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Old 11-05-2009, 12:07 PM   #1
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Default Saturn and the Cassini mission

So I was working with one of the scientists here yesterday (at work) assembling a vacuum chamber I designed for a new ice deposition experiment. At one point in the afternoon, he says he has to leave for a meeting to talk about the Cassini mission; We have some instrumentation on the spacecraft.

NASA - Cassini

He says the meeting was to make a decision on how they are going to alter the trajectory of the spacecraft to fly through this huge plume of gas erupting from Enceladus, one of Saturn's moons, and that they had to make it soon, since the spacecraft was about to fly through the plume. So I am thinking, OK that is pretty cool, and we talk briefly about orbital trajectories.

Then I come in this morning and see this...
Successful Flight Through Enceladus Plume

Awesome...
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Old 11-05-2009, 12:10 PM   #2
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Really dude...you are my new role model.

I used to love this kind of thing as a kid...and it's neat to see a career engineer living it.

I need to hurry up and graduate and excel in my career.

Thanks for sharing. Will make studying my **** off a little easier for the rest of the day.
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Old 11-05-2009, 12:18 PM   #3
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haha. wow that is pretty awesome. I'm surprised they could change the path so quickly. Doesn't it take half a day to just send a message there?
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Old 11-05-2009, 12:50 PM   #4
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I'm doing a presentation for astronomy class on this very thing. Been reading about it since the mission began.
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Old 11-05-2009, 02:37 PM   #5
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Yes, study hard, get through school, and you too can do cool Engineering stuff. Seriously, I know that sounds a little gay. But getting my Bachelor's is the hardest thing I have ever done. However it was completely worth it and then some looking back on it now. I love my job, and work with some great people, doing really difficult, but interesting work.

OK to answer the message time question, or the time it takes to get a message to Saturn from Earth:
Mean distance from Sun to Saturn - 1430M km
Mean distance from Sun to Earth - 152M km (we are in MUCH closer)
Closest possible distance from Earth to Saturn = 1430 - 152 = 1278M km
Farthest possible distance from Earth to Saturn = 1430 + 152 = 1582M km
Speed of light = 299,792 km/sec
Therefore:
Fastest possible message time = 4262 seconds = 1.18 hours
Slowest possible message time = 5276 seconds = 1.46 hours

Check me on my math here but I think that is right. The message travel (between fastest and slowest) depends on where Earth and Saturn are in their orbits when the message is sent. If they are in alignment (lined up with the Sun on the same side of the Sun) the message travel time is the shortest. If they are on opposite sides of the solar system, with the Sun between, then the message time is the longest.

But usually no one does real-time feedback flight control over a data link with that kind of latency. Plus there is a chance the connection could be lost at a critical moment. What they do is check current position (star tracker data, whatever) which is monitored constantly anyway, then send a command to the spacecraft telling it how to change its orbit, or in other words, go here. Then the spacecraft onboard control system takes care of the real-time control with onboard, autonomous, closed-loop feedback control, like rate gyros and star trackers.

I work with another Engineer that was deeply involved in the Deep Impact mission. This is the big comet impact mission that was in the news. We had some avionics (computers) on that spacecraft. He told me about the moments of terror he and the team experienced as they stood in Mission Control and watched the video feed from the spacecraft while it made its own final course corrections right before impact. There was nothing they could do but watch and hope. Of course it all went perfectly, and the rest is history.

Last edited by ZX-Tex; 11-05-2009 at 03:23 PM. Reason: Typo
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Old 11-05-2009, 03:28 PM   #6
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Quote:
Originally Posted by NA6C-Guy View Post
I'm doing a presentation for astronomy class on this very thing. Been reading about it since the mission began.
That is really cool BTW. I know a little about the science side of the Mission. You may know more than I do. On the Engineering side, an interesting tidbit; Cassini is the Mission that carried its own power in the form of a nuclear power source (http://en.wikipedia.org/wiki/Radiois...tric_generator). The reason being that Saturn is sooooooo far out from the Sun, that it would have required massive, impractical solar arrays to meet the power requirements. Inverse square law. If I have it right, Cassini would have required arrays that are 100 times bigger than what you would need here at Earth, all else being equal. Mass is everything.

That was in the news too. Everyone was freaking out, mostly sensationalist media types, about launching 'a nuke' into space.

Anyway, I suppose that is enough Aerospace for today. Thanks for letting me geek out
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Old 11-05-2009, 06:38 PM   #7
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Quote:
Originally Posted by ZX-Tex View Post
That is really cool BTW. I know a little about the science side of the Mission. You may know more than I do. On the Engineering side, an interesting tidbit; Cassini is the Mission that carried its own power in the form of a nuclear power source (Radioisotope thermoelectric generator - Wikipedia, the free encyclopedia). The reason being that Saturn is sooooooo far out from the Sun, that it would have required massive, impractical solar arrays to meet the power requirements. Inverse square law. If I have it right, Cassini would have required arrays that are 100 times bigger than what you would need here at Earth, all else being equal. Mass is everything.

That was in the news too. Everyone was freaking out, mostly sensationalist media types, about launching 'a nuke' into space.

Anyway, I suppose that is enough Aerospace for today. Thanks for letting me geek out
Thanks, I will plug that tidbit into my presentation for extra cool/geek points.
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Old 11-05-2009, 11:39 PM   #8
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Interesting, I just went through a lecture that included Enceladus. Astronomy is the ****.
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Old 11-05-2009, 11:52 PM   #9
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Wow, good job! Astronomy fascinates me, but I'm nowhere near smart enough to work in that field
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Old 11-06-2009, 12:11 AM   #10
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Quote:
Originally Posted by NA6C-Guy View Post
Thanks, I will plug that tidbit into my presentation for extra cool/geek points.
"Yeah, I was just talking to an engineer that works on this project. We're tight. He told me that..."

Serious geek points.
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Old 11-06-2009, 12:14 AM   #11
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Quote:
Originally Posted by Stein View Post
"Yeah, I was just talking to an engineer that works on this project. We're tight. He told me that..."

Serious geek points.
Yeah, I've know him since birth He didn't even need tell me, we are so close I can finish his sentences after the first word. I dunno, that may be taking the exaggeration to the extreme.
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Old 11-06-2009, 03:04 AM   #12
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LOL. Yeah no worries. Drop by my office and I'll lend you one of our 6-figure prototypes for show and tell. If they try to stop you at the front gate, just keep going.
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Old 11-06-2009, 03:20 AM   #13
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Originally Posted by ZX-Tex View Post
LOL. Yeah no worries. Drop by my office and I'll lend you one of our 6-figure prototypes for show and tell. If they try to stop you at the front gate, just keep going.
How early do you get there? I could leave in about an hour and be there by 9AM. I promise I won't damage it.
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Old 11-06-2009, 11:32 PM   #14
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Bring a deposit in the form of a ubiquitous commodity. Diamonds are more portable than cash.
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Old 11-07-2009, 02:16 AM   #15
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Originally Posted by ZX-Tex View Post
TOn the Engineering side, an interesting tidbit; Cassini is the Mission that carried its own power in the form of a nuclear power source
I thought that RTGs had been in use for quite some time on deep-space probes, going all the way back to the Voyager era.

(Pause for Wiki)

Older than that, actually. First one went up on Apollo 12, to run the Lunar Experiments Package.

Why all the fuss suddenly?
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Old 11-07-2009, 02:28 AM   #16
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Its the media... they want ratings...
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Old 11-07-2009, 02:36 AM   #17
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Actually, Apollos 12-17 all carried one. And the unit which was aboard Apollo 13 obviously re-entered the atmosphere along with the rest of the LM package. Oh, the horror! Nuclear holocaust raining down from the sky!

There's a part of me that wants to start a new brand of beer. It would just be plain ole' Budweiser, repackaged in a fancy bottle which promotes such facts as that it is gluten-free and contains no trans-fats.
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Old 11-07-2009, 07:50 AM   #18
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Why all the fuss suddenly?
Exactly...

Last year I was (am) the Spacecraft Structural, Thermal, Mechanical (SMT) Lead Engineer on development of a scientific observatory during the Concept Development phase. This was a low earth orbit (LEO) mission that used solar arrays for a power source, so no RTG needed. But like all other missions, we had to do an orbital debris analysis as required by NASA. It is pretty extensive.

I and the other leads on the team had to provide a lot of information about the spacecraft design to the Engineer doing the analysis. This was fed into a standardized NASA model that helped predict what would happen when the satellite inevitably re-entered the atmosphere, how it would break up, and what the little pieces would look like when they fell back to Earth, if any survived reentry that is. The resulting report went to NASA for analysis.

The report was not trivial; it took a lot of work to get it done properly. And we did not have any hazardous materials onboard, not even a propulsion system (once separated from the launch vehicle), not even beryllium; beryllium alloy is a fantastic metal by the way, just hazardous if not handled correctly. Point being, a lot of thought goes into this sort of thing, driven by analysis and empirical data from past missions. Plus, in the linked wiki article, you can see what happened with the Apollo 13 RTG in the safety section. In general, NASA and the community pay close attention to what works and what does not. I'll bet there are a lot of Engineers who would really, really like to get their hands on the Apollo 13 RTG to do a post-mortem analysis, looking for how well it did.

We (my division) did not develop the spacecraft for the Cassini; we did the scientific instruments. So I do not know about nor can I comment on the details of its RTG.

Last edited by ZX-Tex; 11-07-2009 at 08:10 AM.
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Old 11-08-2009, 01:44 AM   #19
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I'll bet there are a lot of Engineers who would really, really like to get their hands on the Apollo 13 RTG to do a post-mortem analysis, looking for how well it did.
An interesting point. I wonder why they've never done an actual test. Substitute an inert metal of similar mass for the regular fuel, take it up into orbit either on the shuttle or as a piggyback payload on another satellite mission, and then throw it at the state of Nevada.

Well, OK, I do know the answer to that. Money. Still, it'd be a neat test.

Quote:
We (my division) did not develop the spacecraft for the Cassini; we did the scientific instruments. So I do not know about nor can I comment on the details of its RTG.
Here's the unit that went up on Cassini. Same unit that was used on New Horizons, Galileo, and Ulysses: GPHS-RTG - Wikipedia, the free encyclopedia

http://saturn.jpl.nasa.gov/spacecraft/safety/eisss2.pdf
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