DARMSTADT, Germany — European and American scientists celebrated here Jan. 14 what they characterized as the resounding success of the Huygens descent probe, whose voyage to the surface of Saturn’s moon, Titan — and continued functioning after landing — will give scientists raw data to work on for years.
The first of about 350 images taken of Titan by Huygens appears to show a series of drainage channels at the side of what scientists said is likely to be a lake of tar or super-cold hydrocarbons. Another image, taken either on the surface or just above it, shows a flat, rocky terrain.
Huygens’ success was met with occasionally emotional reactions on the part of European Space Agency (ESA) and NASA officials, many of whom had worked for more than a decade — and some as long as 22 years — on a project whose mission was designed to last only about two hours.
In fact, indications from the Huygens data received by NASA’s Cassini orbiter and sent to Earth Jan. 14 were that Huygens’ descent through the thick atmosphere of Titan lasted 147 minutes, or 27 minutes longer than expected.
Of even greater surprise was Huygens’ stubborn refusal to die immediately on landing. The probe continued to send data more than two hours after landing. It continued to transmit after Cassini had disappeared beneath Titan’s horizon and out of line-of-sight contact with Huygens.
In an effort to capture as much data as possible, Jean-Pierre Lebreton, Huygens project manager at ESA , said the agency was scrambling to bring additional ground-based antennas into the exiting fleet of 18 that were already trained on Titan to receive Huygens data.
There was one glitch in the mission. One of the two largely redundant communications channels aboard Huygens did not work because the Cassini orbiter was not programmed to receive the data, according to European officials.
“Early indications suggest it was our fault,” said one European government official. “It was not the fault of JPL [ NASA’s Jet Propulsion Laboratory, which built Cassini]. We wrote the code for switching on the communications channels, and the way we wrote it was confusing and led to only one of the channels being switched on. We could have corrected the error up to Dec. 23 [two days before Huygens separated from Cassini after a seven-year voyage], but we were not aware of it.”
How much Huygens data will be lost remained unclear late Jan. 14. A part of the crop of an expected 750 Titan atmospheric and surface images appears to have been lost, but whether this will much diminish the scientific harvest could not be determined. Other data traveling only on the failed communications line was at least partially received by the network of ground antennas, although several officials said the wind measurements received directly by the ground antennas were less precise than those that would have been received by Cassini.
ESA Director-General Jean-Jacques Dordain embraced NASA Associate Administrator Al Diaz — who spent years working on Cassini and Huygens before assuming his current post — and said the relief in seeing Huygens’ results on mission control computer screens here had brought tears to many. Diaz himself struggled to hold back tears as he reported on Huygens’ initial results.
Huygens ground teams were still piecing together the packets of data late in the evening of Jan. 14. But based on the initial data transmitted via Cassini and the global network of ground telescopes, they said the mission seemed to go almost exactly as planned, or better.
“This is bigger than Giotto, and Giotto is what really put us on the map in planetary exploration,” ESA Science Director David Southwood said, referring to the agency’s mid-1980s mission to rendezvouse with a comet.
The three-parachute system designed to slow Huygens from a speed of 18,000 kilometers per hour to 1,400 kilometers per hour within three minutes appeared to have functioned as designed. The EADS Space-built heat shield protecting Huygens from atmospheric entry temperatures estimated at 1,800 degrees Celsius protected Huygens as it slowed its descent, and then was jettisoned to expose the instruments and permit them to begin collecting data on Huygens’ atmosphere.
One surprise was that after the heat shield’s separation, Huygens was able to maintain an interior temperature of 25 degrees Celsius despite outside temperatures of minus 120 degrees Celsius.
Claudio Sollazzo, Huygens mission operations manager, said this is one reason why Huygens’ batteries appeared to last far longer than expected. Officials said that even after a slightly longer-than-expected atmospheric descent of 147 minutes, Huygens continued to send signals for two hours after its landing and after Cassini’s orbit had sent it below the Titan horizon. Cassini then turned toward Earth and began sending what it had received from Huygens.
The long after-landing survival time was music to the ears of Huygens’ Surface Science Package team, whose experiments are designed to deliver data about Titan’s surface characteristics. John Zarnecki, the principal investigator of that suite of instruments, recalled that Huygens managers at ESA initially resisted the idea of investing in a surface-evaluation package. It was thought that Huygens’ mission would be just about completed by the time it hit the surface.
Weighing 349 kilograms including its heat shield, Huygens was built by a European contracting team led by Alcatel Space of Paris. The probe was launched aboard the Cassini Saturn orbiter in October 1997 and spent the next seven years in hibernation before being switched on in December and sent on its Titan trajectory on Dec. 25.
Europe’s investment in Huygens and Cassini totaled 605 million euros ($786.5 million at current exchange rates), including 360 million euros from ESA for Huygens project management and assembly, 100 million euros from European laboratories contributing to Huygens’ six-sensor experiment payload and an additional 145 million euros spent by the Italian Space Agency to provide Cassini’s high-gain antenna and instruments on Cassini and Huygens.
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