More Moons Found at Climax of 12-Year Journey : Voyager Hits Bull’s-Eye at Neptune

Neptune embraced Voyager Thursday night, using its gravity to boost the tiny spacecraft’s speed up to 60,000 m.p.h. in the grand climax of a spectacular expedition through the outer solar system.

After traveling 4.4 billion miles over the past 12 years, Voyager arrived “within 20 miles of the point we were aiming at,” said Norm Haynes, Voyager project manager.

“That’s not bad shooting,” he said, especially considering the fact that Neptune is 2.7 billion miles away. “The navigation team nailed it right down the middle.”

That was good enough for Voyager to discover two more moons Thursday and add a few more chapters to the history of space exploration. That brings the total number of Neptunian moons discovered by the spacecraft to six.

At 8:56 p.m., Voyager made its closest approach to Neptune, passing just 3,098 miles above the planet’s cloud tops. That is about 20,000 miles closer to Neptune than the distance between the Earth and its communications satellites, but it is about 15 times higher than the orbit the shuttle normally flies.

Unbelievably, the Voyager arrived at Neptune within 1.4 seconds of its schedule, which was set several years ago.

Scientists at NASA’s Jet Propulsion Laboratory in Pasadena devoured images sent back by Voyager Thursday and wondered among themselves when, if ever, they were going to get any sleep.

“This is the day,” said a buoyant Edward Stone, the Voyager’s chief scientist.

Scientists like Stone, a Caltech physicist, are beginning to show a little wear and tear as they sit glued to video monitors, savoring every moment. The fact that all of them know this will not happen again in their lifetime makes the sleepless nights easier to bear.

Meanwhile, researchers who have been struggling to explain why Neptune had a partial ring, known as a ring arc, got a little help when Voyager revealed that the arc may be a complete ring after all, making it the second complete ring found around the planet. Now all they have to worry about is why the ring should be so different from any other ring in the solar system.

“This is a very peculiar ring,” said Bradford Smith of the University of Arizona, leader of the Voyager’s imaging team.

The ring is much denser in some areas than in others, and no one so far has been able to explain that. One would expect the small particles that make up the ring to be fairly uniformly distributed.

Neither of Neptune’s rings would be worth bragging about to any of the other outer planets, all of which have flashier rings. Neptune’s rings, Smith said, are so faint that even if someone were riding aboard Voyager the rings would be almost impossible to see.

The planet itself, however, is no shrinking violet.

It is a dazzling blue orb with light, fleecy clouds drifting over fierce, dark-blue hurricanes--a commanding presence if ever there was one.

Much of the attention today will be focused on Neptune’s largest moon, the mysterious Triton, and what used to be its second largest satellite, a tortured hunk of real estate known as Nereid. Voyager makes its closest approach to Triton this morning.

On Thursday, Voyager demoted Nereid, one of the previously known moons. It turns out that Nereid is smaller than the first moon Voyager discovered at Neptune in June. So Nereid is now the third largest, not the second.

Nereid has been supplanted by the “June moon,” known for now only as 1989N1. Scientists said Thursday that Voyager data revealed that the moon is 260 miles in diameter, which is twice the size of Nereid.

Nereid, which travels through Neptune’s most distant neighborhood, had been seen from Earth, but 1989N1 orbits at a mere 73,000 miles from Neptune and was hidden from Earth’s prying eyes by Neptune’s glare.

Nereid may have been demoted Thursday, but Voyager’s snapshots of the moon that supplanted it were hardly flattering--1989N1 looks somewhat like a potato, battered around the edges and darkened with age.

Neptune, by contrast, draped itself in finery Thursday, showing off in ways that no other planet in the solar system has been able to match.

Throughout the encounter, scientists have been fascinated by the wispy, white clouds that hover over the dark storm centers that lie deep inside Neptune’s thick atmosphere.

But scientists studying images from Voyager were startled when they discovered they could actually see shadows from the cirrus-like clouds on the dense clouds below them.

“This is the first time Voyager has been able to see cloud shadows on any planet,” Smith said.

That discovery was exciting because it enabled scientists to determine the height of the clouds, and vertical measurements are essential if they are to understand the dynamics of Neptune’s stormy atmosphere.

“We know the angle the sun is shining at,” Smith said. So he and his colleagues can measure the distance between the clouds and their shadows and determine the height through fundamental geometry.

“It’s quite extraordinary,” he said.

The clouds, he added, are from 30 to 45 miles high, much higher than the highest clouds on Earth.

Between the high clouds and the dense atmosphere below, he said, the region seems to be relatively clear, which is why the shadows show up.

The thick lower atmosphere, which is blue because of the presence of methane gas, includes several severe storm centers. One is about the size of the Earth, and in images sent back by Voyager it looks like a dark eye peering out into space.

One scientist, watching the eerie image on a monitor Thursday, quipped: “I’m glad there aren’t two.”

White clouds above the storm centers suggest that there are strong convection currents that carry moisture up to form the clouds, Smith said. That is similar to the process that creates tornado-packing thunderheads on Earth.

Although the closest approach was Thursday night, today in some ways is the most important day of all because of the number of images and the amount of data that Voyager has sent back over the past few hours.

Much of today belongs to Triton, the cold, icy moon that has tantalized scientists for years.

“I’m anxious to see Triton,” said Lynnard Fisk, head of space science for the National Aeronautics and Space Administration. “I like solid bodies myself.”