Posted on Sunday 7 November 2004

I have this vision of two alien astronauts in a spaceship looking down through our planet's thick atmosphere and saying to one another "Ewww...it must be really mushy down there, I sure hope we land on something hard."

Currently, Cassini is in the process of being positioned for the most exciting part of its mission, the detachment of the tiny Huygens space probe which will descend into the Titan atmosphere. Judging from the information that has already been garnered about Titan's geologically active surface, this is likely to be very interesting indeed.

According to this 1997 press release from the European Space Agency, the space-probe Huygens was scheduled to detach from Cassini today (6th November, 2004) and start its slow decent into Titan's murky atmosphere. But if you consult the current schedule you'll note that this detachment is actually not planned to happen until the 24th of December.

Why the change in plans? Well, in the words of one John Zarnecki of Britain’s Open University, "We have a technical term for what went wrong here..."

"It’s called a cock-up."

The Cassini spacecraft has a flaw in its basic design which means that if the Huygens probe had been launched as originally planned, the signal sent back would have been hopelessly scrambled and the centrepiece of an otherwise wildly successful $3.2 billion mission would have ended as an embarrassing failure.

The flaw, which was picked up well after the craft had been launched, is in the radio receiver that will pick up signals from Cassini. It was a piece of equipment provided by a contractor which had not been designed to correctly deal with the high velocity differentials that will be experienced between Cassini and Huygens.

If the original plan had been followed then the Huygens probe would have detached from Cassini and at the time of descent there would have been a differential velocity of around 5.5 km/sec between the two craft. This would have created a Doppler shift in the radio signal which would have raised Huygens' radio frequency by 38 KHz. The radio receiver on Cassini was certainly designed to cope with this but in an amazing oversight it wasn't designed to expect an increase in the bit rate of the data coming from the probe which would also be a consequence of the Doppler shift. This would have meant that all the timing would have been out and the sampling frequency of Cassini would have chopped up the data wrongly rendering most of it as garbage.

Unfortunately, the programs that run this part of the process are stored in firmware and can't be reprogrammed in-flight so the only way to save the mission was to change the flight path itself so that Cassini could track Huygens without this differential velocity getting in the way. This was not to be a trivial undertaking, Cassini was already on its way and redesigning its multi-year mission from scratch was out of the question. A compromise solution was arrived at where Cassini would orbit Saturn three times instead of the planned two times. Luckily Cassini has ample fuel on board with which to complete this additional manoeuvre.

For more detail, be sure to read the whole ripping yarn over at the IEEE site. It's a story about the obstinacy one engineer, how he found the bug and how the bureaucracy was forced reluctantly to sit up and pay attention to it.

Finally, enormous kudos really needs to go to all those technicians that worked on the problem and managed to fix it. I'm very much looking forward to what Huygens sends back. Fingers crossed.