European Space Agency's Vehicle Set To Drop In On
Titan
The highlights of the first year of the Cassini-Huygens mission
to Saturn can be broken into two chapters: first, the arrival of
the Cassini orbiter at Saturn in June, and second, the release of
the Huygens probe on Dec. 24, 2004, on a path toward Titan.
The Huygens probe, built and managed by the European Space
Agency (ESA), is bolted to Cassini and fed electrical power through
an umbilical cable. It has been riding along during the nearly
seven-year journey to Saturn largely in a "sleep" mode, awakened
every six months for three-hour instrument and engineering
checkups. In three days, it will be cut loose from its mother ship
and will coast toward Saturn's moon Titan, arriving on Jan. 14,
2005.
"As partners with ESA, one of our obligations was to carry the
Huygens probe to Saturn and drop it off at Titan," said Robert T.
Mitchell, Cassini program manager at NASA's Jet Propulsion
Laboratory, Pasadena, CA. "We've done the first part, and on
Christmas Eve we will release Huygens and tension-loaded springs
will gently push it away from Cassini onto a ballistic free-fall
path to Titan."
Once freed from Cassini, the Huygens probe will remain dormant
until the onboard timer wakes it up shortly before the probe
reaches Titan's upper atmosphere on Jan. 14. Then it will begin a
dramatic plunge through Titan's murky atmosphere, tasting the
chemical makeup and composition as it descends to touch down on its
surface. The data gathered during this 2-1/2 hour descent will be
transmitted from the probe to the Cassini orbiter. Afterward,
Cassini will point its antenna to Earth and relay the data through
NASA's Deep Space Network to JPL and on to ESA's Space Operations
Center in Darmstadt, Germany, which serves as the operations center
for the Huygens probe mission. From this control center, ESA
engineers will be tracking the probe and scientists will be
standing by to process the data from the probe's six
instruments.
Currently, both the orbiter and the probe are on an impact
trajectory with Titan. This is the only way to ensure that Cassini
delivers the probe in the right location. A confirmation of
successful release is expected to be received from NASA's Deep
Space Network tracking stations at Madrid, Spain and Goldstone, CA,
shortly before 2000 PST on Dec. 24. A team of JPL engineers and ESA
mission managers will be monitoring spacecraft activities at JPL
during the release phase of the mission.
On Dec. 27, the Cassini orbiter will perform a deflection
maneuver to keep it from following Huygens into Titan's atmosphere.
This maneuver will also establish the required geometry between the
probe and the orbiter for radio communications during the probe
descent.
Two of the instruments on ESA's Huygens probe, the descent
imager and spectral radiometer camera and the gas
chromatograph-mass spectrometer, are contributions from NASA and
American academia.
The imaging camera will take advantage of the Huygens probe's
rotation, using two imagers to observe the surface of Titan during
the late stages of descent for a view of the regions around the
impact site. A side-looking imager will view the horizon and the
underside of any cloud deck. More than just a camera, the
instrument is designed to measure concentrations of argon and
methane in the atmosphere and determine the size and density of
particles. The instrument will also determine if the local surface
is a solid or liquid, and if solid, its topography. The principal
investigator is Dr. Martin G. Tomasko of the University of Arizona,
Tucson, AZ.
Although Titan's atmosphere is primarily nitrogen and methane,
scientists believe it contains many other gases that are present
only in small amounts. These trace gases can reveal critical
details about the origin and evolution of Titan's atmosphere.
Because trace gases are rare, they are difficult or impossible to
observe remotely, so direct measurements must be made.
The gas chromatograph-mass spectrometer instrument will sample
gas directly from Titan's atmosphere as the Huygens probe descends
by parachute. Data from the instrument will allow researchers to
investigate the chemical composition, origin and evolution of the
atmosphere of Titan. The instrument was designed and built by
NASA's Goddard Space Flight Center, Greenbelt, MD, and is led by
the principal investigator, Dr. Hasso Niemann.