IT was March 13, 1781. A professional musician and amateur astronomer, who went on to become a world-class scientist, was scanning the sky with a homemade telescope. It was one of the most powerful instruments of its day. A hazy disk swam into view. William Herschel took it to be ``a curious . . . nebulous star or perhaps a comet.'' It was neither. Professional analysis of its orbit soon showed it to be a previously unknown planet -- the planet we now call Uranus. At a stroke, Herschel had more than doubled the known extent of the solar system. On Jan. 24, the hardy 1-ton spacecraft Voyager 2 is due to double the extent of the solar system that has been explored firsthand by scientific instruments from Earth.

James Elliot of the Massachusetts Institute of Technology -- who led the team that, in 1977, first discovered a ring around the distant planet -- describes the significance of that exploration this way: ``Turn on a flashlight for one second. The light produced in that moment exceeds the total amount of light detected from Uranus by astronomers since its discovery by William Herschel in 1781.''

Reviewing in Sky & Telescope magazine what little knowledge astronomers have gleaned from those glimpses of the planet, he says, ``For us Earth-based observers, who have been painstakingly solving the crossword puzzle of the Uranus system bit by bit, the impending Voyager 2 encounter will be like turning to the back of the book for the answers.''

Voyager will be returning those answers at a rate of 21,600 bits a second -- a picture every four minutes of the enigmatic planet and of its coal-black rings and little-known moons -- as it zips through the Uranian system at more than 45,000 miles an hour.

Those pictures will take 2 hours, 44 minutes, and 50 seconds to cover the approximately 1.8427 billion miles to Earth. It will be afternoon at the Voyager control center of NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif., before the Voyager scientific team will know what the spacecraft finds as it passes within 50,600 miles of Uranus's cloud tops -- and 66,500 miles from its center -- at 10 a.m. Pacific standard time on Friday.

Uranus is 19.218 times farther from the sun than is Earth. It receives only 1/400 of the sunshine we enjoy on Earth. It's little wonder that Dr. Elliot could coin his flashlight analogy. Uncovering planet's secrets

However, as Voyager closes in on the Uranian system, it has begun to penetrate its mystery. Some of the nine rings now arc across the television monitors that display Voyager images. And, as planetary scientists have expected, new moons in addition to the five already known have begun to appear. At this writing, seven such small satellites already had been found. Early estimates of their diameters range from 20 to 40 miles. However, Voyager Project scientist Edward C. Stone says the moons may be darker than scientists have assumed. If this is the case, they may turn out to be larger than this preliminary estimate as Voyager gets clearer views of them.

These moons lie, unexpectedly, outside the nine Uranian rings. Scientists had expected to find some quite small moons within the rings. As is the case at Saturn, such moons would have a ``shepherding'' effect on the rings because their gravitational influence would keep ring particles in line. The seven nonshepherding moons so far found, though, have been a surprise.

Any shepherd moons will be quite dark and hard to see, Dr. Stone says. But they may leave wakes like ships at sea or produced kinks in the rings which would reveal their presence.

Uranus is an unusual planet. The third largest in the solar system, it's the only one that rolls along its orbit like a wheel. Its axis of rotation lies within a few degrees of the orbital plane. (In astronomical terms, the obliquity of the pole is 97.92 degrees.) No one knows why this is. Perhaps an Earth-size body struck the planet and tipped it on its side early in its history. Thus, as the planet circles the sun once every 84 years, first its northern hemisphere is in the sunshine for 42 years and then its southern hemisphere is sunlit. As Voyager approaches, the south pole points sunward. Great gas bag

Like Jupiter and Saturn, Uranus is a great gas bag, although it is considerably smaller than those giants. Its diameter is estimated to be about 31,900 miles. That's the diameter of a sphere whose surface coincides with an atmospheric level where the pressure is one ``bar,'' which is roughly sea-level air pressure on Earth. That compares with 89,000 miles for Jupiter and 74,600 miles for Saturn. The Uranian diameter is about four times larger than that of Earth, while the planet has about 14.5 times Earth's mass.

Astronomers think Uranus may be the twin of Neptune -- the next planet on Voyager 2's assignment list. Like Jupiter and Saturn, both planets may have relatively small rocky cores. Computer modeling suggests the Uranian core is about the size of Earth. This is overlain by an ocean of water, which was originally created as ice and which probably has ammonia dissolved in it, Dr. Stone says. A 5,000-mile thick atmosphere covers the planet, according to these models. It consists mainly of hydrogen, although it has been suggested recently that 40 percent may be helium. Noting that this ``would be quite remarkable if it's true,'' Stone says that Voyager's survey will help settle the question of the Uranian composition.

The planet is large enough to command the allegiance of five major satellites. Until now, though, astronomers have seen them only as pinpoints of light, so estimates of their sizes are uncertain while estimates of their masses have proved very questionable. Now the moons are beginning to show bright and dark features in Voyager images. Stone says the flyby should yield good estimates of their size and mass.

The rings, of which nine are known, also present puzzles, although astronomers now have fairly good orbits for these features. They are very, very dark. The outermost (Epsilon) ring -- the only one whose reflectivity has been measured -- reflects only 5 percent of incident light. This implies that the ring particles lack ice or snow coating. They may have coverings of carbon compounds formed when sunlight or energetic particles decompose methane from the Uranian atmosphere. But scientists need a close look to see what's actually there.

Since the rings and moons orbit more or less in the planet's orbital plane, Uranus and its retinue face the oncoming spacecraft like the rings and bull's eye of an archery target. Voyager 2 will zip through that target like an arrow. Its instruments must work quickly, sharply, and move with precision if James Elliot and his scientific colleagues are to get those back-of-the-book answers. That's asking a lot of a spacecraft that was never intended to go to Uranus and which is well past its expected operational lifetime.

``Voyager is in its post-retirement years,'' says Project Manager Richard P. Laeser. But, he adds, ``it is very healthy for its age.'' At this writing, project officials expect the craft to complete its Uranian assignment.

Voyager is one of a pair of what were then advanced space explorers launched in 1977. Ellis Miner calls them ``the most complex and adaptable robot craft ever launched from Earth.''

Voyager 2 was launched first on Aug. 20, 1977. Its twin, Voyager 1, followed on Sept. 5. It visited Jupiter March 5, 1979, and passed by Saturn Nov. 12, 1980. Voyager 1 now is traveling upward out of the plane of the ecliptic -- the plane in which the planets orbit. It should cross the heliopause, where the sun's influence yields to interstellar space, in the late 1990s. It's heading in the direction of the Raselhauque, the main star in the constellation Ophiuchus.

Voyager 2 also surveyed Jupiter and Saturn respectively on July 9, 1979, and Aug. 25, 1981. Its swing by Uranus will put it on course to intercept Neptune on Aug. 25, 1989. Then it, too, will pass beyond the heliopause in the late 1990s, heading in the direction of the ``Dog Star'' Sirius, the brightest star in the sky.

Between them, the twin spacecraft sent back more than 70,000 pictures of the Jovian and Saturnian systems, plus volumes of other data on such phenomena as magnetic fields and energetic particles. Their major discoveries at Jupiter included active volcanism on the Jovian satellite Io, thin rings of dust and gas circling the planet, and three new satellites. At Saturn, they found several new satellites. They determined that the big Saturnian moon Titan has an atmosphere consisting primarily of nitrogen with some simple organic compounds and with a surface pressure 1.5 times sea-level pressure on Earth.

Among the unexpected phenomena in Saturn's rings, the spacecraft found thousands of wave-like features, some of which seem to be due to gravitational influence of some of the satellites. There also are spoke-like features, which may be electrically charged dust particles levitated above the main ring plane. Voyager 2 a survivor

Voyager 2 has survived its eight-year, 3-billion-mile looping journey among the outer planets well. Its main radio receiver failed within a year of launching. Then the backup receiver lost its ability to make certain essential frequency adjustments to receive JPL signals as it moved toward the outer planets. It, in effect, became tone deaf. JPL engineers learned to work around that problem years ago. They can communicate well with the spacecraft.

Also, the scan platform, which carries TV cameras and other vital instruments jammed during the Saturn flyby. The platform later freed up. It now is performing so well that a backup procedure developed to work around any jamming won't be needed, Dr. Stone says. This would have involved rotating the entire spacecraft to point the instruments.

As Voyager 2 nears its target, the mission scientists' anticipation is at a peak. JPL Director Lew Allen Jr. notes that discovery of the new moons is already pushing scientists beyond what they had predicted from planetary theory. Also, there had been concern that the major moons might have bland, uninteresting surfaces. But early glimpses of them are already beginning to show some features. So, Dr. Allen says, ``From the little bit of data which is now coming back from Uranus prior to the encounter, the one thing one has already learned is the one thing we were certain of before we started and that is that Uranus was going to have a lot of surprises. . . . It'll be an exciting encounter.''

Uranus has five major moons whose sizes and masses are poorly defined. In the following table, the uncertainty in a moon's diameter is given in parentheses. Mass estimates are so unreliable it would be misleading to list them. Astronomers should come up with better numbers after Voyager surveys the Uranian system Jan. 24. Name Diameter (miles) Distance from planet (miles) Miranda 310 (140) 81,000 Ariel 826 (80) 119,000 Umbriel 690 (60) 166,000 Titania 995 (75) 272,000 Oberon 1,010 (85) 364,000

Voyager 2 will zip through the Uranian system like an arrow shot through a hoop. It will be moving almost perpendicular to the plane of Uranus's rings. That's the plane in which the planet's moons also orbit.

In the following list of main events, all times are Pacific standard time and refer to what happens at Uranus. The planet is slightly more than 19 times farther from the Sun than is Earth. It will take 2 hours and 44.5 minutes for news of each event to reach Earth even though Voyager's signals travel at the speed of light. Closest approach to planet:

Time of encounter: 10 a.m.

Distance from cloud tops: 50,600 miles.

Distance from planet center: 66,500 miles. Approaches to major moons:

Titania: 227,000 miles; 7:10 a.m.

Oberon: 300,000 miles; 8:13 a.m.

Ariel: 79,000 miles; 8:22 a.m.

Miranda: 18,000 miles; 9:04 a.m.

Umbriel: 202,000 miles; 12:53 p.m. Voyager 2 speed:

Relative to sun: 49,200 m.p.h.

Relative to Earth: 80,500 m.p.h.

Relative to Uranus: 45,000 m.p.h.

Nine rings of variously sized particles orbit Uranus. They're blacker than coal dust and, generally, have slightly variable widths. First discovered in 1977 by a research team led by James Elliot of the Massachusetts Insititute of Technology, some rings are numbered while others bear Greek letters. The mixed nomenclature reflects the preferences of different research group which discovered individual sets of rings. Most rings are somewhat eccentric (elongated) in shape, although Eta, Gamma, and Delta are very nearly circular. Epsilon, the outermost ring, is unusual in being quite elongated with its width varying by tens of miles.

The following table begins with the innermost ring. Ring name Radius of orbit (miles) Width (miles) 6 26,002 0.3-1.2 5 26,249 0.5-1.2 4 26,458 0.4-1.2 Alpha 27,792 3.6 Beta 28,377 3.7 Eta 29,319 0.3-1.2 Gamma 29,597 2 (?) Delta 30,016 1.5-2.0 Epsilon 31,787 12-60

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