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Bound for the crown of Neptune.

Bound for the crown of Neptune

A decade ago, when Voyager 1 and 2took off from Florida's Cape Canaveral, the official mission plan called only for both probes to fly close to Jupiter and Saturn. There was hope that Voyager 2 would then go on to Uranus in 1986, but the craft would have to survive more than twice as long to do so, and speculation about its lasting yet another three and a half years to reach Neptune produced even more cautious prognoses.

The Uranus encounter in 1986 was aringing success, however, and the long-lived vehicle is looking ready and able for Neptune. So it was not concerns about the probe's longevity that prompted engineers last week to fire Voyager 2's rocket engine for a slight increase in speed hastening its Neptune arrival by 12 hours, on Aug. 24, 1989.

The reason was to improve the receptionof the craft's radio messages from what is now the solar system's most distant known planet. It is a matter of getting to Neptune when the earth is turned as to allow the signals to be picked up from Australia, where one of the antennas of NASA's Deep Space Network has been electronically arrayed with the big dish of the Parkes Radio Astronomy Observatory, about 200 miles away. The result will be a larger, more sensitive antenna.

And when the Neptune "flyby' passes3,100 miles from the cloud tops, Voyager 2 will be going about 16 times closer than it went to Uranus, 20 times closer than it went to Saturn and 55 times closer than Voyager 1's visit to Jupiter.

Voyager 2's trajectory past Neptunehas been dubbed the "polar crown,' approaching from the south, swooping up through the plane of the planet's equator and then bending back down over the North Pole on a path that will carry the probe about 25,000 miles from Neptune's big moon, Triton. One official called the maneuver the Neptune encounter's "holy grail.'

But the quest for the grail has involvedtrying to be as sure as possible that it will not risk Voyager's life by sending the probe through material from the planet's rings, which have been detected only as incomplete "ring arcs' in earth-based occultation studies. Another concern has been possible danger from Neptunian trapped radiation belts, concern that prompted a special meeting of Voyager scientists on Jan. 16 to see if they felt that the polar crown would be a safe route. The group reached a favorable consensus after evaluating the possibility of hazardous radiation on the basis of last year's experience with Uranus.
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Title Annotation:Voyager 2 mission
Author:Eberhart, Jonathan
Publication:Science News
Date:Mar 21, 1987
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