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Philae landing on the nucleus of Comet 67P C-G
MarsInMyLifetime
post Sep 24 2014, 04:11 AM
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QUOTE (Hungry4info @ Sep 23 2014, 10:51 PM) *
Was there anything similar planned for Beagle 2?

Beagle's airbag-wrapped landing process precluded the need for descent imaging. But this discussion is drifting from the subject of Philae itself. Philae will be a free-falling object without the ability to use the descent images for terrain avoidance; figuring out where it landed after the fact is much more important, so deferred imaging is fine. The issue is less about precedents than about requirements for the mission, with science and engineering both as stakeholders in the outcome.


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Explorer1
post Sep 24 2014, 04:20 AM
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A big difference from Beagle 2 is that OSIRIS will be able to resolve Philae on the surface right away, so even loss of contact would be a learning experience. This will not be a shot in the dark.
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SpaceScout
post Sep 24 2014, 03:37 PM
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QUOTE (Explorer1 @ Sep 24 2014, 06:20 AM) *
[...] so even loss of contact would be a learning experience. This will not be a shot in the dark.

and I think this is the most important point. Whatever happen to the lander, it will be possible to reconstruct its approach and touchdown.




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Phil Stooke
post Sep 24 2014, 05:37 PM
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One aspect of landing which I don't think has been mentioned is that it takes 7 hours for the lander to descend from release to the surface. That's over half a rotation. So it's not like hovering over the landing site and letting go - the landing site is far away, and as the lander falls the landing site moves until, at the appointed time, it's just under the lander. Like one of those action movies where somebody jumps off a bridge to land on the top of the train car as it passes underneath ... only a bit slower. It's easy to see that any uncertainty in the descent rate translates into a change in location on the surface.

Phil



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vikingmars
post Sep 24 2014, 06:12 PM
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QUOTE (Phil Stooke @ Sep 24 2014, 07:37 PM) *
One aspect of landing which I don't think has been mentioned is that it takes 7 hours for the lander to descend from release to the surface. That's over half a rotation. So it's not like hovering over the landing site and letting go - the landing site is far away, and as the lander falls the landing site moves until, at the appointed time, it's just under the lander. Like one of those action movies where somebody jumps off a bridge to land on the top of the train car as it passes underneath ... only a bit slower. It's easy to see that any uncertainty in the descent rate translates into a change in location on the surface. Phil

Well seen Phil ! This is not a descent, but a low-speed free-fall, hoping that the lander will not gain too much horizontal velocity at touchdown...
PS : the 7 hrs descent, from "T0" to "T0 + 7h", was duly mentioned inside the "PDF" timetable attached to the 1st post wink.gif
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centsworth_II
post Sep 24 2014, 06:51 PM
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I'm wondering what percent of Phillae's descent rate is due to the mechanical push toward the comet it receives and how much is due to gravity and how that changes over the course of the descent. Has this been described?

Edit: After some more reading and thought, I've concluded that the mechanical shove is almost entirely devoted to counteracting orbital velocity (and not to push the lander toward the comet) and that the descent is pretty much 100% due to gravity.
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charborob
post Sep 24 2014, 07:49 PM
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Is there an animation somewhere that shows the complete Philae landing sequence, including the comet rotating in the background? From the descriptions I read, I can't figure out how exactly it's going to work out.
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Gerald
post Sep 24 2014, 08:42 PM
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QUOTE (vikingmars @ Sep 23 2014, 05:40 PM) *
... I should have put the timeschedule in German also, but my writing with Goethe's style is somewhat shaky : all my apologizes ! ...

Far from competing with Goethe (I'm sure he would be excited by the mission), here my amateur translation of your appreciated survey into German:
Attached File  philae_landing_schedule_de_utf8.txt ( 2.96K ) Number of downloads: 365

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MahFL
post Sep 25 2014, 12:08 AM
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QUOTE (charborob @ Sep 24 2014, 07:49 PM) *
Is there an animation somewhere that shows the complete Philae landing sequence, including the comet rotating in the background? ..


Yes, it's at 12:40 on this presentation, the comet actually hits the lander, albeit softly. If they did not have the hold down thruster, the harpoons and screws on the legs the lander would bounce away from the comet.

Landing Site Announcement.
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charborob
post Sep 25 2014, 03:11 AM
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That's exactly what I wanted to see. Thanks. Unfortunately, I didn't have the time to watch the presentation.
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vikingmars
post Sep 25 2014, 07:12 AM
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QUOTE (Gerald @ Sep 24 2014, 10:42 PM) *
Far from competing with Goethe (I'm sure he would be excited by the mission), here my amateur translation of your appreciated survey into German:
Attached File  philae_landing_schedule_de_utf8.txt ( 2.96K ) Number of downloads: 365

Thanks a lot Gerald for this nice work !
I keep the German translation in my records.
We may have also some German visitors in Cite des Sciences in Paris and they may like some documentation in German too (I'll put your name as translator then)... smile.gif
Attached Image

"Goethe mit uns"
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TheAnt
post Sep 26 2014, 01:09 AM
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Landing site J with C as backup if they spot anything untoward on the first selection.'

(Edit: Since I heard it verbally I got it wrong still think they really said 'G'.)
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vikingmars
post Sep 26 2014, 03:14 PM
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A new set of 2 Rosetta NavCam images was released this afternoon by the Rosetta team... (sorry, again, no OSIRIS images).
This NavCam image is a very interesting one : it shows the backup "C" landing ellipse (see the superimposed yellow ellipse I draw on the left-hand side of the image)...
Wow ! Look carefully at it : its the only "safe" place around, among truly chaotic terrain ! What a GREAT mission indeed smile.gif
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Gerald
post Sep 26 2014, 10:28 PM
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The timing of the landing options has been pinned down for November 12 (GMT) :
QUOTE
Two robust landing scenarios have been identified, one for the primary site and one for the backup. Both anticipate separation and landing on 12 November.

For the primary landing scenario, targeting Site J, Rosetta will release Philae at 08:35 GMT/09:35 CET at a distance of 22.5 km from the centre of the comet, landing about seven hours later. The one-way signal travel time between Rosetta and Earth on 12 November is 28 minutes 20 seconds, meaning that confirmation of the landing will arrive at Earth ground stations at around 16:00 GMT/17:00 CET.

If a decision is made to use the backup Site C, separation will occur at 13:04 GMT/14:04 CET, 12.5 km from the centre of the comet. Landing will occur about four hours later, with confirmation on Earth at around 17:30 GMT/18:30 CET. The timings are subject to uncertainties of several minutes.


The 22.5 km look rather distant. I'm not quite sure about the reason. Might be it turned out, that Philae is more predictable than Rosetta regarding drag due to outgassing? Or it has something to do with the observability of Philae during the descent and communication early after landing.
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MahFL
post Sep 26 2014, 11:10 PM
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QUOTE (Gerald @ Sep 26 2014, 11:28 PM) *
The 22.5 km look rather distant.


Remember you do not want the orbiter to accidentally hit the comet too, so someone decided 22.5 km was the distance to use.
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