A NASA spacecraft once all but given up for dead has provided the best close-up view of the nucleus of a comet, project officials and scientists announced Tuesday.

In this highest resolution view of the icy, rocky nucleus of comet Borrelly, (about 45 meters or 150 feet per pixel) a variety of terrains and surface textures, mountains and fault structures, and darkened material are visible over the nucleus's surface. This was the final image of the nucleus of comet Borrelly, taken just 160 seconds before Deep Space 1's closest approach to it. Photo: NASA/JPL

The preliminary analysis of images and other data returned by Deep Space 1 (DS1) during its recent flyby of comet Borrelly, released at a press conference Tuesday afternoon, show that the comet has a bowling pin-shaped nucleus with an unusual jet of gas and dust emanating from one region of it.

"I have the distinct pleasure today of introducing Earth to another member of the solar system family, named comet Borrelly," said DS1 project manager Marc Rayman, as he and several scientists unveiled high resolution images and solar wind data collected by DS1 as it passed 2,200 kilometers from Borrelly last Saturday.

Of particular interest were the images from DS1's camera, which show that Borrelly appears vaguely like a bowling pin, eight kilometers long and up to four kilometers in diameter. Those images, with a maximum resolution of 45 meters per pixel, are the best images ever taken of a comet nucleus, besting those taken of comet Halley by the European Space Agency's Giotto spacecraft in 1986.

The images also showed that the comet has a mottled appearance, with a mixture of bright and dark regions. The bright regions, while appearing white in the images, are actually as dark as chimney soot, explained Larry Soderblom of the U.S. Geological Survey, leader of the DS1 imaging team, but are still two to three times brighter than the darker regions.

Mission scientists also found that a large bright area in the middle of the nucleus, as well as a smaller bright region at the small end of the nucleus, are linked to several distinct jets of gas and dust emanating from the nucleus. Those jets may be linked to cross-cutting fractures seen in the "neck" of the nucleus, a sign that the comet may one day split.

"It's mind boggling and stupendous," said Soderblom. "These pictures have told us that comet nuclei are far more complex than we ever imagined. They have rugged terrain, smooth rolling plains, deep fractures and very, very dark material."

This very long exposure was taken by NASA's Deep Space 1 to show detailed structures in the faint parts of comet Borrelly's inner coma. As a result, the nucleus has been greatly over-exposed and its shape appears distorted. The main jet extends to the lower right about 30 degrees off the Sun-line. Faint structures in the coma stretch from the nucleus in all directions. The surface of Borrelly is composed of a mixture of dust and water ice, and as the comet approaches the Sun the ice sublimes. Photo: NASA/JPL

Those jets may also explain another puzzling aspect of the comet found by DS1's ion and electron spectrometer. Preliminary analysis of that instrument's data showed that, unlike other comets, the nucleus was not at the center of the coma of gas and dust that surrounds it, but rather off to one side. Such an asymmetry could be caused by the jets seen in the images, if they are the major source of the gas and dust that make up Borrelly's coma.

"Most of the charged particles formed to one side, which is not what we expected," said University of Michigan scientist David Young, leader of DS1's space physics team. "We've got some explaining to do, and a lot of modeling to do."

Mission scientists stressed that the data presented Tuesday was the result of only a very preliminary analysis, and that a great deal of additional study was planned to better understand the nature of comet Borrelly. For example, they noted that spectroscopic data collected by DS1 -- which may provide more information about the composition of Borrelly -- still required a few weeks of analysis before scientists can draw any conclusions from it.

"This encounter was not only exciting, it was an unqualified success," said Don Yeomans, manager of NASA's Near Earth Objects program office at JPL.

Planning for future missions
Those involved with the mission hailed both the scientific and technical accomplishments of the DS1 flyby of Borrelly. For scientists, it gave them the first detailed look at a distinct class of comets not previously studied up close. About two-thirds of known comets, including Halley, are believed to have come from the Oort Cloud in the distant reaches of the solar system, while the remainder, including Borrelly, come from the less-distant Kuiper Belt. In addition to their different orbits, Halley-class comets have considerable amounts of carbon molecules, such as C2 and C3, in the nuclei, which Borrelly-class comets lack.

The flyby also helps blaze the trail for several other comet missions planned for the coming decade. In November 2003 the Comet Nucleus Tour (CONTOUR) spacecraft will fly by the comet Encke, and three months later the Stardust spacecraft will fly by Wild 2, collecting samples of comet dust for return to Earth. On July 4, 2005, the Deep Impact spacecraft will fly by Tempel 1 and release a probe that will crash into the nucleus. In 2006 CONTOUR will fly by another comet, Schwassmann--Wachmann 3. In 2011 ESA's Rosetta spacecraft will perform the first soft landing on a comet nucleus when it touches down on comet Wirtanen.

DS1 showed that flying close to the nucleus of a comet might not be risky as once thought. Mission planners feared that high-speed collisions with dust could damage or disable the spacecraft, but no such encounters were noticed. Because most of Borrelly's dust was concentrated into a set of collimated jets, DS1 was apparently in far less danger than expected, which may bode well for future missions if other comets are like Borrelly.

"The lessons learned from this cometary encounter at Borrelly will be put to good use for the coming five encounters with comets in the next decade," said Yeomans. "We'll be a bit smarter for the next encounter, and smarter still for the following encounters."

"The little spacecraft that could"
The flyby was also hailed as a final triumph for a spacecraft that once appeared to have been lost. Launched in October 1998 on a one-year mission to demonstrate a set of advanced technologies, including ion drives and autonomous navigation and control, the spacecraft was given an extended mission in 1999 to fly by two comets, Wilson-Harrington and Borrelly, in 2001.

However, in November 1999 DS1's star tracker, an instrument used to determine the orientation of the spacecraft, failed. At that time NASA was tempted to end the mission, as DS1 had already fulfilled its objectives to test new technologies. However, the small project team persevered, and spent several months writing software that allowed DS1's science camera to also serve as a star tracker, pumping new life into the mission. While the star tracker failure kept DS1 from visiting Wilson-Harrington, the spacecraft was fixed in time for it to keep its date with Borrelly.

An artist's concept of Deep Space 1 during comet encounter. Photo: NASA/JPL

Project officials maintained very low expectations about the flyby, noting the star tracker and other problems experienced by DS1 as well as the low levels of hydrazine propellant used by the spacecraft's attitude control systems. In addition, NASA recalled the mixed results from DS1's flyby of the asteroid 9969 Braille in July 1999, when technical glitches produced a disappointing set of images of the asteroid.

"To be quite honest, I didn't believe that this little spacecraft was up to the complex and risky assignment," said Rayman Tuesday. "The encounter did not go the way we expected. It went perfectly."

DS1 will continue to operate for at least the near future, conducting additional technology demonstrations as part of a "hyperextended" mission, according to Rayman. However, when the spacecraft runs out of hydrazine -- which may come at any time -- the spacecraft will no longer be able to control its attitude and the mission will then end.

Until then, Rayman and colleagues plan to make the most out of this amazing spacecraft. "This is really the little spacecraft that could, and did," he said.