High expectations for Voyager 2 at Neptune.

High expectations for Voyager 2 at Neptune

Scientists last week reported findings with exciting implications for the Voyager 2 spacecraft's flight past Neptune next August. Though the probe's photos so far show little more than a fuzzy ball, measurements from Earth suggest Neptune has a dynamic atmosphere with details that will not be hidden by haze, and a magnetic field that might produce auroras, radiation belts and more.

"When I showed them the 6,190-angstrom images, everyone started to applaud," says Heidi B. Hammel of Jet Propulsion Laboratory in Pasadena, Calif., describing the response of members of Voyager 2's camera team. Yet the photos had been taken not from space but through the University of Hawaii's 2.24-meter telescope on Mauna Kea. The object of the excitement was a diffuse bright spot on several of the pictures -- a large, cloudy feature high in the Neptunian atmosphere. The feature's detectability from Earth suggests that finer details should appear in Voyager's closeups.

The pictures, presented last week in Austin, Tex., at the meeting of the American Astronomical Society's Division for Planetary Sciences, were taken at a wavelength of 6,190 angstroms to seek brightness variations in Neptune's methane atmosphere. They excited Voyager scientists because the spacecraft has a filter to take photos at that wavelength.

Scientists can track a cloud such as Hammel photographed as it circles the planet, gaining a clue to the length of Neptune's day. A similar cloud seen in 1986 and 1987 was farther south and circling more slowly, indicating that the winds at different latitudes move at different speeds, creating wind shear that may drive a host of other visible circulation features. By comparison, Uranus -- Voyager 2's previous target -- kept its atmospheric details masked by haze until the spacecraft was just five days away, and even then, details appeared in images only after much computer processing.

Also reported at the meeting were measurements of Neptune's radio emissions, which Imke de Pater of the University of California, Berkeley, says provide strong evidence the planet has a magnetic field. Detected by the antennas of the Very Large Array in Socorro, N.M., the emissions proved powerful enough at a wavelength of 20 centimeters to suggest they were produced by electrons from the solar wind, trapped on the lines of a field whose strength is about 1 gauss -- less than a fourth Jupiter's strength but about twice as strong as Earth's.

Such a field could mean Voyager 2 will encounter diverse electromagnetic phenomena ranging from auroras to trapped radiation akin to Earth's Van Allen belts.

Also intriguing are Neptune's possible rings, which scientists believe to be short arcs rather than whole rings because of the way they block the light of stars that pass behind them. The rings' positions are not precisely known, but several stellar occultations are expected before Voyager 2 flies past Neptune, and mission officials have readied new computer commands that can be radioed to the craft to re-aim its cameras in hopes of taking pictures of the rings as the stars pass behind the planet.