N° 39–2001: ESA and NASA agree new mission scenario for Cassini-Huygens

After six months of investigations and analysis by a joint ESA/NASA Huygens Recovery Task Force (HRTF), senior management from the two space agencies and members of the Cassini-Huygens scientific community have endorsed several modifications to the mission. These will ensure a return close to 100% of the Huygens science data, with no impact on the nominal prime Cassini tour after the third Titan encounter.

The modifications have been introduced because of a design flaw in the Huygens communication system. This problem meant that the Huygens receiver was unable to compensate for the frequency shift between the signal emitted by the Probe and the one received by the Orbiter, due to the Doppler shift (**). This would have resulted in the loss of most of the unique data returned from the Probe during its descent through Titan’s dense atmosphere.

To ensure that as much data as possible is returned from the pioneering Probe, the HRTF proposed a new schedule for Cassini’s first orbits around Saturn.

The agreed scenario involves shortening Cassini’s first two orbits around the ringed planet and adding a third which provides the required new geometry for the Huygens mission to Titan.

In the new scenario, the arrival at Saturn on 1 July 2004 remains unchanged. However, Cassini’s first flyby of Titan will now occur on 26 October, followed by another on 13 December. The Huygens Probe will be released towards Titan on 25 December, for an entry into the moon’s atmosphere 22 days later, on 14 January 2005, seven weeks later than originally planned.

To reduce the Doppler shift in the signal from Huygens, the Cassini Orbiter will fly over Titan’s cloud tops at a much higher altitude than originally planned – 65,000 km instead of 1,200 km. This higher orbit has the added advantage that Cassini will be able to preserve the four-year baseline tour through the Saturn system, by resuming its original orbital plan in mid-February 2005.

“In any complex space mission problems may arise,” said John Credland, Head of ESA’s Space Science Projects Department. “The measure of an organisation is the manner in which it recovers.”

The new mission scenario will have some impact on Cassini’s propellant supply, consuming about a quarter of the Orbiter’s reserve fuel by the end of the four-year mission. It also involves 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 force the receiver into non-Doppler mode, and changes in the Probe’s on-board software.

"I am very happy that we have found a good engineering solution,” said Kai Clausen, ESA’s Integral Project Manager and co-chairman of the HRTF. “But a lot more work still needs to be done. Now we need to complete the detailed design, implementation, validation and testing over the next few years." “There are still some uncertainties, for example the exact definition of the landing site, but these are minor problems,” said Jean-Pierre Lebreton, ESA’s Huygens Project Scientist. “What is important is that we have found the solution. It is now time for fine tuning.”

The ESA Director of Science, David Southwood, and the NASA Associate Director for Space Science, Edward Weiler, have jointly agreed to the new mission approach and have asked the HRTF to hand over to the project teams in July for implementation of the joint recommendations.

(**) The Doppler shift is a measure of the difference in tone between an emitted and a received wave (e.g. radio) when the transmitting source and the receiver move one with respect to the other.

Note to editors

Cassini-Huygens is a joint NASA/ESA mission with the participation of the Italian Space Agency (ASI) launched by a Titan IVB/Centaur launch vehicle on 15 October 1997, that will reach Saturn in 2004. It consists of NASA's orbiter Cassini and ESA's probe Huygens. While Cassini continues to explore Saturn and its rings, the Huygens probe will be released to parachute through the atmosphere of Titan. Shrouded in an orange haze that hides its surface, Titan is one of the most mysterious objects in our Solar System. It is the second largest moon (only Jupiter’s Ganymede is bigger), and the only one with a thick atmosphere. It is this atmosphere that excites scientific interest, since it is thought to resemble that of a very young Earth.

For further information please contact:

ESA – Communication Department
Media Relations Office
Tel: +33(0)1.53.69.7155
Fax: +33(0)1.53.69.7690
John Credland, ESA – Head of Space Science Projects Department
Tel: +31 71 565 3430
Email: John.Credland@esa.int
Dolores Beasley, NASA Headquarters, Washington DC
Tel. + 1 202/358-1758
Guy Webster, Jet Propulsion Laboratory, Pasadena, CA
Tel. + 1 818/354-6278