Question About the Huygens Probe

A

Aladdin

Registered Senior Member
I recently read a bit about the Cassini-Huygens mission of Saturn exploration and was a bit surprised by the fact that the Huygens probe, which successfully landed on Titan (a saturnian moon), was only supposed to be „alive” (i.e. transmit data to the orbiter Cassini) for a maximum of 30 minutes on the titanian soil (source). Contrasting this with the almost seven years it took the spacecraft to reach Saturn those 30 minutes look to me like an incredibly short time interval.

Does anyone know more about why the scientists behind this mission decided on such a short life for the Huygens probe?
 
The Huygens probe had about three hours of battery life. Much of that would be used during descent. Bear in mind that the probe would be transmitting during descent so the 30 minutes you refer to only relates to the expected time taking data on the surface after landing. I believe it actually transmitted for about an hour and a half.

That may not seem like much time, but that depends on what you get for that time.

So why batteries? Solar cells would have been pretty much useless obviously. This left only a nuclear power source as being viable. Huygens designers were not strongly familiar with nuclear power sources. The US is building a probe for Titan that is nuclear power sourced which is planned to drift on Titan for years- I'll have to look up what that is... can't remember right now.
So three hours for batteries- this is because of the added weight batteries brings to the probe. To limit weight included to limit batteries and that means limiting battery life. It was a trade off, all the way around.
 
Yes, the batteries used by Huygens were designed to last for about two-and-a-half to three hours, with most of this time spent in descent through the Titanian atmosphere. And I believe they actually outperformed a bit powering the instruments for something like four hours overall. But even so, to spend almost seven years in a journey and then only four hours collecting data seems utterly unbalanced, doesn't it?

As for the power source -- Cassini (the probe's transporter) is equipped with plutonium-based radioactive generators, so I guess the designers could have used the same for Huygens, too, which should have provided a lot more power than those four hours, I suppose. There must have been some reasons why Huygens was designed to have such a short active life. (Maybe... the landing itself had low expectations of being successful, hence the focus on the collection of data while in descent?)
 
One reason not to use radioactive power generators on landing probes is that if there is a failure of one sort or another you end up contaminating the place you are trying to study. A heck of a lot of effort goes into sterilising the probe to make sure no bacteria or other microbe is transferred from Earth, as at the very least that could give you a false positive life reading.
 
Aladdin said:
Yes, the batteries used by Huygens were designed to last for about two-and-a-half to three hours, with most of this time spent in descent through the Titanian atmosphere. And I believe they actually outperformed a bit powering the instruments for something like four hours overall. But even so, to spend almost seven years in a journey and then only four hours collecting data seems utterly unbalanced, doesn't it?

As for the power source -- Cassini (the probe's transporter) is equipped with plutonium-based radioactive generators, so I guess the designers could have used the same for Huygens, too, which should have provided a lot more power than those four hours, I suppose. There must have been some reasons why Huygens was designed to have such a short active life. (Maybe... the landing itself had low expectations of being successful, hence the focus on the collection of data while in descent?)
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Cassini and Huygens were not designed by the same team. Some of what you speculated here may be some of those reasons, as well. And Prometheus pointed out agreement on landing probes- contamination.

But more interesting to me is your comment about how unbalanced it seems. It's what you get out of it that matters most. For example, the fuel used to facilitate Huygens landing: Does it seem silly for that fuel to get packed in there, transported up to space on a huge rocket, then fly millions of miles, just to burn it up in a matter of seconds descending to the moon? I mean, seconds seems silly next to the travel time, money and effort to get that fuel there...

This may not be the best comparison, but I'm hoping it kind of makes you think...
 
Neverfly said:
But more interesting to me is your comment about how unbalanced it seems. It's what you get out of it that matters most. (...)
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I understand what you're saying and, in principle, would agree with you. But :)...

As a purely hypothetical exercise, picture yourself as the head of the team designing the first trip to explore Alpha Centauri, and let's also assume that space travel technology is capable of something like 0.1c average over long distances, so that the one-way trip there takes something on the order of 40-50 years. Now, would you consider sitting down and discussing a probe that's only going to be active for about a day or two once it gets there? Guess you would if that's the only option on the table (i.e. take it or leave it kind of thing), but I'm also guessing that first you'll take a long hard look into options that could possibly give you a much longer active live for such an hypothetical probe. Am I wrong?

Just out of curiosity I looked up the weights of the Cassini orbiter (5,712kg) and Huygens probe (320kg); together they are a bit over 6,000kg with Huygens taking about 5% of that. Well, Cassini is still in orbit more than eight years after reaching Saturn's system, while Huygens has only had a few hours of fame.
 
That's just it, at the time that was one of the options. It's not like any country is sinking a lot of money into space exploration, so the few projects that get funded and sent have to do the best they can. There's another reason to not put a lot into such a probe though...what if you invested a bunch of money, time, and resources to get a very elaborate probe there, and after all that time during the landing (or before even) something goes wrong that wasn't known about. If Huygens had failed during entry, it would have been a loss, but even through that loss we would learn a bit more for future missions. Better to start small and understand your target. And don't put eggs in a single basket.
 
What I don't understand is why cassini released hughens so early in it's mission. Huyghens landed abouth 5 months afther the cassini craft arrived at saturn and at that time it was a complete mystery on how the surface would be. If they would have waited a year(2006) they would have discovered the lakes if they waited until 2012 they would have discovered the equatorial lakes that could have been aimed for and I'm sure that between now and the end of the mission their will be a change for a equatorial lake descend days or weeks afther it had rainend in stead of this apparent blind landing.
 
That's an interesting question, too, orcot. Who knows, maybe Carolyn Porco (one of the lead scientist involved with this mission) will address it in her forthcoming book...?

In the meantime National Geographic has just published a news piece on Cassini with some amazing photos. From the article:
„After completing its extended mission in 2017, Cassini will likely be commanded to plunge into the gas-giant planet to prevent contamination of its moons, including Titan and Enceladus.”
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Prevent contamination? So... are there some bacterium left on the orbiter? If so, I wonder what are the chances of it already being spread to Titan via Huygens delivery.
 
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