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.

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.

More information about Cassini-Huygens is available online

and at:

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.