Voyager 2 preparing for flyby of Uranus

Remember Voyager 2, the spacecraft that explored Jupiter and Saturn half a decade ago? At noon today, Eastern daylight time, it is 1,500,917,705.419 miles from Earth, 231,416,838.405 miles from Uranus, and speeding toward its January rendezvous with that little-known planet at 40,760.770 miles an hour. To be precise, the US National Aeronautics and Space Administration (NASA) points out that, at this point, Voyager's speed is measured relative to the sun, not Uranus. The spacecraft's controllers can't be too careful about these things as they make final preparations for yet another epochal close-up exploration of a distant planetary system.

Only a decade ago, planetary scientists in the United States had all but abandoned hope of such an achievement when NASA budget austerity had curtailed plans for what they called the Grand Tour. The planets were coming into a rare alignment that would allow spacecraft to be deflected by an individual planet's gravity so that a probe could be ``handed on'' from Jupiter to Saturn to Uranus to Neptune. This meant that a relatively modest launch rocket could send an unmanned explorer throughout the outer solar system.

Regretfully, the space agency could not fit such a project into its budget. But it did fund twin Voyager spacecraft to explore Jupiter and Saturn. Then, thanks to the heartiness of the spacecraft and to careful in-flight planning by the Voyager team at the NASA Jet Propulsion Laboratory (JPL), a way was found to send Voyager 2 on the Grand Tour anyway, and at little extra expense.

Now the JPL team is preparing to explore what is largely a mystery planet during the period from Nov. 4 through Feb. 25, 1986, as the spacecraft approaches, passes through, and recedes from the Uranian system.

Uranus -- the seventh planet from the sun and the third largest (diameter: 32,000 miles, or more than four times that of Earth) in the solar system -- is something of a maverick. It alone among the planets lies on its side, with its axis in its orbital plane. It is so distant that, when Voyager flies by within 66,000 miles, the pictures radioed back will take two hours and 45 minutes to reach Earth. Uranus has five moons and at least nine thick rings. The rings, first discovered in 1977, are among the darkest objects known in the solar system.

Voyager scientists hope the flyby will reveal the composition of the rings and perhaps of the moons, about which little is known. They also hope to pin down basic facts about the planet itself, such as its rotation rate and the composition of its atmosphere.

Meanwhile, Voyager 1 -- the twin of this spacecraft pair -- is exploring a different space frontier. Its swing around Saturn flung it upward and out of the ecliptic plane in which the orbits of most planets lie. Now more than 2 billion miles from Earth, it is heading for interstellar space through an unexplored solar-system region.

Both Voyagers still have enough fuel for their attitude control rockets to assume and hold any desired orientation. When launched in August and September of 1977, respectively, they each had 103 kilograms of fuel. Voyager 1 still has 39 kg of fuel, while Voyager 2 has just under 64 kg. The fact that the craft still have healthy fuel reserves after such long trips through space, and after executing many maneuvers, is a tribute both to the Voyager design and to the skill of the JPL controllers.

Voyager 2 will need plenty of fuel. It will approach Uranus and its system of rings and moons head-on instead of traveling more or less in the same plane as the moons and rings as it did at Saturn. Like an arrow shot through a hoop, it will pass quickly through the Uranian system -- zipping past at a speed, relative to the planet, of 45,000 m.p.h. This means its cameras must pan as the spacecraft swings by a target to avoid motion blur in the pictures. One of the gears in the mechanism that pans the camera platform is sticking. JPL controllers are programming the Voyager computer to maneuver the entire spacecraft to compensate for this loss of camera-platform agility.

There should be enough fuel both for this extensive maneuvering and to leave a reserve for later exploration of Neptune. As Voyager passes by Uranus, the planet's gravity will fling it onto a course for Neptune and give it a boost in speed. Voyager is expected to reach the more distant planet on Aug. 24, 1989 -- just about 12 years after it left Earth.

If the spacecraft continues to operate well when it arrives, it will have fulfilled one of the most ambitious dreams of the early space planners -- a Grand Tour of the outer planets. Even to make it as far as Uranus and return useful data will be a major achievement.

Halley's comet will pass through our corner of the solar system this winter -- an event that has already stirred public anticipation. This time, the famous visitor will have to share attention with another interplanetary traveler -- a man-made ``comet.'' And if Voyager's close-up views of Uranus are anything like the earlier Jupiter and Saturn pictures, the spacecraft may turn out to be a scene stealer.

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