Durable Voyager 2 Unveils Secrets of Mysterious Uranus

Aging, durable Voyager 2, plunging onward through unexplored territory, has started sending information back to Earth about a mysterious planet that has baffled astronomers for two centuries.

Designed for a life expectancy of no more than four years, Voyager has been “rebuilt” electronically while more than 1 billion miles from Earth. Now, in the eighth year of its life, the trusty bird is speeding toward a rendezvous with Uranus.

One of two spacecraft that sent back spectacular photographs of Saturn and Jupiter on its journey through the solar system, Voyager 2 began earlier this month sending back information on the seventh planet from the sun. The craft is still 54 million miles from Uranus--too far to provide details--but the data is helpful to scientists and engineers at the Jet Propulsion Laboratory near Pasadena who want to be sure that the instruments aboard Voyager are in good working order.

Much New Information

The amount of data will grow dramatically in the coming weeks as Voyager approaches Uranus, now 1.8 billion miles from Earth, peaking on Jan. 24 when the craft darts through the Uranian system of moons and rings. Voyager will send back more information in the six hours it is near the planet and its moons than astronomers have been able to develop in the more than 200 years since Uranus was discovered.

The distant planet, third largest in the solar system, has both bored and baffled scientists over the years. At times it appears as a gray, featureless dot, but at other times it shows large, changing dark areas. It has at least nine rings and five moons.

So little is known about the planet that almost anything Voyager sends back will be new information.

“We just don’t know what to expect,” said Edward C. Stone, chairman of the physics department at Caltech, who has been chief scientist for Voyager for more than a decade. Uranus, he said, “is not like anything we’ve seen before.”

Not only is it different, it is much farther away, and that makes the mission far more difficult. Engineers at JPL have responded by, in effect, rebuilding the spacecraft remotely.

Whipped Into Orbit

Computers that were on board to serve only as backups have been brought on line and programmed with new software. A new system of compressing data has been radioed up to the craft to make it possible to send back a greater quantity of information. The entire craft has been converted into a camera platform to permit long exposures in the dim light that reaches the planet.

Many of the modifications were needed simply because Voyager was not designed to explore a planet as far away as Uranus. Long after the craft was launched from Cape Canaveral in 1977, engineers determined that they could use the gravity of Saturn to whip Voyager into the orbit of Uranus.

“We’re flying a hybrid,” said Richard P. Laeser of JPL, the project manager.

The tiny amount of light reflected back from Uranus is one of the key problems that engineers have been struggling to overcome.

Prof. James Elliot of Massachusetts Institute of Technology said in an article in Sky & Telescope magazine that an ordinary flashlight, flicked on for one second, produces more light than the total amount that astronomers have detected from Uranus in the countless thousands of hours that they have spent studying the planet since it was discovered in 1781. Elliot led the team of scientists that discovered the rings around Uranus in 1977.

Although Voyager carries 10 scientific instruments, it will be most remembered for the photos it sends back of the Uranian system. But because of Uranus’ distance from the sun--light reaching the planet is only about 1/400th the intensity of light reaching Earth--long exposure times will be required for the pictures.

Difficult Task

Long exposures, however, are difficult to obtain from a spacecraft speeding past a planet at 45,000 miles an hour. It would be like taking a timed photograph of passing scenery from inside a speeding automobile.

The blurring can be reduced substantially if the camera can be locked on the subject and turned to compensate for the vehicle’s movement. That is exactly what JPL engineers have planned for Voyager.

They plan to lock the camera on its target--either Uranus, the rings or one of the moons--and then swing the entire spacecraft so that the camera remains pointed straight at the subject.

That turns out to be more difficult than it sounds. The craft normally uses sensors to lock onto the sun and a star to control its attitude so that its data-transmitting antenna is always pointed at the Earth. Small jets are fired periodically to keep the craft in position.

Even the tiny nudges caused by the jets would result in smeared images, so to get around that problem, the craft will be released from its attitude control system during photo sessions. It will drift through the rotational movement that will be required to keep the camera pointed at its target. After each photo is taken, the system will switch on again automatically and adjust the attitude as needed.

Split-Second Timing

All of that will have to be done with split-second timing. Prime time for the key observations of the planet will total only about six hours, and Voyager’s computers will have to shift it back and forth among many targets, including the planet and its rings and moons.

In a sense, the spacecraft will be on its own during its Jan. 24 fly-by. It will be so far away from the Earth that it will take 2 hours and 42 minutes for a message traveling at the speed of light to reach the craft and the same amount of time for a response to travel back to Earth.

The short duration of the prime viewing time is a function of the peculiar nature of the Uranian system. Unlike the other planets, which revolve around an axis that is perpendicular to the plane of their orbits, Uranus is lying on its side. Thus Uranus is now showing the Earth its south pole, whereas the Earth is showing Uranus its equatorial region. No one knows exactly why Uranus is cockeyed, but scientists have speculated that it was struck by a giant object--possibly even another planet--and tipped onto its side.

The rings and the moons travel around the planet’s equatorial belt, so from the Earth the Uranian system looks like a giant target. At Saturn and Jupiter, Voyager 2 and Voyager 1 (which is on its way out of the solar system) were able to take a more leisurely stroll through those planetary systems, examining moons in outer orbits before eventually reaching the planets.

Like a Wall Painting

Not so with Uranus. The planet and its rings and moons will appear like a painting on a flat wall lying across Voyager 2’s course. Voyager will pass through the “target” like a bullet, traveling between the rings and the outer moons at 45,000 miles an hour.

“We’ll be vulnerable because our time will be short,” said William McLaughlin, Voyager’s chief engineer. “We can’t slip. It’s like a relay race. If you drop the baton you’re in deep yogurt.”

Voyager will pass within about 66,000 miles of the planet, and it will pass much closer to some of its satellites--only 18,000 miles from one of the five known moons, Miranda. That should be close enough to produce good scientific data and sharp photographs, if all goes well.

What will the pictures show?

“We just don’t know,” said Stone, the project scientist.

Virtually anything the craft sends back will add to the meager storehouse of knowledge about Uranus, Stone said.

“I will be very surprised if we are not surprised,” he quipped.

May Be Disappointing

It may well be that the photos of the planet will be disappointing compared to the brilliant snapshots from prior visits to Jupiter and Saturn.

Uranus, he said, “may just be a fuzzy object” covered with dull clouds and something that could look a lot like Southern California’s smog.

“It could be very bland, in a sense a dead object with no dynamics,” Stone said.

But the black rings around the planet, and the five known moons, could prove extremely interesting, he added.

Uranus and its moons are not highly reflective, and astronomers over the years have reported seeing large black lines and dark spots on the planet.

Content to Wait and See

“It’s possible the surface is black charcoal from methane gas,” Stone said. Voyager could help answer questions like that, but Stone is content to wait and see.

“Both Jupiter and Saturn humbled us,” he said. “Nature is much more inventive than the human mind.”

Voyager’s journey will not end at Uranus. The craft will use the gravitational field of the huge planet to redirect it toward a 1989 rendezvous with Neptune.

VOYAGER’S PASS BY URANUS

Uranus, the third largest of the planets, is the seventh from the sun.Uranus, at left, is circled by at least nine charcoal-black rings. Unlike the Earth, which rotates on a north-south axis, Uranus lies on its side, with its “south pole”facing the sun. Early in its formation, the planet may have been tipped from its axis in a violent collision with another body.

Voyager 2 will fly past Uranus--2 billion miles from Earth--on Jan. 24, 1986, passing within 64,500 miles of the planet. Uranus will be the third planet encountered by the spacecraft, which will pass near Neptune on its 3-billion-mile, looping trajectory before leaving the solar system.

EARTH August 20, 1977

JUPITER July 9, 1979

SATURN August 25, 1981

URANUS January 24, 1986

NEPTUNE August 24, 1989