The silver-and-black Voyager 2 spacecraft flew by Uranus today after traveling almost 2 billion miles over eight years, arriving only 68 seconds earlier than predicted years ago.
A stream of photos and data is expected to swamp scientists over the next few days as they pore through the information beamed from Voyager 2 during a six-hour sojourn past the seventh planet from the sun and its major moons.
Flying more than 33,000 miles an hour, Voyager 2 raced by Uranus 51,000 miles above its cloud-tops and only 18,000 miles from Miranda, one of 14 moons known to circle the planet, most of them discovered just days earlier by Voyager 2. The historic encounter took place a few seconds before 12:59 p.m. EST, 1 minute and 8 seconds off the time scientists predicted almost five years ago.
"Voyager 2 is the first spacecraft to fly by a planet that was not known in antiquity, a planet discovered by someone whose name we know," Voyager Project scientist Edward C. Stone said at the Jet Propulsion Laboratory where Voyager's flight is directed. "This flight commemorates the discovery of Uranus in 1781 by Sir William Herschel, a talented British musician and astronomer who built better telescopes than anybody who lived before he did."
Pictures and radio signals from Voyager took 2 hours and 45 minutes to reach the laboratory because of Uranus' great distance from Earth. Voyager returned photographs of the planet, its nine known rings and five largest moons in close-up images never before seen on Earth.
"We're seeing things today that are very different than things we saw before at Jupiter and Saturn," said Bradford A. Smith of the University of Arizona. "Everything at Uranus is colder, darker and more mysterious than anything else we've seen in the solar system."
One color picture taken head-on of the south pole of Uranus showed ovals of white, orange and blue-green. Smith said the white oval was a layer of methane haze covering most of the south pole, which now faces the sun. He said the blue represented a part of the planet's atmosphere near the equator that was free of haze.
"The orange color is an area along the planet's limb that is a mix of haze and smog," Smith said. "We don't know the chemistry of what makes the orange color, but it's sure to be methane that has been changed chemically by ultraviolet radiation from the sun."
Smith displayed several pictures of the nine dark rings known to surround Uranus that showed its outermost ring as a bright, wide band that seemed to be two rings fused together. Smith said it was too soon to tell whether the outer ring was a single ring or two rings separated by less than a few miles.
"The outer ring is as dark as coal dust," Smith said. "At least two of the inner rings look like they might have a different color, but we still don't know what these rings are made of or what the dark coating is that causes the outer ring to look so black."
Pictures of the five largest moons suggested each is different. Titania looked like a spear covered by confetti. Oberon has a surface streaked by bright rays while the small Miranda had a strange chevron-shaped feature that Smith suggested may be caused by an internal heat source that is causing surface faults and ridges.
"We'll have much better images in the next 48 hours to unravel some of these mysteries," Stone said. "We fully expect to get more science out of this flight than astronomers have been able to get about Uranus in the last 200 years."
All five moons appear dark except when streaked with what appeared to be patches of ice. Scientists said they do not understand the causes of the streaks, suggesting that meteorites may have collided with the moons and punched out icy material. One geologist said a few of the moons might have their original internal heat sources.
"My own bias is that some of these moons are still active at their centers," said Harold Masursky of the U.S. Geological Survey.
Scientists confirmed today that Uranus has an unexpectedly strong magnetic field that is half the strength of the Earth's. This means that the planet has a hot liquid core that is rotating with the planet and generating a magnetic field much like a dynamo.
Radio commands rolled Voyager 2 into five different positions as it passed Uranus, aiming its cameras and instruments properly.
The first roll targeted Titania, a second locked the spacecraft back on its guiding star, Canopus, and a third slued Voyager onto the moon Ariel. A fourth roll locked the spacecraft on the planet's equator, and the fifth roll pointed the spacecraft at the star Femelhaut allowing Voyager a clear look at the planet as it speeds toward a 1989 encounter with Neptune.