The Ulysses Mission; January 1998; Scientific American Magazine; by Smith, Marsden; 6 Page(s)

Although explorers have been traveling around the world for the past 500 years, it was not until the 20th century that a few hardy souls first trudged across the frozen wastes of the Arctic and Antarctic to reach the North and South poles. Curiously, exploration of the solar system has followed a similar pattern. For much of the past four decades, the scientific probes sent into space stayed relatively close to the equatorial plane of the sun, which contains the orbits of Earth and other planets. But a few years ago a single craft, Ulysses, ventured out of that thin zone and into the "polar regions" of interplanetary space.

The reasons researchers waited so long to investigate this realm have more to do with the vagaries of spaceflight than a lack of attention. Indeed, scientific interest in making such a journey has been quite keen. Astronomers have known for decades that the sun is surrounded by a diaphanous outer atmosphere (called the solar corona) that extends past the orbit of Earth. And they realized that the gases in the tails of comets always point away from the sun because they are pushed by the corona as it streams rapidly outward, a flow called the solar wind. Until recently, however, scientists have been unable to sense how the material emanating from around the poles of the sun courses through the interplanetary vastness.