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.

Additional Media Contacts:

Dr. Leonid I. Gurvits, JIVE
Phone: +31 (0)521 596514
lgurvits@jive.nl

Dr Chris Phillips, CSIRO Australia Telescope National Facility
Office: +61-(0)2-9372-4608
Mobile: +61-(0)439-487-601
Chris.Phillips@csiro.au