AT the planet Neptune, the music of the spheres has become what Edward Stone calls ``the final movement of the Voyager symphony.'' And as the spacecraft covers the last miles to its target, ``the tempo is picking up,'' the Voyager project scientist says.
At noon Eastern daylight time Thursday, Aug. 24, Voyager 2 will be nearly 2.748 billion miles from Earth and just under 470,000 miles from what at present is the most distant planet. At midnight, it will swoop over Neptune's north pole within 3,000 miles of the highest cloud tops in the closest approach to a solar-system body it has yet made. It will pass 29,000 miles away from the large moon Triton five hours and 14 minutes later.
The tempo of the ``Voyager symphony'' will beat fastest during those close-encounter hours, if the spacecraft continues to work as well as it has done up to this writing. The planet, by then, should have unveiled many of its secrets.
The increasingly detailed views of the cloud forms that cross Neptune's blue-green disk have enthralled and perplexed mission scientists here at the NASA Jet Propulsion Laboratory. In their excitement, the atmospheric specialists have been giving pet names to their favorite features.
There is, for example, the ``Great Dark Spot,'' named by analogy to Jupiter's famous Great Red Spot. Then there is ``the Scooter.'' This is a relatively small cloud system, whose motion outpaces that of other features to suggest that Neptune has very fast winds in its upper atmosphere.
Imaging-science team leader Bradford Smith of the University of Arizona notes that the Jovian Red Spot is a giant cyclonic storm system. Mission scientists don't yet know the nature of the Neptunian Dark Spot, which also seems to be a major planetary feature. While much smaller than the Jovian spot, it still is large enough for Earth to fit snugly inside it.
Dr. Smith says it is intriguing that Neptune's spot has the same size relationship to the planet's diameter that Jupiter's spot has to Jupiter's diameter. Moreover, he adds, both spots are at the same 20 to 23 degree southern latitude.
Jupiter's atmosphere has many smaller spots that are also storm systems. They form between - and are sustained by - jet-stream winds that flow in opposite directions. Dr. Stone says that one objective of the present encounter is to try to determine whether winds on Neptune can sustain similar storm systems.
Indeed, he explains that one of the major objectives of Voyager 2's ``grand tour'' of the four giant outer planets is to make a comparative study of these bodies. Neptune is a key factor in that study.
It is the only one of those planets so far from the sun on which any methane present may be frozen. The white cirrus clouds so prominent in the Voyager pictures are likely to consist of methane ice particles rather than the water ice crystals that constitute cirrus clouds on Earth.
As of this writing, scientists studying Neptune's weather were perplexed by the question of what drives this dynamic system. Bradford Smith points out that the energy available to drive it - the total of sunshine and internal heat - is only 5 percent of that available to Jupiter.
Stone adds that, while Neptune has an active atmosphere like Jupiter, the distribution of winds with latitude is similar to that of the much less active atmosphere of Uranus. This is ``another mystery to be solved'' in this encounter, he says.
The key to solving this and many other Neptunian puzzles may lie in the better knowledge of the planet's makeup that the Voyager team expects to gain. Stone explains that theories held up to now suggest that the planet may be mainly a mixture of water and rocky material overlaid by a thick gaseous blanket containing the methane that gives Neptune its blue-green cast. Indeed, the planet may be fluid down through its core. In that case, Stone speculates, Neptune's magnetic field may result from the circulation of electrically charged water molecules in that core.
With Voyager about to survey Neptune at close range, the time for such speculation to yield to knowledge is at hand.
``I think we're in for a good ride,'' says Stone.