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[Fwd: Press Release Nr 39-2001 - ESA and NASA agree new mission scenario for Cassini-Huygens]

Date: Fri, 29 Jun 2001 17:43:50 +0200
From: ESA Media Relations <ContactESA@esa.int>

Paris, 29 June 2001
Press Release
N° 39-2001 

ESA and NASA agree new mission scenario for Cassini-Huygens 

A new mission scenario has been identified 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 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
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

 "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)
Fax: +33(0)

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

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