Radio Telescopes Will Add to Cassini-Huygens Discoveries

http://www.nrao.edu/pr/2004/huygens/

December 22, 2004

Contact:

Dave Finley, Public Information Officer
Socorro, NM
(505) 835-7302


Radio Telescopes Will Add to Cassini-Huygens Discoveries

When the European Space Agency's Huygens spacecraft makes its plunge
into the atmosphere of Saturn's moon Titan on January 14, radio
telescopes of the National Science Foundation's National Radio
Astronomy
Observatory (NRAO) will help international teams
of scientists extract the maximum possible amount of irreplaceable
information from an experiment unique in human history. Huygens is the
700-pound probe that has accompanied the larger Cassini spacecraft on a
mission to thoroughly explore Saturn, its rings and its numerous moons.

The Robert C. Byrd Green Bank Telescope (GBT)
in West Virginia and eight of the
ten telescopes of the continent-wide Very Long Baseline Array (VLBA),
located at Pie Town and Los Alamos, NM, Fort Davis, TX, North Liberty,
IA, Kitt Peak, AZ, Brewster, WA, Owens Valley, CA, and Mauna Kea, HI,
will directly receive the faint signal from Huygens during its descent.

Along with other radio telescopes in Australia, Japan, and China, the
NRAO facilities will add significantly to the information about Titan
and its atmosphere that will be gained from the Huygens mission. A
European-led team will use the radio telescopes to make extremely
precise measurements of the probe's position during its descent, while
a
U.S.-led team will concentrate on gathering measurements of the probe's
descent speed and the direction of its motion. The radio-telescope
measurements will provide data vital to gaining a full understanding of
the winds that Huygens encounters in Titan's atmosphere.

Currently, scientists know little about Titan's winds. Data from the
Voyager I spacecraft's 1980 flyby indicated that east-west winds may
reach 225 mph or more. North-south winds and possible vertical winds,
while probably much weaker, may still be significant. There are
competing theoretical models of Titan's winds, and the overall picture
is best summarized as poorly understood. Predictions of where the
Huygens probe will land range from nearly 250 miles east to nearly 125
miles west of the point where its parachute first deploys, depending on
which wind model is used. What actually happens to the probe as it
makes
its parachute descent through Titan's atmosphere will give scientists
their best-ever opportunity to learn about Titan's winds.

During its descent, Huygens will transmit data from its onboard sensors
to Cassini, the "mother ship" that brought it to Titan. Cassini will
then relay the data back to Earth. However, the large radio telescopes
will be able to receive the faint (10-watt) signal from Huygens
directly, even at a distance of nearly 750 million miles. This will not
be done to duplicate the data collection, but to generate new data
about
Huygens' position and motions through direct measurement.

Measurements of the Doppler shift in the
frequency of Huygens' radio signal made from the Cassini spacecraft, in
an experiment led by Mike Bird of the University of Bonn, will largely
give information about the speed of Titan's east-west winds. A team led
by scientists at NASA's Jet Propulsion Laboratory in Pasadena, CA, will

measure the Doppler shift in the probe's signal relative to Earth.
These
additional Doppler measurements from the Earth-based radio telescopes
will provide important data needed to learn about the north-south
winds.

"Adding the ground-based telescopes to the experiment will not only
help
confirm the data we get from the Cassini orbiter but also will allow us
to get a much more complete picture of the winds on Titan," said
William
Folkner, a JPL scientist.

Another team, led by scientists from the Joint Institute for Very Long
Baseline Interferometry in Europe (JIVE), in
Dwingeloo, The Netherlands, will use a world-wide network of radio
telescopes, including the NRAO telescopes, to track the probe's
trajectory with unprecedented accuracy. They expect to measure the
probe's position within two-thirds of a mile (1 kilometer) at a
distance
of nearly 750 million miles.

"That's like being able to sit in your back yard and watch the ball in
a
ping-pong game being played on the Moon," said Leonid Gurvits of JIVE.

Both the JPL and JIVE teams will record the data collected by the radio
telescopes and process it later. In the case of the Doppler
measurements, some real-time information may be available, depending on
the strength of the signal, but the scientists on this team also plan
to
do their detailed analysis on recorded data.

The JPL team is utilizing special instrumentation from the Deep Space
Network called Radio Science Receivers. One will be loaned to the GBT
and another to the Parkes radio observatory. "This is the same
instrument that allowed us to support the challenging communications
during the landing of the Spirit and Opportunity Mars rovers as well as
the Cassini Saturn Orbit Insertion when the received radio signal was
very weak," said Sami Asmar, the JPL scientist responsible for the data
recording.

When the Galileo spacecraft's probe entered Jupiter's atmosphere in
1995, a JPL team used the NSF's Very Large Array (VLA)
radio telescope in New Mexico to directly
track the probe's signal. Adding the data from the VLA to that
experiment dramatically improved the accuracy of the wind-speed
measurements.

"The Galileo probe gave us a surprise. Contrary to some predictions, we
learned that Jupiter's winds got stronger as we went deeper into its
atmosphere. That tells us that those deeper winds are not driven
entirely by sunlight, but also by heat coming up from the planet's
core.
If we get lucky at Titan, we'll get surprises there, too," said Robert
Preston, another JPL scientist.

The Huygens probe is a spacecraft built by the European Space Agency
(ESA). In addition to the NRAO
telescopes, the JPL Doppler Wind Experiment will use the Australia
Telescope National Facility and other radio telescopes in Parkes,
Mopra,
and Ceduna, Australia; Hobart, Tasmania; Urumqi and Shanghai, China;
and
Kashima, Japan. The positional measurements are a project led by JIVE
and involving ESA, the Netherlands Foundation for Research in
Astronomy,
the University of Bonn, Helsinki University of Technology, JPL, the
Australia Telescope National Facility, the National Astronomical
Observatories of China, the Shanghai Astronomical Observatory, and the
National Institute for Communication Technologies in Kashima, Japan.

The Joint Institute for VLBI in Europe is funded by the national
research councils, national facilities and institutes of The
Netherlands
(NWO and ASTRON), the United Kingdom (PPARC), Italy (CNR), Sweden
(Onsala Space Observatory, National Facility), Spain (IGN) and Germany
(MPIfR). The European VLBI Network is a joint facility of European,
Chinese, South African and other radio astronomy institutes funded by
their national research councils. The Australia Telescope is funded by
the Commonwealth of Australia for operation as a National Facility
managed by CSIRO.

The National Radio Astronomy Observatory is a
facility of the National Science Foundation,
operated under cooperative agreement by Associated Universities, Inc.