Voyager 2 builds suspense for Neptune.

Voyager 2 Builds Suspense for Neptune

As Voyager 2 approaches Neptune, its newest findings are creating great expectations for the craft's Aug. 24 encounter with the planet. With six weeks and 37 million miles to go, Voyager's photos already have led to the discovery of only the third Neptunian moon identified since German astronomer Johann Galle found the planet itself in 1846. The images also reveal an oval cloud, wider than Mars, that may mark a huge atmospheric storm cell like Jupiter's Great Red Spot.

A scientist with the Voyager camera team, Stephen P. Synnott of NASA's Jet Propulsion Laboratory in Pasadena, Calif., first saw what turned out to be the satellite as a small, bright smudge in photos taken in mid-June. He confirmed the discovery on July 5, after later images showed it right where it should be according to an orbit calculated from the initial pictures.

Temporarily known only as 1989 N1, designating the first Neptunian moon discovered this year, it moves in a nearly circular orbit about 73,000 miles from the planet's center and some 57,600 miles above the cloud tops. It is also the only known moon that circles Neptune's equator. Nereid follows a long elliptical path tilted 28[deg.] from the equator, while Triton's circular orbit tilts 160[deg.] so that it actually travels backward (clockwise). Some scientists believe one or both of these moons may be former asteroids, or even ex-satellites of other planets, that somehow got knocked out of their original paths and captured by Neptune's gravity.

Although the surface appearance of the newly discovered moon remains unclear, Synnott suggests its diameter measures between 125 and 400 miles.

Besides the oval cloud, the photos reveal dark bands around Neptune's south pole, possibly consisting of carbon-rich compounds arising from chemical reactions driven by heat escaping from the planet's interior and by incoming charged particles.

Synnott notes that the pictures have yielded "not a hint" of the short, comma-shaped arcs that may be Neptune's odd version of planetary rings. These arcs have not been seen from Earth either. But astronomers have inferred their existence from Earth-based observations of the way starlight flickers before and after Neptune gets in the way. Also absent so far from Voyager 2's data, says Michael L. Kaiser of the NASA Goddard Space Flight Center in Greenbelt, Md., is evidence of radio emissions indicating a magnetic field. Last year's Earth-based detection of possible radio emissions from Neptune (SN: 11/12/88, p.310) may be real but at a frequency too low for Voyager's instruments.