Artist's concept of Ulysses

After a seven-and-a-half-year flight covering 3.8 billion kilometers (2.4 billion miles), Ulysses -- the only spacecraft ever to explore the Sun's polar regions -- has completed its first full orbit.

Having passed under and over the Sun's poles, Ulysses has returned to the region of space occupied by Jupiter, which is more than five times the distance of the Earth from the Sun. Ulysses flew past Jupiter in February 1992 and used that giant planet's gravity to achieve the spacecraft's current orbit. The spacecraft is now on the opposite side of the Sun from Jupiter and poised to return to the Sun's poles during the peak of solar activity.

"Before the Ulysses mission, very little was known about the regions above and below the solar equator because our solar observations were restricted," said Dr. Edward J. Smith, project scientist of the joint NASA-European Space Agency mission. "We knew that the Sun and solar system were enveloped in a huge, tenuous cloud of gas and dust, known as the heliosphere, but we did not know how this vast space around the solar system was structured."

Like its mythological namesake, Ulysses, launched in 1990, ventured into unknown territory to conduct the first ever investigation of this huge magnetic bubble shielding the solar system from interstellar space. When the spacecraft arrived, the Sun was in the quiet phase of its 11-year solar cycle, with a relatively low amount of activity. In this phase, circumstances were ideal for revealing the underlying structure of the Sun's atmosphere and the solar wind in their simplest form. Improved knowledge of these solar dynamics helps scientists better understand the origin of such events as auroras and magnetic storms in Earth's atmosphere.

Ulysses, which is powered by a radioisotope thermoelectric generator because of the distance it needs to travel from the Sun during the most distant phase of its orbit, carries nine instruments that have gathered data continuously since launch. Some instruments detect the outward-blowing solar wind and its magnetic field, which create the heliosphere. Others record cosmic rays coming in from the galaxy, which are strongly influenced by the solar wind. Ulysses picks up natural radio signals emitted by the Sun, the planets and the heliosphere itself. Innovative techniques can detect foreign atoms and dust particles entering this protective magnetic balloon.

During the first orbit, Ulysses data revealed that the gas consists principally of energetic atoms from which one or more electrons have been removed to form ions. These ions become positively charged when they lose their electrons. In addition, three classes of charged particles have been identified on the basis of their energy and place of origin.

"At the lowest energy, but dominant in number, are the particles that originate on the Sun and continuously stream outward, forming the solar wind," Smith said. "More energetic but less numerous particles originate beyond the orbit of Earth. These accelerated particles occur in bursts and are found near 'weather fronts,' which develop several times a month. At the highest energies, and even lower in number, are the cosmic rays, which originate outside the solar system in the Milky Way galaxy."

Ulysses scientists found that the space between the Sun's equator and poles can be divided into distinct regions, just as the Earth can be divided into tropical, temperate and arctic zones.

The speed of the solar wind is divided into two zones, one extending from the equator to about 30 degrees latitude. The typical wind flow in the equatorial region consists of particles traveling at variable rates, alternating between speeds of 350 kilometers to 400 kilometers per second (nearly 1 million miles per hour). Above this zone, extending all the way to the highest latitude, is fast wind traveling at double the speed -- about 750 kilometers per second (about 2 million miles per hour) -- and at a relatively steady flow. These winds come from coronal holes in the Sun, which are close to the poles and fairly large when the Sun is in a quiescent state. The speeds in the high latitude zones north and south are nearly equal, despite their separation by slow wind around the equator.

"This division of the wind into three zones -- north, equatorial and south -- is also seen in the magnetic field measurements," Smith said. "In the fast, high-latitude wind, large amplitude waves are continuously present, traveling outward from the Sun. These unusually large, strong waves are similar to waves moving along a taut rope. They move outward along a large scale solar magnetic field that is stretched outward into space by the solar wind."

Energetic particle bursts also divide space into two regions. The bursts occur from the equator to between 40 and 70 degrees latitude, after which they disappear. Particles in this region of fast wind surprised scientists, who then had to develop new models of the Sun's global magnetic field. Since the particles are guided along the large-scale field, these field lines appear to be transported from high to low latitude through the Sun's equatorial region.

Unlike the particle bursts, galactic cosmic rays occupy a single zone stretching from the equator to the poles, without a significant increase in number. Magnetic field observations made by Ulysses explain this relatively uniform distribution, Smith said. "Although the cosmic rays might have easier access to the poles by following the converging magnetic lines of force, the outward-traveling waves in the fast wind oppose their entry and compensate for this potential increase."

As Ulysses begins its second solar cycle, the Sun is now becoming increasingly active, with more sunspot activity and solar eruptions leading to the next peak around 2000. Conditions will be dramatically different when the spacecraft reaches the polar regions again at the turn of the century. In this stormy phase, Ulysses, along with an international fleet of Sun explorers -- including NASA's Advanced Composition Explorer, Wind spacecraft and joint missions such as Polar and SOHO -- will allow scientists to better understand the connections between activities on the Sun and their potential for impacting atmospheric conditions on Earth.

Ulysses is managed jointly by NASA and the European Space Agency to study the regions above and below the Sun's poles. The Jet Propulsion Laboratory manages the U.S. portion of the mission for NASA's Office of Space Science, Washington, DC. JPL is a division of the California Institute of Technology, Pasadena, CA.

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