An artist's concept of Huygens dropping toward the moon Titan. Photo: ESA

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 November 16-17 and 19-20, 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.