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Neptune Orbiter, Another proposed mission
edstrick
post Nov 28 2005, 06:06 AM
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In a conversation some 10? years ago, maybe at the MagicCon World SF convention in Orlando, I chatted with Gentry Lee and asked him something about the reverse motors on the Galileo antenna, having heard some references to that. *AS I RECALL*, he indicated they had needed that command capability for something else, maybe sun-shade deployment for revised mission that took Galileo inward to Venus where it wasn't designed to survive. He also stated that simple reversing the motor direction would not have backed the pins out of the position in which they'd jammed and would have in fact made the problem worse. I don't understand the design well enough to see how that would happen, but I can imagine design conditions where it would. Everything I'm told is that the failure was more complicated and less intuitively preventible (or fixable -- "if only") than everybody wants to imagine.

The Galileo antenna deploy failure was simply one of those "damn things" that should never force you to not use a proven design or concept because of one terrible example.
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mchan
post Nov 29 2005, 07:51 AM
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QUOTE (BruceMoomaw @ Nov 27 2005, 05:32 AM)
Just keep in mind that Galileo's antenna was exactly the same design used on the TDRS satellites, each of which carried four of them.  Out of (I believe) a total of 28 on them, not one has ever shown any trouble unfolding -- which is why the Galileo failure caught virtually everyone by shock.  No one had ever anticipated truck vibrations as a cause.  Now they do.  And so, while I distrust moving parts in space as much as anyone, I see no reason to flee screaming from the idea of an unfolding antenna.  It only requires making sure that the deployment springs have enough of a margin this time.
*


Galileo's folding HGA design was used on the first series of TDRS satellites, each of which carried two antennas. There were 7 satellites built and launched, but one was destroyed with Challenger. The last 3 satellites launched after the Galileo deployment (or non-deployment as it was). An antenna of very similar design was also used on the US Navy FleetSatCom satellites, of which 6 were successfully launched and deployed before Galileo. A total of 12 unfurlings, all successful, occurred prior to Galileo.

Furled dish antennas of newer designs continue to be used in commercial comsats. NASA certainly has not backed away from furled structures. One need not look further than the James Webb Space Telescope.
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edstrick
post Nov 29 2005, 08:44 AM
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Deployments have ALWAYS been one of the big mission-killers. Without the ability to actually test fly a spacecraft in orbit before boosting it to it's destination, there's no way to do a really adequate pre-flight test of in-space deployment. They've gotten DAMN good at faking things in 1 G and 1 Atmosphere, but the ghods of space still have nasty tricks up their sleeves.
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tasp
post Dec 5 2005, 05:23 AM
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Just in case NASA/JPL finds themselves looking for a 'flagship' type mission idea (I hold no illusions regarding the probability of that), how about this:

Send a probe to Neptune that can aerobrake in Neptunes' atmosphere to acheive Neptune orbit. During the decel phase, have some instruments do some direct sampling and analysis of the Neptune atmosphere.

After achieving Neptune orbit, use Triton's gravity to modify the probe orbit to allow close examination of everything in the Neptune system deemed interesting.

Towards the end of the mission, use aerobraking in Triton's atmosphere to decel into Triton orbit (probably can't use the same heat shield for both Neptune and Triton, but some effort would be expended to make sure of that, handy if you can). Examine Triton from orbit, use more aerobraking to circularize the orbit (or, if possible, use a steerable ballute for orbital plane changes) to examine interesting landing locals on Triton.

Then, again using the ballute or aerocapture shell for re-entry, land on the best spot found on Triton. Hopefully with almost dry fuel tanks at touch down.



I'm thinking the advantages of this will be a great savings in fuel needed for a very demanding mission, utilization of various instruments in all phases of the mission, difficulty in approving 3 missions to Neptune can be avoided by having 1 mission do all 3, probe will potentially have a long life on the surface of Triton from the nuclear batteries needed for the probe.

Difficulties would be great complexity of the probe, cost and technical risk.
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dvandorn
post Dec 5 2005, 07:35 AM
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Does Triton *really* have enough of an atmosphere to allow for efficient aerobraking? At least, without a gazillion passes before you're significantly slowed down?

-the other Doug


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“The trouble ain't that there is too many fools, but that the lightning ain't distributed right.” -Mark Twain
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Toma B
post Dec 5 2005, 08:34 AM
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QUOTE (dvandorn @ Dec 5 2005, 10:35 AM)
Does Triton *really* have enough of an atmosphere to allow for efficient aerobraking?  At least, without a gazillion passes before you're significantly slowed down?

-the other Doug
*


NO IT DOES NOT!
The atmospheric pressure at Triton's surface is about 15 microbars , 0.000015 times the sea-level surface pressure on Earth...that's not enough for any kind of aerobraking...
Mars has average presure of 7 milibars, that is why MRO will have to spend many months aerobraking and don't forget that it uses only upper parts of atmosphere where presure is much less than that...
The average pressure on the Earth surface (sea level) is 1000 millibars....
In order to use that little atmosphere on Triton for aerobraking our unlucky spaceprobe would have to fly verry,verry,verry dangerously close to surface...
Edit:
Some approximate calculatins based on these facts:
Mars atmosphere is 143 times less efficient in aerobraking then atmosphere of Earth is...
Triton atmosphere is 467 times worst then Mars... sad.gif
So it would take gazillon passes through it to slow down significantly... sad.gif
Solution : USE ROCKET ENGINES!!! smile.gif


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The scientist does not study nature because it is useful; he studies it because he delights in it, and he delights in it because it is beautiful.
Jules H. Poincare

My "Astrophotos" gallery on flickr...
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chris
post Dec 5 2005, 10:49 AM
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QUOTE (Toma B @ Dec 5 2005, 08:34 AM)
Solution : USE ROCKET ENGINES!!! smile.gif
*


Which could get exciting if you landed on a patch of methane or nitrogen ice.
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edstrick
post Dec 5 2005, 11:07 AM
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Note that the surface of Triton may be very rough on spacecraft scales, in some places.

When a moon's been cratered "till the rubble bounces", the surface's largely pulverized and while large scale topography is rough, churned regolity or ice-rubble is locally smooth. A moon like Enceladus or Europa, with active geology, can be very very rough, with sharp local topography and lots of broken ice.

Add glacial activity possible in nitrogen and/or methane ices, and sublimation eroding processes, you may get a complicated and very rough surface indeed.
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Toma B
post Dec 5 2005, 11:42 AM
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QUOTE (chris @ Dec 5 2005, 01:49 PM)
Which could get exciting if you landed on a patch of methane or nitrogen ice.
*


What did you meen by "exiting" ?
What's exiting about landing on a patch of nitrogen or methane ice?

Edit:
Also what does it have to do with rocket engines?


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My "Astrophotos" gallery on flickr...
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helvick
post Dec 5 2005, 12:02 PM
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QUOTE (Toma B @ Dec 5 2005, 09:34 AM)
Mars atmosphere is 143 times less efficient in aerobraking then atmosphere of Earth is...
*


Not really. Despite having only a few millibars surface pressure Mars' atmosphere is denser at high altitudes than the earth's and extends further so it is not any worse than the earth's for orbital aerobraking. The cross over point is at around an altitude of around 70km if I recall correctly and orbiter aerobraking takes place much further out than that. The problems arise for landing craft as the lower density near surface atmosphere leads to much higher terminal velocities. Again I'm just going by recollection here but I think martian terminal velocities are about 5-10x Earth ones so from a lander aerobraking perspective I reckon it would be more accurate to say that Mars was about 10 times less efficient.

Triton's atmosphere does seem to be just too thin to be of much use but it would be worth running some calculations on it to be certain. More on this later when I can find my drag calculations spreadsheet.
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chris
post Dec 5 2005, 12:52 PM
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QUOTE (Toma B @ Dec 5 2005, 11:42 AM)
What did you meen by "exiting" ?
What's exiting about landing on a patch of nitrogen or methane ice?

Edit:
Also what does it have to do with rocket engines?
*


If you land on a patch of such ice, and the rocket exhaust is hot enough to vaporise the ice, then you might get an explosive release of gas, which would be dangerous to the lander. The surface of Triton, at -235 Centrigrade, is only 20 degress lower than the freezing point for nitrogen, so it wouldn't be that hard.

Chris
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Toma B
post Dec 5 2005, 01:27 PM
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QUOTE (chris @ Dec 5 2005, 03:52 PM)
If you land on a patch of such ice, and the rocket exhaust is hot enough to vaporise the ice, then you might get an explosive release of gas, which would be dangerous to the lander. The surface of Triton, at -235 Centrigrade, is only 20 degress lower than the freezing point for nitrogen, so it wouldn't be that hard.
*

Question:
Will methane explode without oxygen?
NO IT WILL NOT !!!
...there's not much oxygen on Triton isn't it?
About Nitrogen...
How long would rocket engine work when verry close to surface...2-3 seconds perhaps?
Without any atmospheric presure there might be some sublimation but I'm sure nothing exiting....

Here's a deal:
Why don't we make a bet and we will see when NASA send that probe in the next 20 to 150 years... smile.gif


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The scientist does not study nature because it is useful; he studies it because he delights in it, and he delights in it because it is beautiful.
Jules H. Poincare

My "Astrophotos" gallery on flickr...
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paxdan
post Dec 5 2005, 01:47 PM
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QUOTE (Toma B @ Dec 5 2005, 01:27 PM)
Question:
Will methane explode without oxygen?
NO IT WILL NOT !!!
*

Err yes it will, the mechanism is descibed in the post you quoted. Explosive does not just refer to combustion. Careful with the caplocks key and the exclamation marks when making such bold statements.
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ljk4-1
post Dec 5 2005, 02:57 PM
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Drop a probe right into one of the Triton geysers. That way we get a quick and relatively easy access to the moon's subsurface.

I presume we can make a probe tough enough to survive such a journey long enough to relay back useful data?


--------------------
"After having some business dealings with men, I am occasionally chagrined,
and feel as if I had done some wrong, and it is hard to forget the ugly circumstance.
I see that such intercourse long continued would make one thoroughly prosaic, hard,
and coarse. But the longest intercourse with Nature, though in her rudest moods, does
not thus harden and make coarse. A hard, sensible man whom we liken to a rock is
indeed much harder than a rock. From hard, coarse, insensible men with whom I have
no sympathy, I go to commune with the rocks, whose hearts are comparatively soft."

- Henry David Thoreau, November 15, 1853

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tasp
post Dec 5 2005, 03:05 PM
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Check out the last half of the Uranus orbiter thread. Bruce Moomaw has posted reference to a paper outlining use of ballutes in thin atmospheres.

Seems very exciting missions involving Trtion and Pluto are more feasible than almost anyone dared hope for. Decels of up to 40 to 50 gees are possible in the thin upper atmospheres of these objects for orbit mods and landing missions.

Stunning information, really. {love this site for that very reason}

cool.gif
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