GREENBELT, Md. — What some had thought would be a suicide mission turned instead into a victory march Wednesday as an aging U.S. spacecraft shot through the surprisingly turbulent tail of comet Giacobini-Zinner.
The International Cometary Explorer spent about 20 minutes in the highly charged tail before emerging unscathed from humankind's first probe of a comet. As it did so the spacecraft sent back enough data to revise thinking about comets, according to members of the team that put the bold project together.
"It was a remarkable achievement," said James M. Beggs, head of the National Aeronautics and Space Administration.
The mission was so successful, Beggs said, that he regrets that NASA decided against sending a spacecraft to Halley's Comet.
"The money we would have spent would not have been missed over the years," Beggs said shortly after the successful completion of the mission to Giacobini-Zinner, named for its two discoverers. He said the American people probably "would have enjoyed Halley more if a U.S. spacecraft had been in the armada" of five probes on their way to Halley.
That was about the only note of disappointment sounded on a day of jubilation at the Goddard Space Flight Center here. It was a day that began lyrically. As the craft approached Giacobini-Zinner, loudspeakers at Goddard carried the sound of its transmitters sending electronic data back to Earth. It was a low, haunting sound, like the gentle cooing of a dove.
The biggest surprise of the rendezvous was produced soon afterward when the craft passed safely through the comet's tail. Apparently, scientists said, the 5-foot-tall spacecraft was never in danger of being destroyed by dust particles contained in the tail.
Scientists had speculated that dust could have coated solar energy collectors, thus shutting down the craft's power supply. Larger particles could have sheared off its antenna or even tipped it over so that the transmitter no longer pointed toward the Earth.
"Obviously, I was a lot more pessimistic than I should have been," said Robert Farquhar, the "astrodynamicist" who figured out the extremely complex maneuvers that sent the craft on a wild flight through space toward the encounter.
Sent on New Mission
The recycled craft originally was launched into a fixed position between the Earth and the sun in 1978 to study solar winds. On its new mission it was sent looping past the moon five times--passing once within 75 miles of the lunar surface--to pick up enough speed to reach the comet. In effect, it used the moon as a slingshot.
During the 20-minute trip through the electrically charged core of the tail, the instruments sent back enough data to keep scientists busy for months and will force many revisions in theories about comets. The craft carries no cameras, since it was not designed for cometary exploration.
"A lot of people are going to see that the data doesn't support their predictions," said Edward J. Smith, a physicist at the Jet Propulsion Laboratory in Pasadena and a principal investigator on the Giacobini-Zinner encounter. "There will be a lot of rethinking."
One such revision almost certainly will involve past beliefs about electromagnetic fields surrounding comets. Previously, scientists here said, there was little evidence that comets had such fields.
The encounter by the spacecraft showed, however, that Giacobini-Zinner, which passes the Earth every 6 1/2 years, is surrounded by a strong magnetic field. "The scope and the magnitude (of electromagnetic activity) was completely unexpected," said Frederick L. Scarf of TRW, a principal investigator on the project and an early booster.
The magnetic field surrounding Giacobini-Zinner is not generated by the comet itself, since it has no "internal dynamo" similar to the Earth's core. Rather, the field is picked up as the comet passes through "solar wind" caused by activity on the sun. That interrelationship is known as "space plasma physics" and it underscores the importance many scientists attach to the study of comets.
The comet's tail apparently was also a noisy place. Aboard the craft were instruments that measured sound waves during the fly-through and, according to Scarf, the readings were so high the instruments were nearly "saturated."
A Few Surprises
Scientists working on the project knew they were in for a few surprises more than seven hours before the spacecraft reached the comet's tail at 3:53 a.m. PDT. The craft's instruments picked up signs of very turbulent electromagnetic activity, giving some credence to the theory that comets create "bow shocks" as they speed through space. Such shocks are similar to sonic waves generated by high-speed aircraft.
The existence of a cometary bow shock has been debated among scientists, and the encounter with Giacobini-Zinner will add to that debate. Even though the first signs suggested that the bow shock does exist, other instruments produced conflicting information.