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Rosetta's Landing on 67P (Read 14417 times)
frankuitaalst
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Rosetta's Landing on 67P
08/06/14 at 11:57:54
 
Rosetta was today inserted in a large orbit around 67P . Congrats to Esa and all other involved agencies !  
In the months to come Esa will look for an appropriate landing site for the lander .  
Then the hard job may come to set the lander to the ground .  
Given te complex geomtery of the comet this may require some good navigation skills .  Smiley
Suppose Esa decides to land in the "neck" of the comet . How to do this ?  
 
I've set up a simple GravSim of  the geometry of the comet consisting of a very close binary of two small bodies .  
Masses and dimensions are in the order of magnitude of 67P .  
In this sim the revolution period is about 3 hours , where in reality the period is about 12 hours.  
Rosetta is represented as the small yellow probe orbiting at about 20 km as the sim starts .  
Enjoy the landing ...using the navigition bottons !  
 
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Re: Rosetta's Landing on 67P
Reply #1 - 08/06/14 at 13:59:53
 
Here's a animation of a simulated landing on 67P .  
 
After some trial and error ( at which the probe crashes ) it is even possible to navigate through the narrow gap between the simulated 2 major bodies.  
The trick is to trottle "towards" and "away " alternatively . In this way the probe stays more or less "stationary" while the comet is rotating .
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RosettaLandingSim.gif
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Re: Rosetta's Landing on 67P
Reply #2 - 08/06/14 at 14:02:06
 
For the above simulation I used the following GravSim ( in which I reduced the gap between the two bodies of the comet )
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Rosetta's Landing on 67P
Reply #3 - 08/07/14 at 00:49:49
 
Here's a new updated sim which is more accurate :  
dimension 67P : 4 km , divided into 2 bodies , first being 2400 m , the second 1600 m .  
Density was choosen to 2000 kg/m³ which may be exagerated . Maybe 1000 kg/m³ may be a better figure .  
The rotation speed then becomes ca. 6 hours which still is to big .  
I added Rosetta at sma 7500 meter from the bigger part .
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Re: Rosetta's Landing on 67P
Reply #4 - 08/07/14 at 13:29:49
 
The sim hereunder is the most accurate I could get till now  .  
The density of the comet was set to 350 kg/m³ which gives a revolution period of about 12 hours , which corresponds highly to the measured value by Rosetta (12.6 h ) .  
 
Yet the comet is still simulated as a pair of two bodies , a very close binary . This means in this sim nothing holds the comet together , except mutual attraction .  This also means the comets consistency must be very very loose in reality , as the orbital periods more or less match  . If the comet would rotate faster there is a good chance it might break in two ...  
 
In the sim I added several possible orbits of the spacecraft.  
The most stable orbit seems to be a polar orbit ( over the main body ) .  
 
Unfortunately , due to the low mass of the comet ( in this sim being 3.512 E+12 kg ) and therfor low gravitational filed I was not able anymore to land on the comet using the trusters , because the trusters are too powerfull   .  It is not easy to manoeuver in such a weak field. The escape velocity is about 0.45 m/s !!  
 
@ Tony : is there a method to lower the thrust in Gravsim  , so that one can attempt a landing in this configuration ?
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Re: Rosetta's Landing on 67P
Reply #5 - 08/07/14 at 13:37:54
 
Here's a screnshot of the sim above .  
 
The sim shows 3 coplanar orbits of Rosetta at about 5000 m initial distance , and one ( the blue orbit ) a polar orbit , also at 5000 m from the bigger part of the comet .  
The picture shows the result after 10 days orbiting .  
Clearly visible is the precession the comet will give on the orbit of Rosetta.
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RosettaSimLandingv3.jpg
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Re: Rosetta's Landing on 67P
Reply #6 - 08/08/14 at 14:01:29
 
When I first saw the animation of the comet spinning a few weeks ago, I did a quick calculation and found as you did:  The comet is spinning fast enough that it is on the verge of breaking up.  So modeling it at two orbiting lobes is a good idea.  Try including the Sun in your sim.  This comet is of low enough mass that the Sun might also heavily perturb the orbit of the spacecraft.
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Rosetta's Landing on 67P
Reply #7 - 08/08/14 at 14:09:13
 
The comets extraordinal shape did me wonder about the potential field this body has.  
 
Out of interest I've created a simple model of the comet , consisting of two spheres , and did some excel modelling .  
The picture  of this model in annex gives the gravitational potential along the long axis of the comet .  
I generated an envelope of 100 m above the surface of the comet and calculated the potential energy .  
As can be seen the "head of the duck" has the maximum potential .  
The neck has the lowest potential energy .  
This means the neck should be the most stable region .  
This makes sense  by analogy ( valleys  )
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67P_Potential_Model.jpg
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frankuitaalst
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Re: Rosetta's Landing on 67P
Reply #8 - 08/08/14 at 14:32:06
 
Quote from Tony on 08/08/14 at 14:01:29:
When I first saw the animation of the comet spinning a few weeks ago, I did a quick calculation and found as you did:  The comet is spinning fast enough that it is on the verge of breaking up.  So modeling it at two orbiting lobes is a good idea.  Try including the Sun in your sim.  This comet is of low enough mass that the Sun might also heavily perturb the orbit of the spacecraft.

Yes , I think it must be at the edge of breaking down , depending upon the rotation axis . If it rotates along the long axis there may not be a problem .. I'm sure a debate about this will follow soon .  
 
Concerning adding the sun in the sim : I can do this , but I think Rosetta is now well inside Hills Sphere which I calculated to be about 430 km . Rosetta now is at 100 km .  
 
Tony : It would also interest me how I could lower the "thrust" in the thrust boxes in GravSim . Is there a way to get a thrust of let's say 1/10 or 1/100 of the actual minimum value provided ?  
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Re: Rosetta's Landing on 67P
Reply #9 - 08/08/14 at 20:26:20
 
The Moon is well within Earth's Hill Sphere, but the Sun plays some heavy games with it: 18 year precession of nodes, 9-year precession of perihelion.  That's why I suggested to include the Sun.
 
You can use Autopilot to thrust.  Then you can enter any number you want.
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Re: Rosetta's Landing on 67P - mass estimate
Reply #10 - 08/21/14 at 09:00:46
 
A mass estimate arrived today :  
the mass is now estimated at 1 e+12 kg , this corresponds with a density of about 110 kg/m³ ( density is my estimate )  
 
http://blogs.esa.int/rosetta/2014/08/21/determining-the-mass-of-comet-67pc-g/
 
This means that if we simulate the comet as a binary the orbital period would become about 18.8 hours (taking r head = 800 m ; r body = 1200 m) .  
The binary model doesn't work anymore as the orbital period is only 12.5 hours .  
This also means that the comet is held together by other forces , other than gravitation alone , as we could expect  
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Re: Rosetta's Landing on 67P
Reply #11 - 08/21/14 at 21:34:37
 
Are you sure you didn't drop a zero?  Your density estimate is about 10% that of the ice in my margarita  Cool
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frankuitaalst
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Re: Rosetta's Landing on 67P
Reply #12 - 08/21/14 at 23:06:34
 
My margarita has the same density  Smiley
I saw the figure 1 trillion kg and had the same reaction  as you . This must 1*10e12 kg I think ? (1000x1000x1000x1000 kg )
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Re: Rosetta's Landing on 67P
Reply #13 - 08/22/14 at 14:43:35
 
This is funny , reopening the same article on the blog now gives a mass of 10^13 kg , this is 10 times more than a day ago . Must have been a typo which they corrected .  
So the density may be order of 1100 kg/m³ . This is good news .
The comet is better protected from flying apart .
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Re: Rosetta's Landing on 67P
Reply #14 - 10/15/14 at 13:26:15
 
Here's a simulation program , written in PoweBasic I made recently in order to visualize the difficulties the ESA's team might have to land their Philae Lander safely onto Comet 67P .  
I first tried to do this in GravSim , but was'nt succesfull because of the comets complex gravity field.  
In the sim the comet is simulated being two different point masses , rotating around a common barycenter with period of 12.6 hours .  
I couldn't match the period of these point masses and the magnitude of the masses and their distance in GravSim .  
The sim lets you navigate the Rosetta Spacecraft around 67P by the "NAV" button ( click ) .  
Further the probe can be released , and reattached again. .  
Feel free to play with it .  
The Help menu might be usefull .  
I tried to make ths sim as realistic as possible ( best guesses for masses , distances...aso ) , so it visualizes in 2D well what Rosetta has to expect at 12/11/2014 .  
You can unzip the folder , keep the files together and press the .exe . All the rest is just clicking the right buttons.  
The .bas file contains the code which is compilable in a PBWin10 compiler .  
 
Although it's not a GravSim code I thaught I might share it here for those interested.  
 
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