Spacecraft Watch for
Comet Hale-Bopp Tail Disruption
Press Release
Donald Savage
Headquarters, Washington, DC               May 5, 1997
(Phone:  202/358-1547)

William Steigerwald
Goddard Space Flight Center, Greenbelt, MD
(Phone:  301/286-1085)

RELEASE:  97-89


     A fleet of spacecraft for the International Solar Terrestrial 
Physics (ISTP) program is watching for a break in Comet Hale-
Bopp's plasma ion tail.

     "Preliminary estimates indicate that it may happen in the next few
days," said Dr. Mario Acuna, lead scientist for ISTP at NASA's Goddard
Space Flight Center (GSFC), Greenbelt, MD.  Goddard is the focal point 
for many of the ISTP investigations.

     Amateur astronomers around the world were put on watch last 
week when Dr. Bill Farrell, co-investigator for NASA's Wind spacecraft at 
GSFC, placed a notice on an Internet E-mail list, after scientists 
studying data from ISTP spacecraft estimated that Comet Hale-
Bopp's ion tail likely would be disrupted when it enters a region 
around the Sun known as the "current sheet."  Observations from 
amateur astronomers monitoring changes in the comet's tails will 
provide near-real-time data to scientists to complement 
observations from spacecraft.

     Scientists explain the disruption as a complicated 
interaction between the comet and the Sun's influence and magnetic 
fields.  As a comet comes closer to the Sun, ices from the nucleus 
(a porous structure of dust and ice composed of frozen gases) are 
continually vaporized, dislodging the dust, which is formed by the 
comet's weak gravity into a cloud, called a coma, surrounding the 
comet.  While pressure from the visible sunlight "pushes" the coma 
dust into a diffuse dust tail, the ultraviolet portion of the 
sunlight gives the coma an electrical charge, or ionizes it, 
turning it into a plasma of electrically charged particles of ions 
and electrons.

     The solar wind (also a plasma), flowing from the Sun at 
speeds from 240-450 miles per second and carrying an embedded 
magnetic field, smashes into the coma gas, causing additional 
ionization.  The magnetic field in the solar wind picks up comet 
ions and accelerates them into a long, blue plasma tail.  Since 
this tail is stretched very long, it is much fainter than the dust 
tail and consists mostly of long-lived (stable) ionized carbon 
monoxide.  The magnetic field is draped around the comet coma and 
controls the formation of the plasma tail.  If the magnetic field 
is disrupted, the plasma tail may be disconnected.

     Hale-Bopp's orbit is tilted relative to the Sun's equator 
with the comet moving from the Sun's northern hemisphere to its 
southern hemisphere, crossing the Sun's equatorial plane.  This 
plane is the location of the "current sheet," a place where the 
Sun's magnetic field lines change direction.  As Hale-Bopp passes 
through this plane, its ion tail may disconnect because of the 
change in direction of the magnetic field. 

     "Other events on the Sun may disrupt Hale-Bopp's tail," adds 
Dr. Farrell.  "For example, at any time, the Sun may eject large 
amounts of hot, electrically charged material in the form of 
plasma, called Coronal Mass Ejections, or CME's.  The magnetic 
fields associated with a CME may disrupt the ion tail, 
particularly if the CME is from the Sun's eastern limb in the 
direction of Hale-Bopp.  Also, the solar wind is more gusty around 
the equatorial regions, and this could cause a disruption as 
well," he said.

     "Monitoring this comet tail disruption is more than 
anticipating an intriguing astronomical phenomenon," said Dr. 
Farrell.  "The stronger solar events can have a tremendous impact 
on Earth.  The plasma ejected by these events smashes into the 
Earth's magnetic field and compresses it.  This generates a 
magnetic storm which can disrupt power grids and radio 
communications.  Additionally, the effects can damage 
microcircuits in satellites.  With ISTP, if we can monitor 
disruption events for comets, we can do the same for Earth, 
providing a warning when they occur," he said.

     When Hale-Bopp crosses the current sheet, it will provide 
additional data about its structure where no ISTP spacecraft 
exist.  "It could cost about a billion dollars to build and place 
a spacecraft where Hale-Bopp is," said Dr. Adam Szabo, senior 
scientist with Raytheon STX on the Wind project.  "Comet Hale-Bopp 
will give us interesting information about this region of space 
for virtually no cost, except our time to watch and study it.  
It's a bonus which can really help us understand the most powerful 
forces which are affecting the Earth." 

     The ISTP spacecraft involved in this study are NASA's Polar 
and Wind missions and the European Space Agency/NASA Solar and 
Heliospheric Observatory mission.