More than eight years after leaving Earth, the Voyager 2 spacecraft on Friday may unlock some of the mysteries of Uranus, the seventh planet from the sun, circled by at least nine rings and 12 moons and one of two planets (with Pluto) to rotate on its side.
Voyager 2, with one radio inoperative and the other barely working, flew past Jupiter in 1979 and Saturn in 1981, sending back dramatic pictures and greatly increasing the knowledge of the solar system. Now the spacecraft, which has outlived its design life, will home in on one of the most mysterious of the nine planets.
In the six hours that Voyager will traverse the Uranian system, it is expected to return more information about the third-largest planet than has been learned in more than 200 years of earthbound astronomical observation.
Almost 2 billion miles away, Uranus is so far from Earth that astronomers do not know the length of its day. Even more of a mystery is why Uranus travels around the sun on its side, with one pole in sunlight for 42 years and the other in sunlight for the next 42. Its south pole now faces the sun.
"We think Uranus is tilted on its side because it collided with a body the size of Earth early in the life of the solar system," Voyager Project scientist Edward C. Stone said. "There is some suggestion that the rings and moons we see around the planet today were formed from the material left over from the collision, the way some people believe our own moon formed."
Voyager has already found seven small moons circling the planet, raising to 12 the number of known Uranian moons. The planet's five largest moons range in size from 310 to 1,010 miles in diameter. Named Titania, Oberon, Ariel, Umbriel and Miranda after characters in English literature, they are the only moons other than Earth's not named for figures in Greek and Roman mythology.
Voyager 2's fly-by of Uranus is something of a miracle. The spacecraft, which left Earth in 1977, was not built to reach Uranus.
Voyager 2's instruments and cameras are still working, although haltingly, after more than eight years and 3 billion miles in space, long beyond the craft's projected useful life. Voyager has only one radio in operation, and the "scan platform" that moves its cameras and instruments from one target to another has been jammed since the craft flew by Saturn, and can be used only at low speeds as it passes Uranus.
"We need the platform for photographs, and we will be able to use it," Stone said. "It's just that we won't be able to go from one target to another as quickly as we did at Saturn, and on some of the longer scans we'll have to roll the entire spacecraft to get the pictures we want."
In 1979 Voyager flew by Jupiter, taking 17,000 pictures before using Jupiter's enormous gravity to speed it toward Saturn. Two years later, the craft swept by Saturn, taking 15,000 pictures before using the ringed planet's gravity to speed it toward Uranus, whose gravity will give Voyager another push to target it to a 1989 encounter with Neptune, the next-to-last planet in the solar system.
"These gravity-assisted trajectory changes have been the key to Voyager's grand tour of the four big outer planets," Stone said. "Even with a more powerful launch rocket than we used in 1977, it would have taken a spacecraft 30 years to travel to Uranus."
When Voyager encounters Uranus at 1 p.m. Friday, the 1,200-pound spacecraft will be 64,000 miles from the planet's cloud tops and 18,000 miles from Miranda, the smallest of Uranus' five large moons. All the moons are dark, reflecting no more than 30 percent of the light they receive from the sun.
"We believe there is quite a bit of methane ice in these moons, which have been darkened over time by ultraviolet radiation from the sun," Stone said. "What we don't know is how much ice and how much rock are in the moons."
Discovered in 1977, the planet's nine known rings are charcoal black, reflecting about 5 percent of sunlight, making them the darkest bodies in the solar system. Like the moons, the rings are thought to be made mostly of methane ice, blackened by solar radiation.
The planet is blue-green, reflecting the methane haze that scientists say they think covers the planet and obscures any feature that may lie below that haze.
"This is why we don't know the length of the Uranian day," Stone said. "People believe it's about 16 hours long, but we've never seen a feature we can track on the planet's surface to measure how long it takes for Uranus to make one complete rotation."
Under the haze, scientists say they believe there are clouds of methane, which could show through the haze like the large colored ovals of ammonia that Voyager saw in the atmosphere of Jupiter and Saturn.
"There's no question the atmosphere is going to be different on Uranus," Stone said. "It's much colder on Uranus than it is on Jupiter and Saturn; there's a lot more methane in the atmosphere and a mix of hydrogen, helium and ammonia we hope Voyager can tell us about when it flies by the planet."
One mystery Voyager should unravel is the Uranian weather. "There are a lot of scientists who want to know what the meteorology is like on a planet where the sun shines on one pole for 42 years while keeping the other pole in the dark for the same length of time," Stone said.
Another Uranian mystery will almost surely remain a mystery. Scientists speculate that under the methane haze and clouds lies an ocean made up of the melted ices of water, ammonia and methane kept warm by its own pressure and whatever radioactive heat is left in the planet's interior.
"If it exists, the ocean is too far below the clouds for Voyager to see it," Stone said. "We'll be lucky if we see the clouds through the methane haze, and nobody expects to see through the clouds."
Scientists say they expect to find more than nine rings and 12 moons. "My own bias," said Stone, "is that if you had that much debris there to form so many rings and moons, you're almost sure to have other small objects besides those we've already seen."