Comet Studies (March 1997)
PUBLIC INFORMATION OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIF. 91109. TELEPHONE (818) 354-5011
Diane Ainsworth, Jet Propulsion Laboratory
Jim Scott, University of Colorado, Boulder, 303-492-3114
FOR IMMEDIATE RELEASE
SCIENTISTS BEGIN CAPTURING UNIQUE VIEW OF HALE-BOPP
on a joint NASA-European Space Agency mission to study the
poles of the Sun are using the Ulysses spacecraft's unique,
high latitude orbit to help understand changes in comet
Hale-Bopp as it nears the lower latitudes of the Sun while
spewing its outer layers of gas and dust.
solar wind data from the spacecraft, a team of interdisciplinary
scientists at NASA's Jet Propulsion Laboratory, the University
of Colorado at Boulder and an international group of amateur
astronomers known as the "Ulysses Comet Watch"
have been observing other comets in the same way as they
return from the far reaches of space. This unique, collaborative
research effort focuses specifically on what happens to
comets as they are exposed to different solar wind conditions
at various solar latitudes.
recently, no spacecraft had flown through high latitudes
above the Sun's equator and the properties of the solar
wind at these latitudes could only be surmised," said
Dr. Edward Smith, project scientist of the Ulysses mission
at JPL. "In the last two years, Ulysses has studied
the steady, high-speed winds at high latitudes, and alternating,
slow and fast winds near the Sun's equator. Hale-Bopp is
about to enter the lower latitude zone, where the disturbed
solar wind resides, and where dramatic changes in the comet's
plasma tail are expected to occur."
Ulysses Comet Watch group, spearheaded by Drs. John C.
and Martin Snow of the University of Colorado's Laboratory
for Atmospheric and Space Physics, and Carolyn Collins
now of Sky Publishing Corp. in Cambridge, MA, will provide
worldwide observations of the returning comet as it descends
from the polar regions of the Sun. Images from more than
200 observers around the world will be posted on the
Comet Watch home page on the Internet at http://lasp.colorado.edu/ucw/.
Observations will continue to be posted well after the comet
makes its closest approach to the Sun on April 1.
Ulysses group is not watching for changes in the comet's
more visible, wider and yellowish dust tail, though, but
rather for changes in its narrower, paler, often bluish
plasma tail, which consists of ionized gas being emitted
by the comet and picked up by the magnetic field being swept
along by the solar wind.
are particularly interested in the comet's plasma tail,
which 'turned on,' or began to outgas, when Hale-Bopp was
about 1.5 astronomical units (140 million miles) from the
Sun, the equivalent of 1.5 times the distance between the
Earth and the Sun," Brandt said.
plasma tails change constantly due to their interactions
with slow and high-speed solar wind. One of the more dramatic
changes that can occur is the abrupt disconnection of the
tail, which then drifts away. Often a new tail will form
lose their plasma tails when they are subjected to abrupt
reversals in the direction of the solar wind magnetic field,"
Smith said. "This magnetic field reversal typically
happens in the equatorial regions twice per solar rotation,
or about every 13 days. They don't occur at high latitudes."
Brandt and his team expect to see such dramatic changes
in the comet's plasma tail only in the equatorial regions
of the solar wind -- between 20 degrees north and 20 degrees
south of the Sun's equator.
after the very successful international Halley Watch observation
networks, which operated during 1985 and 1986, the Ulysses
Comet Watch network has been operating since late 1992.
Observers have studied a number of comets, and have supplied
sequences of high-quality images of comets de Vico, which
returned in September 1995, and Hyakutake, which was discovered
and observed last year.
first discovered that the plasma tails of these comets change
and drop off according to their latitudes with respect to
the Sun during these two comet returns. Observers are now
beginning to submit early images of Hale-Bopp, which, according
to Brandt, are looking equally as promising. "In fact,
we expect the network's output of Hale-Bopp images to be
fantastic," he said.
equatorial latitudes, the solar wind moves at an average
speed of about 450 kilometers per second (970,000 miles
per hour) with large variations in speed and density,"
Brandt continued. "This type of wind apparently comes
from the equatorial streamers so clear at solar eclipses.
The plasma tail of a comet experiencing this part of the
solar wind has a distinctly disturbed appearance which varies
over time, and undergoes disconnection events as it experiences
reversals of the magnetic field.
contrast, when the same comet travels through the polar
latitudes, it encounters a more steady, less dense and faster
solar wind, moving at about 750 kilometers per second or
1.6 million miles per hour," Brandt said. "There
are smaller variations in speed and density and no magnetic
field reversals. Consequently, the plasma tail looks much
less turbulent and does not have disconnection events. So,
the comet, by acting as a 'solar wind sock,' can be used
to map the conditions in different latitudes of the solar
Hale-Bopp is ideally suited to show these types of tail
changes because of its high-latitude orbit. Ulysses' measurements
of the solar wind from the same latitude, combined with
ground-based observations of comet tails, will help scientists
better understand the physics involved in cometary gases
and their interaction with the outward-flowing solar wind.
From this information, they may be able to understand the
solar wind in regions that have never been accessible to
spacecraft before, such as very close to the Sun or at much
higher latitudes above and below the Sun's equator.
interested in further information about the Ulysses Comet
Watch network may contact John C. Brandt at his e- mail
address: firstname.lastname@example.org, or write to him at the
Laboratory for Atmospheric and Space Physics, Campus Box
392, University of Colorado, Boulder, CO 80309. His office
telephone number: (303) 492-3215, or by fax at (303) 492-6946.
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,
[Note to Editors: NASA TV will broadcast a Ulysses video
file featuring brief interviews with Drs. Bruce Goldstein,
Ulysses deputy project scientist at JPL, and Jack Brandt,
Ulysses Comet Watch team leader at the University of Colorado,
during its regularly scheduled video file programming at
9 a.m., 12 noon, 3 p.m. and 6 p.m. Pacific Standard Time
today, March 19. The program also includes animation of
a comet losing its plasma tail and footage of the Ulysses
mission to the poles of the Sun. NASA Television has switched
to a new satellite and is available on GE-2, transponder
9C, 85 degrees longitude, vertical polarization, audio frequency
3880 megahertz, audio 6.8 megahertz.]