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#880
by
Jim
on 03 Dec, 2011 20:07
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Does the If you're firing blindly to slow it down, you still have no idea where it'll come down,
Strongly disagree. The Pacific is vast, if you start firing at certain locations you are pretty damn sure it would still come down in the Pacific regardless how long the engine fires( assuming minimum delta-v of 80 m/s). But like always we are wasting words, no firing will ever occur, I could bet my house on it, this whole discussion is just about as constructive and academic as discussing when an asteroid will hit the Earth. I think more fun is to start taking bets when it will actually come down and when.
You are wrong and Robotbeat is right. It is blind. solar fixed attitude is useless for deorbit. Deorbit orientation is -velocity vector and solar fixed is seldom that
Length of burn determines delta V.
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#881
by
alk3997
on 04 Dec, 2011 00:38
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Does the If you're firing blindly to slow it down, you still have no idea where it'll come down,
Strongly disagree. The Pacific is vast, if you start firing at certain locations you are pretty damn sure it would still come down in the Pacific regardless how long the engine fires( assuming minimum delta-v of 80 m/s). But like always we are wasting words, no firing will ever occur, I could bet my house on it, this whole discussion is just about as constructive and academic as discussing when an asteroid will hit the Earth. I think more fun is to start taking bets when it will actually come down and when.
Not sure what you are betting your house on, but if it is all space objects re-enter in the Pacific, I'll take that action.
Here's the way it works...If you are in a low Earth orbit, you are traveling about 17,500 miles per hour. The apogee and perigee of the orbit are above most of the atmosphere, which is why you don't slow down and stay on-orbit. To deorbit, some part of the orbit ends up within the atmosphere. To target a particular spot, the orbit targeting (important word) puts the spacecraft's perigee far into the atmosphere then atmospheric drag does the rest after entry interface.
If you attempt to deorbit wrong, you'll probably skip off the atmosphere or have entry interface somewhere where you don't want it. BTW, Shuttle deorbits were done by just slowing down about 200 - 250 miles per hour. But that was done to lower perigee. If we had done the same thing to lower apogee we would not have deorbited on many missions and we certainly would have been in trouble for landing.
Andy
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#882
by
olasek
on 04 Dec, 2011 01:22
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Not sure what you are betting your house on,
I bet no burn will happen so the discussion is purely academic. If I were Russians and knew more about the spacecraft and its exact state I would carefully evaluate probabilities since it may turn out totally blind firing will only makes things worse.
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#883
by
i2000s
on 04 Dec, 2011 03:36
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And now we wait.
From Heavens-Above:
Orbit: 208 x 304 km, 51.4° (Epoch Dec 2)
207 x 302 km, 51.4° (Epoch Dec 3)
Is the orbit raised up or pulled down? It has been in an orbit around 100+km or so before?
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#884
by
Sparky
on 04 Dec, 2011 04:17
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According to
http://twitter.com/#!/PhobosGruntApparently a couple of objects of fairly small size drifted away from Phobos-Grunt on Nov 29 and were catalogued by USSTRATCOM
If parts of the spacecraft are already falling off, it's probably a safe bet that it's not going anywhere except where it's current orbit takes it.
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#885
by
Robotbeat
on 04 Dec, 2011 05:40
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If they regain control (with a functional propulsion system) but cannot save the mission at all, one possibility instead of destructive reentry could be to send it on a heliocentric disposal orbit, on an Earth-escape trajectory. They should have plenty of delta-v for such a maneuver, and it may pose even less risk than a guided reentry would.
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#886
by
Kaputnik
on 04 Dec, 2011 09:45
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Any quesses as to what bits have fallen off the spacecraft? Presumably the torodial tank is an obvious one, or the sample return module?
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#887
by
kevin-rf
on 04 Dec, 2011 11:03
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How about an insulation blanket?
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#888
by
Apollo-phill
on 04 Dec, 2011 11:40
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Just a thinking process here but how about this hypothesis :-
Did the commands of Nov 29 actually get received and caused -
the engine tanks to pressurise and attempt to ignite engines but didn't . But sufficient s/c dynamics due pressurisisation too "shake/disturb" s/c causing small items be dislodged and enter their own orbits which are being detected now ?
or a somewhat similar scenario where Nov 29 commands caused some disturbance of s/c ?
A-P
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#889
by
Orbiter
on 04 Dec, 2011 12:38
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And now we wait.
From Heavens-Above:
Orbit: 208 x 304 km, 51.4° (Epoch Dec 2)
207 x 302 km, 51.4° (Epoch Dec 3)
Is the orbit raised up or pulled down? It has been in an orbit around 100+km or so before?
Being dragged down by atmospheric drag, eventually it's going to burn up soon, sometime next year.
Orbiter
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#890
by
ChileVerde
on 04 Dec, 2011 14:24
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...eventually it's going to burn up soon, sometime next year.
Sometime next year currently looks like the first half of January, five or six weeks from now.
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#891
by
Ben the Space Brit
on 04 Dec, 2011 14:26
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Did the commands of Nov 29 actually get received and caused -
the engine tanks to pressurise and attempt to ignite engines but didn't . But sufficient s/c dynamics due pressurisation too "shake/disturb" s/c causing small items be dislodged and enter their own orbits which are being detected now ?
This lends itself to my current theory that the fault is in the propulsion system rather than anything control system-based.
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#892
by
Chris Bergin
on 04 Dec, 2011 16:57
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...eventually it's going to burn up soon, sometime next year.
Sometime next year currently looks like the first half of January, five or six weeks from now.
Is that pretty much the fate now? The manual attempts - as previously noted on the Russian side - is throwing me off a bit.
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#893
by
Ben the Space Brit
on 04 Dec, 2011 17:21
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...eventually it's going to burn up soon, sometime next year.
Sometime next year currently looks like the first half of January, five or six weeks from now.
Is that pretty much the fate now? The manual attempts - as previously noted on the Russian side - is throwing me off a bit.
Every indication is that every attempt to communicate with the probe before the infamous 'bleep' picked up in Perth have failed, Chris. Even then, there is some evidence of a MPS failure of some kind that is preventing any burns from taking place.
IMHO, the probe is doomed. Everything else is just Roscosmos trying CYA.
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#894
by
seshagirib
on 04 Dec, 2011 17:32
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Did the commands of Nov 29 actually get received and caused -
the engine tanks to pressurise and attempt to ignite engines but didn't . But sufficient s/c dynamics due pressurisation too "shake/disturb" s/c causing small items be dislodged and enter their own orbits which are being detected now ?
This lends itself to my current theory that the fault is in the propulsion system rather than anything control system-based.
if it is indeed the case, that main propulsion is faulty, and control communication systems are ok, it should be possible to fire the pyros(?) and break up the craft into its component modules, making rentry safer.
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#895
by
ChileVerde
on 04 Dec, 2011 17:46
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...eventually it's going to burn up soon, sometime next year.
Sometime next year currently looks like the first half of January, five or six weeks from now.
Is that pretty much the fate now? The manual attempts - as previously noted on the Russian side - is throwing me off a bit.
That's what's going to happen, +- a few days, if PG remains inert or close to it. And the present indications are, IMO, that it's been pretty inert for more than ten days and is likely to stay that way.
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#896
by
Comga
on 04 Dec, 2011 18:12
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And now we wait.
From Heavens-Above:
Orbit: 208 x 304 km, 51.4° (Epoch Dec 2)
207 x 302 km, 51.4° (Epoch Dec 3)
Is the orbit raised up or pulled down? It has been in an orbit around 100+km or so before?
There was a long discussion starting back on the first page of this thread
http://forum.nasaspaceflight.com/index.php?topic=27289.msg828739#msg828739Perigee increased for a while, then resumed descending. Apogee has always been descending.
207 x 301 km, 51.4° (Epoch Dec 4)
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#897
by
Chris Bergin
on 04 Dec, 2011 20:07
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Thanks Ben, Chile. Very sad.
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#898
by
SimoninRio
on 04 Dec, 2011 20:26
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Hi, my first post: Donīt lost track of the exceptionally low cost of this mission. If you do things on the cheap using tried and trusted tech, you are always likely to to have your share of failures but you can do 10 FG missions for the price of one. I canīt personally remember such a low price tag on any science mission, let alone to Mars. Of course tried and trusted means it doesnīt fail. Consequently, the Russiansībest PR option, once the thing comes crashing down, would be to say that they are already building the new improved version at half the price. Iīd really like NASA to put the existing Mars Roversīspecs in the public domain and invite the rest of the world to put a few dozen Opportunity rovers down there.
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#899
by
Jim
on 04 Dec, 2011 21:07
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the rest of the world to put a few dozen Opportunity rovers down there.
And what good will that do?
A. there are limited types of instruments that MER rovers can carry. They don't have the available power for others.
b. MER was design for a unique orbital alignment that only occurs every 16 years. If the missions were delayed from 2003 to 2005, then Delta II would not have been able to launch them. And yet, if another launch vehicle like Atlas II were to be used, the approach velocity to Mars would be too great for the MER EDL system.
c. P-G was cheap because of low Russian wages.