PASADENA, CALIF. — FOR Voyager 2 scientists, years of anticipation are at last bringing their reward: close-in views of Neptune and its encircling rings and moons. Their planet-scanning spacecraft should have zipped through the Neptunian system and be looking back at it from a distance of 471,553 miles by noon Eastern daylight time Friday. But at this writing, Voyager 2 still was on its final approach to the blue-green planet. And as the distance from this target has narrowed, mission scientists have found themselves on a fast learning curve to become acquainted with the remotest of the giant planets.
For example, on Aug. 11, the imaging-science team had found the arcs of two partial rings. One arc, 38,500 miles from Neptune's center, is just outside the small moon 1989 N4. The other arc is just outside 1989 N3, 32,200 miles from the planet center. By last Tuesday, team leader Bradford Smith of the University of Arizona was reporting that the inner arc is, in fact, a complete ring. Neptune had joined Jupiter, Saturn, and Uranus as a ring-girdled world.
Voyager's course was set to bring it in over the ring plane, across the north pole a mere 3,117 miles from the cloud tops, and out across the ring plane again late last night and early this morning. Thus the imaging team expects to have a good look at ring features too tenuous to be seen from Earth as it puts the Voyager pictures through computer enhancement analysis.
Voyager should have an even better view as it looks back at the planet. Sunlight scattered through the rings should make their structure stand out.
Team members likewise need the sharpest views of Triton that Voyager can deliver to resolve the features suggested by the fuzzy views they have had so far of this large pinkish moon. Voyager was targeted to pass within 29,000 miles of Triton at 5:14 Eastern time this morning. It should resolve features as small as O.6 miles across from that flyby distance. Incidentally, close approaches and other distances may differ from those reported earlier, as Voyager navigators have refined them.
Imaging scientists do have a much more accurate estimate of Triton's size already. Bradford Smith puts the diameter at between 1,680 and 1,740 miles. This is smaller than expected. ``Triton has been shrinking as we approached until we feared that by the time we arrived, it might be gone,'' Dr. Smith observed wryly.
By noon, Friday, Voyager should be receding from Neptune at 38,587 miles an hour. Plans called for it to whiz past the planet at a maximum speed of 61,135 m.p.h. when it skimmed the north pole at midnight Thursday. Yet even at such great speed, it was expected to take pictures and return other data that should allow scientists eventually to construct a detailed profile of the Neptunian system.
They will be looking for indications of atmospheric circulation, among other features. The ``Great Dark Spot'' and white cloud structures so prominent in the images taken during Voyager's approach suggest vigorous weather patterns. But scientists need detailed views to trace the atmosphere's circulation.