"We said, this is something strange, and we need your telescope badly"
January 14, 2015 10:49 AM   Subscribe

January 14, 2005. The Huygens probe was falling to Titan(yt). Released after a seven year trip on Cassini, the tiny lander was mankind's first attempt to land on a moon of another world - and nobody knew what would happen next. Its signals, no more powerful than a walkie-talkie, were to be gathered by the mothership and the science relayed back to Earth. More than a light-hour away back at home planet, radio telescopes were also listening not to decode data - far too weak at that distance, even for the most powerful receivers - but to see whether they could hear Huygens at all. A job for radio engineers, not for heroes. Sometimes, though, you have to be both.

The mission had already dodged one bullet during the cruise phase, when an engineer spotted a problem with the radio link that could have lost every bit from Huygens. It wasn't until after Huygens had touched down and Cassini had started to relay its stored data that mission control realised that half the pictures and all the wind speed data from the descent had been lost - because nobody had send the command to turn on Channel A, one of Cassini's receivers. Most of the science was duplicated on Channel B, but not the data-heavy pictures, and only Huygen's Channel A transmitter was stable enough for the wind measurements.

There were still hundreds of pictures covering the entire descent. But the wind speed data, essential to understanding the structure of the alien atmosphere, was a serious scientific loss. Channel A's secondary job, of locating where on the surface Huygens had landed, was hopeless There was just one chance: if the planetary network of radio telescopes constantly spinning in and out of sight of Titan could be massively reconfigured and pushed beyond what anyone really thought possible. if terabytes of data could be rushed from remote locations. if Huygens stayed alive past its design duration. If everything held together just long enough...

The inestimable Emily Lakdawalla of the Planetary Society (Previously, previouslier... oh, just search MeFi for Lakdawalla) tells the story of how the international brotherhood of radio astronomers scrambled to pull off the impossible - and earn the nickname Channel C.
posted by Devonian (9 comments total) 23 users marked this as a favorite
The inestimable Emily Lakdawalla

Damn straight.

Great post!
posted by Songdog at 11:05 AM on January 14, 2015

I was in High School when this happened and had a really awesome teacher who took volunteers from his classes to come in after school and do some monitoring and listening and charts and graphs and geometry and all sorts of fun things. It was a truly wonderful experience with a fantastic teacher who actually wanted his students to learn and be engrossed in learning stuff, not juts rote memorization.

Anyway, we're all standing there listening to some feed for it, watching the data stream back from the craft.

There is a picture somewhere of my dad looking at the screens and data and charts. He is indistinguishable from the all the kids looking like we were opening Christmas presents.

It truly felt like were some small part of history making happening.

Thanks for this post, it brought back a lot of great memories.
posted by Twain Device at 11:09 AM on January 14, 2015 [3 favorites]

The inestimable Emily Lakdawalla

If this sentence continued with "will now explain the phone book to you," I would sit down and listen to her.

Thanks for this!
posted by rtha at 11:12 AM on January 14, 2015 [2 favorites]

I've always been fond of the descent radar audio conversion.
posted by flyingfox at 2:53 PM on January 14, 2015

I wonder why Emily claimed a 3.5-watt power for Huygens transmitters. Looking around, NASA says 10 watts output power (2.1 GHz). (Huygens had a 250W budget.) (Cassini's transmitter seems to be rated at 20W.)

Also mentioned is 20 kilobits of on-board data storage. Also interesting is the redundant radars, 'each transmitting 60 mW of power at 15.4 or 15.8 GHz'.

Also ran across this 'One Year Later' PDF at NASA, lotsa techy diagrams.
posted by Twang at 3:46 PM on January 14, 2015

Yeah, good question. The best place I've found for the Huygens-Cassini link budget says that the transmitters are 40.2 dBm nominal apiece (which is 10.5 watts), there's 0.2dB loss in the plumbing and the uplink antennas are around 5dB plus or minus polar wobbly, so that's an EIRP of 31 watts per channel. I have no idea where 3.5 watts comes from. (The most powerful walkie-talkies I know of are around 7w output, but what's 1.5dBm between friends?)

There's also this paper evaluating the link performance, if you're really keen. That posits an up-to 7dB extra loss during descent due to metallic bits of the parachute getting in the signal path...

Ho, as Marconi said, ho.
posted by Devonian at 5:20 PM on January 14, 2015

Pictures from the surface of one of Saturn's moons. If that's not mind-blowing, I don't know what is.
posted by InsertNiftyNameHere at 9:13 PM on January 14, 2015

Phil Plait: Ten Years on Titan
posted by homunculus at 2:01 PM on January 18, 2015

Fantastic post, Devonian. The radio telescope effort by itself is a good read, but the Boris Smed story deserves all the attention it can get too. I have a soft spot in my heart for the engineers who fix things before they go wrong — they never get to be the one to snatch victory from the jaws of defeat.
posted by benito.strauss at 4:50 PM on January 18, 2015

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