Ulysses Journeys Into 'New' Space

European-American spacecraft will shift from a planetary orbit to explore sun's polar regions

WHILE Magellan radar portrays Venusian landscapes and Hubble Space Telescope eyes the cosmos, a spacecraft that can't even see is on its way to explore entirely new astronomical territory. It aims to make the first three-dimensional probe of the tenuous web of magnetic fields and material particles that envelops our solar system like a cocoon. Ulysses, the European-American solar explorer, is now heading for Jupiter to use that giant planet's strong gravitational field to make a radical change of course.

For the first time, a spacecraft will move sharply down from the so-called ecliptic plane in which the planets orbit, move back toward the sun, and survey the unexplored space below and above the south and north solar poles.

This is why Gary L. Bennett of the Office of Aeronautics, Exploration, and Technology at the United States National Aeronautics and Space Administration (NASA) has dubbed Ulysses the "Admiral Byrd" of solar-system exploration.

Ulysses researchers aren't waiting for their probe to reach the unsurveyed solar regions two years from now to get down to work, however.

Edward J. Smith, Ulysses project scientist at the NASA Jet Propulsion Laboratory here explains that they need to take data all along the spacecraft's route to help sketch the three-dimensional chart they are after. He says that "while we haven't made any really startling observations, ... we have [already] done some interesting things."

These include monitoring for possible effects, including dust emissions, of Comet Kearns-Kwee, which passed about 7 million kilometers (about 4 million miles) from Ulysses earlier this year. The craft also sampled space near what planetary scientists call Earth's geotail. This region is opposite the sun where the so-called solar wind of charged particles and interlaced magnetic fields stretches our planet's own magnetic field into a long tail-like structure.

While Ulysses can't take pictures, it is the most sophisticated probe yet sent to study the solar-system environment. To borrow Mr. Bennett's figure of speech, Ulysses is "feeling, tasting, and listening" its way through the interplanetary medium with unprecedented sensitivity.

For example, the spacecraft is equipped not only to measure the different chemical elements - including cosmic ray particles - in that medium, but also to measure the relative abundance of different forms of a given element. Elements generally come in two or more slightly different forms, called isotopes, which have slightly different atomic weights but behave the same chemically.

Edward Smith notes that the Ulysses equipment designed for this exacting job "has been working quite well." All instruments on the spacecraft were turned on a few weeks after the shuttle Discovery launched Ulysses last October. Even in the relatively short observing period since then, Dr. Smith says the isotope measurements have already distinguished between two forms of helium - helium-3 and helium-4.

Cosmic chemists have long wanted such information. Smith explains, for example, that, once chemists know the relative abundance of elements and their isotopes in cosmic rays, the information "will be compared with the solar system and with the so-called universal abundance which is supposed to apply to all the stars." He adds that cosmic-ray scientists "believe that, if we see things that are anomalous, that perhaps it will be indicative of the source of cosmic rays" entering the solar system.

To visualize Ulysses' mission, think of the solar system as a three-dimensional "sphere of influence," to use a political term, in which the sun's influence dominates space. The full extent of this volume is not yet known. It probably extends billions of miles beyond the farthest planet. The sun fills this volume with a tenuous, but dominating, solar wind. Magnetic fields stretch out from the sun, sometimes breaking away into independent entities. They are bound to, and carry with them, electrically cha r

ged particles.

Interplanetary spacecraft routinely sample this solar-wind environment. But they have traveled mainly in the plane where planets orbit - a plane that roughly coincides with the equatorial plane of the sun. Launching rockets have not had the thrust needed to send spacecraft north or south out of this plane.

Ulysses overcomes this handicap by making use of the Jupiter switching junction. Like a train passing through a railroad junction, Jupiter will switch Ulysses onto an orbit that will bend down some 80 degrees out of the ecliptic plane and then pass back around the sun, as shown in the accompanying diagram.

Ulysses data will be matched with all other available data taken by Earth-orbiting craft and by the Pioneer and Voyager robot spaceships now heading toward the outer solar system. Together, these data should allow researchers to prepare at least a rough 3-D map of the solar "sphere of influence."

Ulysses also should give new perspective on the sun's activity. Smith explains that much of the solar wind comes out of the sun's polar regions. He says scientists hope to have a simpler view of the physics involved in those regions where the sun's rotation isn't so likely to twist and tangle - and thus confuse - the particle-flow and magnetic-field patterns.

The 370-kilogram (816-pound) spacecraft is a major European contribution to this joint mission. The European Space Agency (ESA) paid $540 million of the craft's $750 million development cost. ESA has a controller team at the Jet Propulsion Laboratory where, with the help of the JPL Deep Space Tracking Network and JPL team members, it manages the spacecraft.

Ulysses' primary mission will end after it finishes the polar surveys in 1995. But Smith says there are enough consumables, such as maneuvering gas, on board to make at least one more pass around the sun if all goes well with the spacecraft. That, he adds, is something "the scientists have always emphasized" on their wish list.

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