Getting a 3-D view of the sun

Scientists around the world are readying experiments for a radically new, three-dimensional investigation of the sun. The sun is the only star accessible enough to be studied in any detail. Though much is already known about this vital star, scientists insist they must know more about it to fit the data they have into a complete working theory. They also want to be able to predict more accurately such phenomena as solar flares and the sun's long-term effects on Earth's weather.

That goal may soon get a major boost through a European space probe known as the International Solar Polar Mission. The spacecraft will attempt to explore the sun, and the particles, gases, and magnetic field surrounding it, from a new vantage point.

The $400 million spacecraft, due to be launched by the National Aeronautics and Space Administration from Cape Canaveral in May 1986, is the first designed to break out of the narrow orbital plane of the planets in the solar system and move into a new orbit crossing the sun's north and south poles.

University of Chicago physicist John Simpson, who can lay claim to having more experiments (25) aboard space missions than any other scientist, has been pushing for a high-latitude sun probe of some sort since 1959. Though he says he was deeply disappointed that the United States decided for budget reasons to break its commitment to send a companion satellite on the mission, he unhesitatingly describes the Solar Polar project as the most significant unmanned space exploration that will take place this century.

The voyage of Pioneer 10, launched in 1972, has proved that Jupiter's strong gravity can be used to change the energy of an orbiting spacecraft and give it the boost needed to escape the solar system.

Banking on Pioneer 10's experience, which included surviving both the asteroid belt and the intense radiation around Jupiter, Solar Polar will start its three-year trip to the sun by swinging around Jupiter for the needed speedup and kick to fling it into a solar polar orbit. At no time will Solar Polar be closer to the sun than Earth is.

Since the intensity of sunlight at Jupiter is only about 4 percent of what it is on Earth, the spacecraft will be powered by nuclear rather than solar energy. A radio dish on the probe will transmit all data gathered back to three Earth tracking stations until money for the experiment runs dry.

Many European experiments were slated to go aboard the planned US companion spacecraft, and were grounded by the cancellation. But Dr. Simpson, a pioneer in the study of cosmic rays, was asked by the European Space Agency to head an international team to develop a cosmic-ray experiment to carry aboard the European spacecraft. The result, an array of sensors and silicon detectors that takes up about one-fourth of the available space, has already been placed aboard Solar Polar in West Germany.

Simpson describes cosmic rays as high-energy nuclear ash from supernova (giant star) explosions and are roughly 20 million years old - young compared with the 5 billion-year-old solar system.

''They're the only sample of matter we have from outside the solar system, and therefore they're fascinating,'' he says. ''You can, for instance, work backward with them and determine the composition of a supernova - one of the prime purposes of the experiments I'm doing.''

Simpson began studying cosmic rays in 1947 aboard modified high-altitude Air Force bombers. But it has been difficult, if not impossible, to study these highly charged particles in anything close to the pristine state in which they rain in from other parts of the galaxy. The magnetic field around the sun which guides the particles is constantly blown away from the sun by gaseous solar winds - particularly near the sun's equator, where most space observations have been made. It is hoped that Solar Polar will offer scientists a breakthrough of sorts, since the effect of the magnetic field should be at a minimum at the sun's poles and the concentration of cosmic rays there could be considerable.

''We expect a whole group of particles from the galaxy to be streaming in over the poles that otherwise wouldn't be accessible,'' Simpson says.

Among the eight other studies aboard the traveling Solar Polar laboratory are: ones aimed at exploring the extent, size, and content of interplanetary dust particles, the speed, temperature, and density of the solar wind, and the strength of the sun's magnetic field.

But Simpson clearly also expects some surprises:

''We're scanning a whole new region of space,'' he says. ''If we knew all the answers to what we were going to learn, we wouldn't be bothering to do this. So many things that have proved to be exciting and beneficial weren't even thought of when missions were set up - it's good to keep a long-range view.''

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