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Well-built, well-managed space probe isn't ready to retire yet

One of the marvels of the space age has been the concept of the long-playing spacecraft. This is the idea -- held by many space engineers -- that high-quality workmanship and clever flight management can keep a satellite or deep space probe working productively well beyond its designed lifetime. Voyager 2, now closing in on the planet Uranus at the rate of 5.5 million miles a week, is one of the most successful embodiments of that concept.

Launched in 1977 on a mission intended to take it only to Jupiter and Saturn, the spacecraft will now add Uranus to the list of explored worlds. And, if all continues to go well, it will head on to visit Neptune in 1989.

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Project manager Richard P. Laeser of the NASA Jet Propulsion Laboratory (JPL) notes that, according to its mission calendar, ``Voyager is in its postretirement years.'' Yet, he adds, the spacecraft ``is very healthy for its age.''

JPL engineers have had to use some tricky navigation and have had to work their way around major equipment problems to get Voyager this far.

To begin with, they used Saturn's gravitational pull to boost Voyager on toward Uranus -- just as Uranus itself will send the craft on to Neptune. It would have taken 30 years to send the probe directly to Uranus from Earth with the rocket power available in 1977.

Voyager 2 has had two major failures that might have crippled the probe, were it not for the ingenuity of JPL engineers.

The main radio receiver failed within a year of launching. Then the backup receiver lost its ability to make certain essential frequency adjustments to receive JPL signals as it moved toward the outer planets.

Voyager, in effect, became tone deaf, as far as JPL's planned signals were concerned, even before it arrived at its first planetary target -- Jupiter -- on July 9, 1979. By that time, however, controllers had learned to communicate with the spacecraft on different frequencies. Now, according to deputy project scientist Ellis D. Miner, the team can routinely predict the precise frequency at which Voyager will ``hear'' a signal and can adjust its transmissions accordingly.

Trouble arose again about 100 minutes after Voyager 2 swept close to Saturn on Aug. 25, 1981. Some of the gearing for the scan platform seized up. This platform carries Voyager's optical instruments. It aims those instruments and pans them as needed to avoid blurring pictures taken as the spacecraft zips past a planet or one of its moons.

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The jammed gears crippled Voyager during its Saturn flyby. Yet, two days later, the gearing was free again. Engineers had been slewing the platform at its highest rate of one angular degree per second. This apparently drove the lubricant out of the stuck bearings. It seeped back in during the two days' rest.

Reviewing the spacecraft's status in Sky and Telescope magazine last month, Miner said that JPL engineers now understand how to cope with this potentially crippling problem. They have studied it extensively with similar equipment in the laboratory. They are confident that, if they don't drive the platform at its highest slew rate, it won't seize up. Indeed, Miner said, they have used the platform extensively at slower slew rates since February 1983 ``with no hint of stickiness.''

Should the platform fail at Uranus, however, flight controllers have already worked out other ways to pan the instruments. They will roll the spacecraft through carefully planned maneuvers using its attitude-control jets.

When Voyager flashes by Uranus at 10 a.m. Pacific standard time on Jan. 24, it will be a long way from home -- nearly 2 billion miles, or twice the distance of when it was at Saturn. Getting Voyager's priceless data back intact over such a distance is a new challenge. It's another instance where engineers have to cope with a situation that wasn't in the original flight plan.

Voyager's signal, which will take about two hours and 45 minutes to travel from Uranus, will be too weak for JPL's worldwide network of powerful tracking antennas to receive adequately. So Australia's Parkes Radio Observatory will help out.

Also, communications engineers have developed a way of encoding Voyager's picture information so that the spacecraft will have to send no more than 40 percent of the usual number of data bits per image. It would have taken 8.8 minutes to send a single picture from Uranus using the old method. That compares with 2.4 minutes per picture at Saturn. With the new method, Voyager should be able to send a picture every four minutes. Since Voyager will spend only a few hours near Uranus, scientists need to get as many pictures as possible to make the most of their opportunity.

Avoiding motion blur in the pictures from Uranus will be even harder than with those from Saturn. Panning the scan platform can't fully compensate for motion at all the photo targets as the spacecraft whizzes past at 45,000 miles an hour. Also, at twice Saturn's distance from the sun, the light is four times dimmer at Uranus than at Saturn. That means longer exposures per picture than Voyager engineers have used before.

Uranus is the most distant object yet visited in the solar system. It will probably ``be unlike anything else we've looked at,'' says project scientist Edward Stone of the California Institute of Technology. Thanks to the concept of the long-playing spacecraft, which the Voyager team works hard to implement, we don't have to wait for a 21st-century space probe to see this planet close up.

A Tuesday column. Robert C. Cowen is the Monitor's natural science editor.