MEDIA RELATIONS OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIF. 91109. TELEPHONE (818) 354-5011
http://www.jpl.nasa.gov
Contact: Diane Ainsworth
FOR IMMEDIATE RELEASEApril 17, 1998
ULYSSES COMPLETES FIRST FULL ORBIT AROUND THE SUN
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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4/17/98 DEA
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