Huygens Titan probe BBC documentary

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Lorenzo von Matterhorn
Jan 31, 2009
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Great documentary, technically detailed (never enough for me, though), great Titan IV launch sequence.

Destination: Titan


Problem 1 mentioned in the video which was not explained in detail was caused by doppler shift, something which was worked around:

How Huygens avoided disaster
by James Oberg
Monday, January 17, 2005

Problem 2 which was not mentioned and not fixed - half of the probe's data was lost because one of the two Huygens receivers on Cassini was never commanded on. Doh!!!:

Huygens: the missing data

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.