Cometary Closeup

Image of Borrelly taken by Deep Space 1.
This was the final image of the nucleus of comet Borrelly, taken just 160 seconds before Deep Space 1’s closest approach to it. Smooth, rolling plains containing brighter regions are present in the middle of the nucleus and seem to be the source of dust jets seen in the coma.
Credit: NASA/JPL

In a risky flyby, NASA’s ailing Deep Space 1 spacecraft successfully navigated past a comet, giving researchers the best look ever inside the glowing core of icy dust and gas.

The space probe’s close encounter with comet Borrelly provided the best-resolution pictures of the comet to date. The already-successful Deep Space 1, without protection from the little-known comet environment, whizzed by just 2,200 kilometers (1,400 miles) from the rocky, icy nucleus of the 10-kilometer-long (more than 6-mile-long) comet.

Exceeding the team’s expectations of how this elderly spacecraft would perform, the intrepid spacefarer sent back black-and-white photos of the inner core of the comet. It also measured the types of gases and infrared waves around the comet, and how the gases interacted with the solar wind.

"Deep Space 1 plunged into the heart of comet Borrelly and has lived to tell every detail of its spine-tingling adventure!" said Dr. Marc Rayman, the project manager of Deep Space 1 at the Jet Propulsion Laboratory (JPL), Pasadena, Calif. "The images are even better than the impressive images of comet Halley taken by Europe’s Giotto spacecraft in 1986."

Rayman added, "After years of nursing this aged and wounded bird along — a spacecraft not structured to explore comets, a probe that exceeded its objectives more than two years ago — to see it perform its remarkably complex and risky assignment so well was nothing short of incredible."

Deep Space 1 encounter with Comet Borrelly (Solar Wind).
A very strong interaction occurs between the solar wind (horizontal red bands to left and right in figure) and the comet’s (Borrelly) surrounding cloud of dust and gas, the coma. Near Deep Space 1’s closest approach to the nucleus, the solar wind picked up charged water molecules from the coma (upper green band near the center), slowing the wind sharply and creating the V-shaped energy structure at the center.
Credit: NASA/University of Michigan

"It’s mind-boggling and stupendous," said Dr. Laurence Soderblom, the leader of Deep Space 1’s imaging team, and a geologist with the U.S. Geological Survey, Flagstaff, Ariz. "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."

Scientists also realized that Borrelly was different than they expected as Deep Space 1 flew through the coma, the cloud of dust and gas surrounding the nucleus. They had expected that the solar wind would flow symmetrically around the cloud, with the nucleus in the center.

Instead, they found that though the solar wind was flowing symmetrically around the cloud, the nucleus was off to one side shooting out a great jet of material forming the cloud that makes the comet visible from Earth. "The formation of the coma is not the simple process we once thought it was," said Dr. David Young of the University of Michigan, Ann Arbor, leader of the team that made the measurements. "Most of the charged particles are formed to one side, which is not what we expected."

Deep Space 1 completed its primary mission testing ion propulsion and 11 other advanced, high-risk technologies in September 1999. NASA extended the mission, taking advantage of the ion propulsion and other systems to undertake this chancy but exciting encounter with the comet.

Deep Space 1, launched in October 1998 as part of NASA’s New Millennium Program, is managed by JPL for NASA’s Office of Space Science in Washington. The California Institute of Technology manages JPL for NASA.