Huygens' mission to Titan has worked almost perfectly.© ESA/NASA/University of Arizona

The Huygens probe's mission to Titan has been universally hailed as a grand success. It has delivered stunning images of the moon's landscape and scientists are now poring over measurements of Titan's weather and soil. And yet it didn't all go exactly to plan. One aspect of the mission that hasn't been trumpeted quite so loudly is the fact that not all of Huygens' data actually arrived on Earth. So what happened to the rest of them? News@nature.com investigates.

Which data went missing?

Scientists on Huygens' imaging team only got half of the pictures they had hoped for during the descent. They expected to have more than 700 images from the 2.5-hour flight, and only got about 350. Data from the Doppler-shift experiment, which measured subtle changes in the wind speeds that Huygens experienced, was also lost.

What happened exactly?

As it fell towards Titan, Huygens transmitted a continuous stream of information to the Cassini mother ship passing overhead, which collected all the data before turning towards Earth to send it to the waiting scientists. The Huygens transmissions were sent on two channels that used slightly different microwave frequencies.

Most of the probe's data were duplicated on each channel, like two different radio stations broadcasting the same programme. "The information is so important you carry it twice, it's a redundant system," explains David Southwood, director of science for the European Space Agency (ESA), which built and operated the probe.

That redundancy saved the mission from failure. Cassini had two different receivers to collect the data from Huygens, and one of them did not work.

Why didn't the receiver work?

The Channel A receiver was simply not turned on during the mission. "There's no mystery why it didn't turn on," says one scientist on the imaging team, who was upset by the loss. "The command was never sent to switch it on."

Whose fault was it?

"That's an ESA responsibility," admits Southwood. Any instructions that need to be sent to the Cassini spacecraft are compiled as a series of software commands by mission scientists, and these are transmitted to the craft from the Jet Propulsion Laboratory in Pasadena, California. All commands relating to the Huygens probe were programmed by ESA.

Southwood says it isn't important who omitted the crucial instruction, because the responsibility runs wider than that: the error should have been picked up during checks. ESA is now mounting an investigation into why the mistake was not spotted. "I'm extremely anxious to learn lessons from this," says Southwood.

Why were the pictures not duplicated like the rest of the data?

The imaging team, based at the University of Arizona, Tucson, sacrificed the redundancy of the system in an attempt to get as much data as possible from Titan. Instead of sending the same pictures twice they interleaved them across both channels, with successive images being sent to alternating receivers on Cassini. If it had worked, they would have got 700 images.

"They chose to fly without a parachute," says Southwood. "But as a scientist, I would have done exactly the same." And because the pictures were alternated between the two channels, the scientists still have a continuous pictorial record of the descent.

What about the wind speed measurements?

The Doppler Wind Experiment was designed to calculate the direction and strength of Titan's winds, to find out more about the moon's weather and to precisely plot Huygens' trajectory and landing point.

As the probe fell away from Cassini, its microwave transmissions were 'stretched' slightly - Cassini would have picked up a lower frequency than Huygens actually sent. This tiny Doppler effect, similar to the changing tone of a police siren as it passes on the street, can be related directly to the probe's motion.

But the effect is so small that it can only be discerned if both transmitter and receiver operate at an extremely regular frequency. To achieve this stability, one of the two data transmitters was fitted with an ultra-stable oscillator (USO).

Weight constraints prevented both transmitters from being fitted with a USO. But this made the experiment dependent on a single channel - the channel that Cassini failed to pick up.

Fortunately, a network of radio telescopes on Earth picked up the signal from the ultra-stable transmitter directly. This should allow scientists to reconstruct the craft's flight path within the next two weeks, "although it will require a huge amount of number crunching," says Southwood. 

Member of the Huygens imaging team

Director of Science, ESA

Director of Science, ESA