ESA and NASA agree a new mission scenario for Cassini-Huygens
2 July 2001The European Space Agency and NASA have identified a new mission scenario in order to solve the Huygens radio communications problem and fully recover the scientific return from the Cassini-Huygens mission to Saturn and its giant moon Titan.
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 allow 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. (The Doppler shift is a measure of the difference in tone between an emitted and a received wave (for example, radio) when the transmitting source and the receiver move one with the respect to the other.) This would have resulted in the loss of most of the unique data returned from the Probe during its descent through Titans dense atmosphere.
In order to ensure that as much data as possible is returned from the pioneering Probe, the Huygens Recovery Task Force proposed a new schedule for Cassinis first orbits around Saturn. The agreed scenario involves shortening Cassinis first two orbits around the ringed planet and inserting an additional orbit which provides the required new geometry for the Huygens mission to Titan.
Under the new scenario, the arrival at Saturn on 1 July 2004 remains unchanged. However, Cassinis 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 2004, for an entry into the moons atmosphere 22 days later, on 14 January 2005, seven weeks later than originally planned.
In order to reduce the Doppler shift in the signal from Huygens, the Cassini Orbiter will fly over Titans cloud tops at a much higher altitude than originally planned 65 000 km instead of 1200 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 ESAs Space Science Projects Department. "The measure of an organization is the manner in which it recovers."
The new mission scenario will have some impact on Cassinis propellant supply, consuming approximately one quarter of the Orbiters estimated fuel reserve 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 Probes on-board software.
"I am very happy that we have found a good engineering solution," said Kai Clausen, ESAs 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, ESAs Huygens project scientist. "What is important is that we have found the solution. It is now time for fine-tuning."
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 requested the HRTF to hand over to the project teams in July for implementation of the joint recommendations.
Note to editors:
Cassini-Huygens is an international 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 an orbiter (Cassini) and a 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 Jupiters 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.
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John Credland, ESA Head of Space Science Projects Dept.
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