The Voyager 2 spacecraft, moving closer to a historic encounter Friday with Uranus, today found two tiny, previously unknown moons there and for the first time observed cloudlike features suggesting that the planet's day may be 15 hours long.
No larger than 12.4 miles in diameter, the moons were discovered inside and outside the planet's outermost ring, raising to 14 the number known to be in orbit around the seventh planet from the sun.
The new moons are the first of the so-called "shepherd" moons that scientists thought would be found at Uranus after Voyager saw similar ones in 1981 between Saturn's rings. The moons keep the rings from colliding and grinding themselves into dust.
"These new moons are quite dark, making them look more like the planet's rings than its other moons," Bradford A. Smith of the University of Arizona told reporters at the Jet Propulsion Laboratory, which is directing Voyager's fly-by. "We don't know why this is so, but we do know it must be telling us something."
Voyager, within 1.5 million miles of the cloud tops in Uranus' methane atmosphere late tonight, was speeding at more than 33,000 miles an hour. The craft -- launched Aug. 20, 1977 -- was on target to become the first manmade machine to reach Uranus, almost 2 billion miles from Earth.
The craft's encounter with the third largest planet in the solar system is due to occur at 1 p.m. EST Friday when Voyager is to sweep past 64,000 miles above its atmosphere.
Voyager's cameras apparently pierced the methane haze, because pictures released today showed three distinct cloudlike features moving around the planet. Uranus' rate of rotation and length of day have been a mystery since its discovery in 1781.
"Uranus is a planet of unfathomable mystery," Smith said. "It is also a planet that is not giving up its secrets very easily."
Twice as far from the sun as Saturn and four times as far as Jupiter, Uranus orbits the sun in an area whose temperatures are 360 degrees below zero Fahrenheit. The amount of sunlight reaching Uranus is 1/350th of the amount striking Earth.
"It's so cold in the orbit of Uranus that the planet's clouds are deeper, the contrasts are lower and the rings and moons are darker," Smith said. "We can compensate for the light levels, but it's the planet itself that gives us trouble observing it."
Photographs released today show a large cloudlike feature at 25 degrees south latitude, another at 33 degrees and a third at 45 degrees in the planet's southern hemisphere, which is now facing the sun. Smith said the feature at 25 degrees orbited the planet once in 17 hours. He said the feature at 33 degrees required slightly more than 16 hours and the one at 45 degreees took 15 hours.
"This says that the atmosphere of Uranus is slippery. In other words, there are high winds there," Smith said. "It also says we don't know its precise rotation period. But it does suggest its rotation time could be as low as 15 hours."
Scientists have long speculated about whether the extreme cold there might produce a planet whose atmosphere was not moved by high winds such as those reaching 1,000 miles an hour near Saturn's equator. The pictures released today appear to have dispelled any notion that high winds do not exist on Uranus.
Another photograph of the planet's south pole, now directly facing the sun and Earth, shows the entire southern hemisphere, with a brown circle inside an orange one. Around both is a blue-green ring that makes the southern hemisphere look like a giant eyeball. Smith said he believes that the brown and orange colors are produced when the methane atmosphere is changed chemically by the sun's ultraviolet radiation. Among the known planets, only Uranus and Pluto lie on their sides, leaving one pole facing the sun for 42 years at a time.
Photographs today also showed for the first time discernible features on the planet's five largest moons -- Miranda, Umbriel, Ariel, Titania and Oberon. Scientists were able to distinguish features on the moons and report that Ariel is the brightest of the five and Umbriel the darkest.
"We still expect to find at least eight and as many as 16 more moons in the Uranian system," Voyager Project Scientist Edward C. Stone said. "But we really don't know what to expect as we get closer . . . ."