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 Hughes 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.