Just over three years from now, ESA’s Huygens probe will
separate from the NASA Cassini spacecraft and plunge into the atmosphere of
Titan, the largest of Saturn’s 30 moons. Far from the tender care of
controllers on the Earth, every precaution must be taken to ensure that the
risks of failure are minimised.

One of the most important tests to be conducted with Huygens since it was
launched four years ago is about to be carried out some 10 000 km away from
ESA’s Huygens Centre in Germany.

The purpose of this test is to validate the revised Cassini-Huygens mission
plan, in particular the new parameters of the telecommunications link
between the Probe and the Orbiter.

“We want to ensure that the all-important communication link between Huygens
and Cassini will not break down in three years’ time, during the Probe’s
two-hour descent towards Titan’s unseen, icy wilderness,” said Huygens
project scientist Jean-Pierre Lebreton.

“This means that we must validate the revised mission scenario and test its
robustness with regard to small variations in the communications link that
we have not been able to model accurately on the ground,” he explained.

“We need to ascertain whether the modified mission will allow the Huygens
radio receivers on board Cassini to operate within the narrow range of
frequencies available to us,” he said. “If the signal drifts outside this
range, it will cause interruptions in communications.”

“Such a communications breakdown would be catastrophic, since we would lose
the unique pictures and other scientific data from this once-in-a-lifetime
mission,” said Lebreton. “So this week and next week we are carrying out a
dress rehearsal to make sure that the signals from Huygens will be
successfully picked up by Cassini and then relayed back home.”

These tests, which are scheduled for 16-17 and 19-20 November, involve joint
ESA-NASA teams based at NASA’s Goldstone facility in California, at the
European Space Operations Centre (ESOC) in Darmstadt, Germany, and at the
Cassini Mission Operations Centre in the Jet Propulsion Laboratory,
Pasadena, California.

Signals will be beamed to the Huygens receiver on board Cassini, using
equipment that has been specially developed at ESOC. If all goes well, the
Probe’s computers should format the decoded data and send it in packets to
computers on Cassini for retransmission back to Earth. The data will then
undergo detailed analysis in Darmstadt.

However, this is not as easy as it sounds. The test signals sent from
Goldstone must attempt to duplicate the signals (and the information) that
will be transmitted from Huygens to Cassini during the Probe’s descent
through Titan’s atmosphere.

Since Cassini-Huygens is currently heading away from the Earth at high speed
(about 20 km/s), the signal sent from Goldstone will have to be very
carefully modified if it is to simulate the signal that will be transmitted
during Huygens’ prolonged parachute drop.

In particular, engineers must attempt to recreate the very small shift in
the frequency of the Probe’s signals that will be caused by the continuously
changing positions of the two spacecraft. This so-called Doppler shift is
rather like the change in the sound from a siren that a pedestrian notices
when a police car approaches, passes by and moves into the distance.

“To allow for this Doppler effect during our test, we will have to adjust
slightly the signal that we transmit from Earth,” explained Dr. Lebreton.
“This means that, when our signal arrives at the Orbiter, it will have the
same frequency as the one that Cassini will receive from the Probe during
its actual descent.”

“This will be a very difficult, delicate operation,” he added. “The
performance of the Probe’s transmitter, which we cannot test in flight, may
be slightly different from what we anticipate, so we need to simulate all of
the possible variables.”

“The test will also simulate the additional drift in signal frequency that
occurs when we preheat the Probe to warm up its radio equipment,” said
Lebreton. “We found that this warming will help us to adjust the clock that
controls the frequency of Huygens’ transmission.”

“However, we need to verify that several hours of preheating will improve
our ability to receive the exciting data that will be sent back during the
descent and touchdown,” he said.

For further information please contact:

Dr Jean-Pierre Lebreton

Huygens project scientist

ESTEC, The Netherlands

Tel: +31 71 5653600

Email: Jean-Pierre.Lebreton@esa.int