Managers for an international mission to Saturn have announced a revised plan to work around a telecommunications problem and avoid loss of scientific data after the Cassini spacecraft releases the Huygens probe to descend to the surface of Titan, Saturn's biggest moon, in 2005.

An artist's concept of Huygens dropping toward the moon Titan. Photo: ESA

The new plan will change the planned release date and geometry for the part of the mission in which the Huygens probe will parachute into the thick atmosphere of Titan. The new date will be Jan. 14, 2005, seven weeks later than originally planned. The plan will also position the Cassini orbiter farther away during that descent.

After six months of analysis by the European Space Agency (ESA)-NASA joint Huygens Recovery Task Force, senior management from both agencies and members of the Cassini- Huygens scientific community have endorsed the mission modifications. The analysis was undertaken after the Huygens probe telecommunications problem was identified last autumn.

The Cassini-Huygens mission was launched in 1997. Engineers last year identified a design flaw in the Huygens communications system. Without a change in flight plans, the Huygens receiver would be unable to compensate enough for the Doppler shift in radio frequency between the signal emitted by the probe and the one received by the orbiter. A Doppler shift happens when the distance between a transmitter and receiver is changing, and Cassini originally would have been rapidly approaching Titan during Huygens' descent. This would have resulted in the loss of important data from the probe during its trip through Titan's atmosphere.

When Cassini arrives at Saturn in July 2004, it will, within the first seven months, complete three flybys of Titan instead of two as originally planned. Then, in February 2005, Cassini will resume the rest of its four-year prime mission as originally planned, studying the planet and its rings, moons and magnetic environment. The changes to the mission plan will use about one-fourth to one-third of Cassini's reserve supply of propellant. The reserve supply is carried for unforeseen needs such as this and for possible use if the mission were to be extended beyond 2008.

"This recovery plan will allow us to meet all of the mission's scientific objectives," said Bob Mitchell, Cassini program manager at NASA's Jet Propulsion Laboratory, Pasadena, CA. "It has the additional advantage of giving us a close look at Titan before releasing Huygens."

This week, European Space Agency Director of Science Professor David Southwood and NASA Associate Administrator for Space Science Dr. Edward Weiler gave the go-ahead for Cassini and Huygens teams to implement the recommendations of the Huygens Recovery Task Force.

To ensure that the pioneering probe returns as much data as possible, the plan shortens Cassini's first two orbits around Saturn and adds an additional orbit that provides the required new geometry for Huygens' descent to Titan. Cassini's arrival date at Saturn on July 1, 2004, remains unchanged. However, its first flyby of Titan will now occur on Oct. 26, 2004, followed by another on Dec. 13. The Huygens probe will be released toward Titan on Dec. 25 for an entry into the moon's atmosphere 22 days later.

To reduce the Doppler shift in the signal from Huygens, Cassini will fly over Titan's cloud tops at an altitude of about 65,000 kilometers (40,000 miles), more than 50 times higher than formerly planned. The new plan also calls for several modifications to ensure maximum efficiency of the Huygens communications system. These include pre-heating the probe to improve tuning of the transmitted signal, continuous commanding by the orbiter to get the best possible performance by the receiver, and changes in the probe's on- board software.

Shrouded in an orange haze, Titan is one of the most mysterious objects in our solar system. It is the second largest moon (after Jupiter's Ganymede) and the only one with a thick atmosphere. The atmosphere excites scientific interest, since it may resemble that of a very young Earth.

The mission is an international collaboration of NASA, ESA and the Italian Space Agency. JPL, a division of the California Institute of Technology in Pasadena, manages it for NASA's Office of Space Science, Washington, DC.