Artist's concept of Huygens descent into Titan's atmosphere. Photo: ESA

Mission engineers at NASA are working with their counterparts at the European Space Agency (ESA) on a concern with the communication system on ESA's Huygens probe, which is attached to the Cassini spacecraft. Huygens is to drop from the Cassini spacecraft in late 2004 onto the large moon of Saturn called Titan as the Cassini orbiter begins its own exploration of the ringed planet and its system of moons.

The concern, which was identified in early September with tests at ESA's Operations Center at Darmstadt, Germany, involves the radio receiver supplied by ESA to receive signals from the Huygens probe as it descends through Titan's atmosphere. According to the tests, the signal sent to Cassini from Huygens will change in frequency as both spacecraft rapidly change position in relation to each other, much as a train whistle appears to change in pitch as it passes by a person standing alongside the tracks (called the Doppler effect). The engineering test found that the ESA-supplied receiver carried on the U.S. Cassini main spacecraft could not receive all the data from the Huygens probe.

ESA's Director of Science, R. M. Bonnet, has initiated an enquiry board to ascertain why these link characteristics were not identified prior to launch and to ensure that the Huygens science will be fully realised and that future ESA missions utilising similar hardware are alerted to the problem.

The link margins are degraded due to Doppler shift on the data sub-carrier being outside the bandwidth of the receiver phase-lock loop. In essence this means that there could be up to 10dB extra loss in the link compared to that assumed for the mission. Verification before launch was achieved by a mix of analysis and ground testing, this parameter was not isolated during the verification process.

Potential options for recovering the situation range from a greater fly-by distance, slower orbiter approach to tracking of the probe, all of which would lead to an increased signal level with a reduced Doppler shift enabling the data signal to remain with the existing Huygens receiver phase lock loop bandwidth. Such changes to the baseline mission scenario need full risk assessment and agreement of both NASA and ESA.

The selected scenario will take full account of the need to maintain the unique science objectives of the Cassini/Huygens mission.

"Cassini has given us the first tantalizing taste of its enormous scientific potential," said Dr. Jay Bergstralh, Cassini Program Scientist at NASA Headquarters, Washington, D.C. "The spacecraft has operated perfectly since its launch three years ago, so we can look forward to even greater things in the coming months. We are, of course, concerned about communications with the Huygens probe, but the best minds in the business are working on solutions."

A plan of action is expected to be ready by next summer for review and approval by officials of ESA and NASA.