Voyager Closes In on Neptune

SPACE RENDEZVOUS

VOYAGER 2 is beginning to fulfill scientists' hopes of learning the true nature of Neptune as the 12-year-old spacecraft nears its Thursday midnight rendezvous with the mystery planet. Monitors here at the NASA Jet Propulsion Laboratory (JPL) are showing ever sharper cloud scenes as Neptune's image fills the screens. The large moon, Triton, is no longer a mere dot, but a large disk with indications of surface features starting to appear.

These pictures and other information, flowing in from Voyager across 2.76 billion miles of space, fuel the sense of excitement that now permeates the JPL campus. They support the Voyager team's prediction that this eagerly awaited visit to what currently is the outermost planet will be a spectacular encounter.

By early August, Voyager's cameras had already found four previously unknown small moons and two partial rings orbiting Neptune. The imaging team says it expects to find more moons and ring arcs this week.

On Friday, the planetary radio astronomy team said it realized that Voyager had been picking up radio noise from Neptune since Aug. 14. Mission scientists have been awaiting such a ``signal'' as a sign that Neptune has a magnetic field. Team leader James Warwick of Radiophysics Inc., in Boulder, Colo., describes the emissions as ``very intense, very impulsive'' over a limited frequency range. He says they are unambiguously ``related to energetic particles interacting in a magnetic field.''

Voyager should provide a detailed map of that magnetic field as it swoops to within 3,000 miles of the planet's north polar cloud tops at midnight Eastern time Thursday. Scientists should learn much about Neptune's deep interior where the field is produced. By tracking field details as the planet rotates, they will be able to fix the true rate of the planet's spin. Cloud motions indicate a 17.7-hour Neptunian day.

Voyager has been working well. It appears to be fit and ready for its planet system survey. But it is well beyond its design lifetime, which ended with its Saturn encounter eight years ago. Mission engineers have had to cope with component failures and instrument degradations.

For example, the craft's infrared spectrometer has taken some of the most important data in recent days. Yet its mirrors have become misaligned over the years, reducing its sensitivity. Mission engineers have learned how to restore some of the lost sensitivity by warming the mirrors with tiny on-board heaters several weeks before a planet encounter.

One of the most crucial - and famous - ``work-arounds'' has averted a catastrophic loss of radio contact. Voyager lost its primary radio receiver shortly after leaving Earth. Its backup receiver failed to the point where it recognizes only a particular frequency that depends on the spacecraft's temperature and speed relative to Earth. Voyager's engineers can anticipate what that frequency is and send the right kind of signal to maintain communication.

One of the most important communications soon to be made is the upload of final instructions to Voyager's computer to guide the craft through its encounter operations. It takes four hours and six minutes for a radio signal to pass between Earth and spacecraft, so real-time guidance from JPL is impossible.

A radio message usually originates at JPL and is transmitted by one or more of the ground stations in the worldwide Deep Space Tracking Network. But this particular upload is so important that JPL has stored copies of the message at each ground station. Thus, the message can be sent on time even if an earthquake were to knock out communications with the laboratory here.