Spacecraft Watch for
Comet Hale-Bopp Tail Disruption
Headquarters, Washington, DC May 5, 1997
Goddard Space Flight Center, Greenbelt, MD
SPACECRAFT WATCH FOR COMET HALE-BOPP TAIL DISRUPTION
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
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.