Voyager 2 has discovered two more moons and has photographed the first clouds ever seen on the planet Uranus, scientists at the Jet Propulsion Laboratory said Wednesday.
The latest findings came on the first day of the weeklong "near-encounter phase" of Voyager's odyssey as the 8-year-old spacecraft speeds toward its historic encounter with Uranus on Friday morning.
The new moons bring the total discovered by Voyager to nine. Five other moons have been known for many years to orbit the seventh planet from the sun.
The latest discoveries came amid growing excitement at the Pasadena laboratory, where scientists from across the country have gathered for the first close encounter with what chief scientist Edward Stone described as "the most remote object ever visited by a spacecraft."
The historic significance of the event has been lost on no one.
"For many of us, this will be the Uranus encounter of our lifetime," said Stone, who is chairman of the physics department at Caltech. An extremely rare juxtaposition of the planets made it possible for Voyager to use the gravitational pull of Saturn to fling itself out toward Uranus.
"If we had launched directly for Uranus, it would have taken us 30 years to get there," Stone said, making it clear that another opportunity like this will not be available for a very long time.
So little is known about the dimly lighted planet, which was discovered only 200 years ago, that scientists involved in the project are expecting many surprises.
"In the next several days, we will have answers to many questions and probably answers to questions that we just aren't smart enough to ask," Stone said. "Those will probably be the most interesting ones."
The planet is so far from the sun that only about one-four-hundredth as much sunlight reaches Uranus as the Earth, making photography extremely difficult. Furthermore, the planet is shrouded in a greenish-blue haze and parts of it are hidden behind something akin to Los Angeles' smog, making the task all the more difficult.
Voyager carries two television cameras that function much like ordinary 35-millimeter still cameras. Instead of recording the images on film, however, the cameras convert light into electrical signals that are transmitted back to Earth. In the laboratory, the signals can be "computer enhanced" by screening out background interference and emphasizing areas that are targeted for study.
The final image is often quite different than what would be seen by a passenger aboard the spacecraft, because it has been filtered and enhanced with computers to meet scientific objectives.
"Uranus is not giving up its secrets easily," said Brad Smith of the University of Arizona, head of Voyager's imaging team.
Smith credited the "really heroic" processing of images by scientists at JPL for the discoveries made so far.
The results are particularly significant in that these are time exposures taken by a spacecraft traveling at more than 40,000 miles an hour. Through a remarkable system that swings the spacecraft so that the camera can remain fixed on its target, Smith said, "we are able to take images of 15 or 16 seconds and hold the camera perfectly still."
The latest moons were spotted on Monday as scientists examined the images from the spacecraft. They are called "shepherding" moons, because one appears on each side of the outermost of nine known rings around the planet.
Smith said other shepherding moons are expected to be found, possibly as many as two for each of the rings, although it is possible that some of the rings may share shepherding moons. Without the gravitational field of the moons, the rings would dissipate as particles by either plunging into the planet or drifting off into space, Smith said.
Project scientists also were intrigued by small "orangish" clouds that Voyager found drifting around the planet. Since the planet itself is obscured by the gases of its atmosphere, the key to what is below the gas could lie largely in being able to observe some form of weather patterns.
"This is the first time clouds have been measured at Uranus," Smith said.
Project scientists are fascinated by the fact that all of the moons recently discovered by Voyager, which lie close to the dark rings, are far darker than the previously discovered moons that lie much farther from the planet. That has led to speculation that Uranus may have benefited from two acts of creation--one resulting in the outer moons and the other resulting in the dark rings and the dark inner moons.
Voyager also turned its cameras again on the first Uranian moon it discovered, known only as 1985U1, and "we find it is very much darker" than the other satellites, Smith said. More photographs will be taken as the spacecraft draws closer in hopes of "showing us a little surface detail," he added.
The moon appears to be elongated and irregular in shape, which Smith attributed to its size. The moon may be "a broken fragment" and smaller objects in space tend to hold their shape better, because they are less subject to impact, he added.