THIS Thursday, at a distance of 246 light minutes (2.75 billion miles) from Earth, a technological miracle is due to happen. Voyager 2 will take us on a tour of the planet Neptune and its satellite system of ring arcs and moons.
The hardy spacecraft has been returning data for 12 to 16 hours a day since it left Earth 12 years ago last Sunday, except when it reported continuously during its encounters with Jupiter (1979), Saturn (1981), and Uranus (1986).
Now it is repeating those virtuoso performances as it surveys what is currently the most distant planetary system in the solar system.
``Even those of us deeply involved in planning Voyager's mission can scarcely believe the remarkable durability of the craft,'' says deputy project scientist Ellis D. Miner, in a review of technical details of the Neptune fly-by in the astronomy magazine Sky & Telescope.
At this writing, Voyager is working well and ready to take on its last planet, according to information officer James Doyle at the Jet Propulsion Laboratory (JPL) in Pasadena, Calif. The lab manages the spacecraft for the National Aeronautics and Space Administration (NASA).
Voyager was speeding toward its target at 42,375 miles an hour on Aug. 22. With 2.37 million miles still to go, it has already begun to reveal Neptunian details about which scientists could only speculate before.
Distinctive cloud patterns now can be seen across Neptune's blue-green disk. Four small moons were found near the planet in July and early August. On Aug. 11, Voyager's camera picked out two incomplete rings partially encircling Neptune.
Voyager scientists expect more moons and ring arcs to appear as the craft nears the planet system.
The planned trajectory brings Voyager over Neptune's north pole at about 60,400 m.p.h. At midnight Aug. 24, the spacecraft skims just 3,013 miles from the cloud tops - 18,132 miles from the planet's center. That's the closest it has come to an exterrestrial body.
Neptune's gravity will bend the trajectory sharply southward, sending Voyager behind the planet (as seen from Earth) and on toward the giant moon Triton. It will pass within 25,000 miles of that moon 5 hours, 14 minutes after its closest approach to the planet, racing by at a speed of about 40,300 m.p.h. On its way into the Neptunian system, Voyager will take a look at the moon Nereid, although it won't come closer than 2,902,000 miles. Triton and Nereid are the only two Neptunian moons astronomers have seen from Earth.
VOYAGER will continue its Neptune study, which began June 5, until Oct. 2. Then, it will take up a new work program - the Voyager Interstellar Mission - to report on space conditions as it heads toward the interface between our solar system and interstellar space. Voyager should be able to pursue this new mission well into the next century, barring equipment failure. Its plutonium-fueled atomic power units have enough energy to last roughly another 25 years, according to JPL.
But for the next few weeks, the Voyager team won't be thinking much beyond Neptune as its members sort through the inflowing data to learn at last the true nature of what has been a tantalizingly mysterious planetary system.
With an equatorial radius of 15,400 miles and a polar radius of 15,100 miles, Neptune is nearly four times bigger than Earth and only slightly smaller than Uranus. Yet it weighs in at 10.5 earth masses compared with 8.7 earth masses for Uranus. Its average density of 1.66 times that of water, compared with the Uranian density of 1.19, suggests that Neptune may have somewhat more rock and less hydrogen and other gases than does its sister planet.
Planetologists think both planets are roughly half water, with the rocky material concentrated in a central core.
In Neptune's case, they think the core's diameter is about 10,000 miles. A mantle made up largely of liquid ammonia, liquid methane, and water ice surrounds this core and underlies an atmosphere rich in helium and hydrogen. That atmosphere obviously also contains methane and ethane, which absorb red light to give the planet its blue-green color.
Astronomer John Davies at Britain's Royal Observatory in Edinburgh offers what he calls the ``interesting, although probably academic, possibility'' that a layer of diamond encases the rocky core. This layer would form as methane decomposes into hydrogen and carbon under the intense pressures and high temperatures deep within the planet, with the carbon crystalizing into diamond.
The members of the scientific team at JPL are using every datum they get from Voyager's imaging camera, its magnetic and electric field sensors, its particle detectors, and its chemical-analyzing spectrometers to try to pin down Neptune's makeup. Among other phenomena, they expect to find a magnetic field.
Indeed, they may have already detected it by Aug. 22. Its features, including its strength, could help scientists fix the planet's rotation rate. Cloud patterns go around once in 17.7 hours. If this is the rate at which the underlying planet spins, its day would be significantly shorter than a day on Earth.
The cloud patterns - including a feature reminiscent of Jupiter's red spot - suggest a vigorous atmospheric circulation. Neptune is warmer than it should be, given its distance from the sun. It has an internal heat source that probably helps drive the atmospheric circulation.
Some of the most intriguing new features to study are the small moons and partial rings that Voyager has already begun to locate. The longer of the two ring arcs, found Aug. 11, is 38,500 miles from the planet's center and is 30,000 miles, or about 1/8th of a circle, long. The smaller arc is 32,200 miles from Neptune's center and 6,000 miles, or about 1/34th of a circle, long.
Scientists think that the small moons probably shepherd material into the ring arcs and may even supply some of that material. The outer ring arc is just outside moon 1989 N4, while the inner arc trails about 50,000 miles behind 1989 N3.
Peter P. Goldreich, Norman N. Murray, Pierre-Yves Longaretti, and Donald D. Banfield of the California Institute of Technology speculate in the journal Science that the large moon Triton may have caused some of the ring debris. They suggest that Triton was an asteroid-like body that collided with a Neptunian moon and was captured by the planet. It then hit and destroyed most of Neptune's other moons, except those orbiting close to the planet. They predict that Neptune may have as many as 10 to 100 ring arcs. Edward Stone, the Voyager chief scientist, has predicted 50 to 100 partial rings.
With a diameter variously estimated at 1,200 to 2,400 miles, Triton is one of the largest satellites in the solar system. It's also the only large satellite in a retrograde orbit, that is, moving in a direction opposite to its planet's spin. Telescopic observations indicate Triton has an atmosphere with a surface pressure as much as 1/10th that on Earth. It may consist largely of nitrogen and methane, plus other hydrocarbons. The moon's surface seems to be covered with a mix of hydrocarbons.
Saturn's giant moon Titan probably has a hydrocarbon ocean too. But atmospheric haze blocked Voyager's view in 1981.
The spacecraft should do better this week. Dale D.P. Cruikshank of the NASA Ames Research Center and colleagues reported new studies last month that predict Voyager cameras will have a good view of Triton's surface.
Voyager's imaging systems should be able to resolve details as small as a few hundred meters across on Neptune and 0.6 miles across on Triton.
Nereid is another story. Voyager's view of that moon will be too distant to resolve features less than 30 miles across. Nevertheless, it may solve the mystery of Nereid's varying brightness. Martha and Bradley Schaefer of the NASA Goddard Space Flight Center have reported variations in brightness of nearly a factor of four. This could imply that Nereid has an elongated shape and presents a varying surface area as it spins. Alternatively, there may be dark areas on an otherwise bright surface, as is the case with the Saturnian moon Iapetus.
The spacecraft should also help scientists measure Nereid's size. Crude estimates range from 100 to 900 miles.
SCIENTISTS have speculated freely as to what Voyager will find. Geophysical Research Letters (GRL) has published a collection of these predictions. They include such wonders as lightning in Neptune's atmosphere and a stratospheric haze of frozen hydrocarbons.
Some suggest Triton has auroras and a cold, bright surface with windswept, methane ice. But, as GRL editor James L. Burch observes: ``Nature's imagination ... is vastly superior to our own.'' He says he looks forward to ``the surprises that Neptune and its satellites surely hold for us.''
VOYAGER 2 PROFILE Launch Vehicle
Height: 160 ft.
Weight: 704 tons
Weight: 1,820 lb.
Antenna diameter: 12 ft.
Scientific package: 254 lb.
Total Mission Cost
Voyagers 1 and 2: