Artist's concept of Ulysses

Scientists 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.

Using 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.

"Until 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."

The Ulysses Comet Watch group, spearheaded by Drs. John C. Brandt and Martin Snow of the University of Colorado's Laboratory for Atmospheric and Space Physics, and Carolyn Collins Petersen, 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 Ulysses Comet Watch home page on the Internet at Observations will continue to be posted well after the comet makes its closest approach to the Sun on April 1.

The 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.

"We 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.

Cometary 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 within hours.

"Comets 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."

Consequently, 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.

Modeled 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.

Scientists 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.

"At 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.

"By 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 wind."

Comet 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.

Astronomers interested in further information about the Ulysses Comet Watch network may contact John C. Brandt at his e-mail address:, 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.

Ulysses 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, D.C.

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