April 25, 1997
RELEASE NO: 96-40
POLAR ALSO DETECTS HALE-BOPP SODIUM TAIL
A sophisticated camera on NASA's POLAR satellite has independently
detected the new type of comet tail that was recently discovered
by European astronomers who studied Comet Hale-Bopp. The phenomenon,
called a "neutral sodium tail" consists of electrically
neutral sodium atoms. The tail glows with the yellow tint of the
flame seen when salt (which contains sodium) is tossed on a fire.
The science observatory is managed by the Goddard Space Flight
Center in Greenbelt, Md.
"The detection of the neutral sodium tail in Hale-Bopp is
a great discovery," said Professor Paul Feldman, an authority
on comets at The Johns Hopkins Univesity in Baltimore, Md. Dr.
Michael Mumma, a comet expert at the Goddard Space Flight Center,
added that the discovery "provides a more complete picture
of the physics in the comet."
The new kind of tail is seen in images obtained with a special
filter, designed to accept light from sodium atoms while blocking
most other light, in POLAR's Visible Imaging System (VIS). The
camera was developed under Professor Louis Frank at the University
of Iowa, in Iowa City. The appearance of the comet in images
obtained through the sodium filter is different from that seen
through other filters in the camera, according to Frank's associate,
Dr. John Sigwarth. Sigwarth reports that the new tail was about
500,000 miles (800,000 kilometers) wide as imaged by POLAR on
March 31, when Comet Hale-Bopp was near its closest approach to
the Sun. It was at least 7,500,000 miles (12 million kilometers)
long, and probably longer, extending beyond the edge of the VIS
camera field of view.
In contrast, when discovered at the Observatorio del Roque de
los Muchachos, La Palma, Canary Islands, Spain by the European
Hale-Bopp Teamon April 16, the neutral sodium tail width had narrowed
to about 410,000 miles (660,000 kilometers). (The new kind of
comet tail was first found by the European astronomers and subsequently
confirmed on images obtained about three weeks earlier and in
process of analysis by the POLAR scientists.) Careful analysis
of differences in observing methods and conditions will be necessary
before scientists draw definitive conclusions from comparison
of the ground based and POLAR satellite data.
POLAR's comet tail detection was made during the interval of
March 27 to April 2, when Comet Hale-Bopp was too close to the
direction of the Sun as seen from Earth to be observed with many
conventional and orbiting telescopes. At that time, ground controllers
at Goddard commanded POLAR to point its complement of three camera
systems off its usual target, the Earth, to record the unique
spectacle of Hale-Bopp's emissions in a variety of wavelengths
(colors of light) of special scientific interest. (Normally, the
POLAR cameras focus on the Earth's northern and southern auroral
zones, while other onboard instruments measure particles, fields,
and wave phenomena in the Earth's magnetosphere.) Because two
of the POLAR cameras are designed to observe faint visible- and
ultraviolet-light emissions of the Earth's atmosphere in close
proximity to the bright dayside of the Earth, they are also capable
of looking closer to the Sun in the sky than most conventional
The special POLAR comet watch was coordinated by Dr. Nicola Fox
of the International Solar-Terrestrial Physics (ISTP) program
staff at the Goddard Center. "This study reflects a tremendous
effort by many members of the POLAR team, and we are delighted
with the results, especially the chance to catch a new kind of
comet tail," Fox said. "This proves how versatile the
POLAR spacecraft is," she added.
The thickness and shape of the new tail distinguish it from previously
known kinds of comet tails, according to Dr. Sigwarth. "We
are excited at the opportunity to study this once-in-a-lifetime
comet with the special analytical filters of our POLAR camera,"
he said, adding "the filters allow us to get new data on
the physics and chemistry of a comet." The new tail is narrower
and straighter than a dust tail, which tends to be smooth and
is often perceptibly curved. And, the neutral sodium tail, although
straight like a comet's ion tail, is not directed along the same
direction in space (which is approximately the anti-sun direction)
as the ion tail. "We can tell one tail from the other by
where they point and how they look," Dr. Sigwarth explained.
The new kind of comet tail is also composed of different material than previously known tails. The neutral sodium tail, as the name suggests, contains ordinary sodium atoms, called "neutral" because they are not electrically charged. The dust tail consists of microscopic solid particles such as silicates that are very effective at reflecting the light of the Sun. (The dust tail of Comet Hale-Bopp is the one most readily seen with the naked eye.) Finally, the ion tail consists of atoms that have each lost one or more negatively charged electrons, so that they have net positive charges, and it glows with a striking blue radiation associated with the individual types of ionized atoms. On ordinary color photographs, the ion tail looks blue and the dust tail is yellowish-white, but the neutral sodium tail may not be distinguished without special filters or other optical devices.
Astronomers around the world have responded to the discovery of a new kind of comet tail in Hale-Bopp by scrutinizing it with many telescopes and instruments. A detailed summary of the numerous investigations of the neutral sodium tail that are in progress can be found on the European Southern Observatory World Wide Web site at the URL
The many POLAR observations of Comet Hale-Bopp are under intensive study, and further findings and images will be announced as results warrant.
CAPTION: Shown here is an image of Comet Hale-Bopp on March 31, 1997 at 11:07 a.m. EST or 16:07 GMT. Arrows show the direction of motion of the comet in its orbit around the Sun, the direction to the Sun, the newly detected "neutral sodium tail" of the comet, and the familiar dust tail. The image was obtained through a special narrow-band filter in the Visible Imaging System on board NASA's POLAR satellite. The filter isolates and accepts the yellow light from sodium atoms. The neutral sodium tail shines in that light, while the dust tail is seen here in the reflected light of the Sun. The comet's ion tail is not readily seen in light of this color.
CREDIT: Louis Frank, John Sigwarth, University of Iowa, and NASA.
Official NASA Contact: Mr. William Mish email@example.com
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Last Updated: 04/30/97