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Message started by frankuitaalst on 08/06/14 at 11:57:54

Title: Rosetta's Landing on 67P
Post by frankuitaalst on 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 .  :)
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 !


Title: Re: Rosetta's Landing on 67P
Post by frankuitaalst on 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 .

Title: Re: Rosetta's Landing on 67P
Post by frankuitaalst on 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 )

Title: Rosetta's Landing on 67P
Post by frankuitaalst on 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 .

Title: Re: Rosetta's Landing on 67P
Post by frankuitaalst on 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 ?

Title: Re: Rosetta's Landing on 67P
Post by frankuitaalst on 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.

Title: Re: Rosetta's Landing on 67P
Post by 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.

Title: Rosetta's Landing on 67P
Post by frankuitaalst on 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  )  

Title: Re: Rosetta's Landing on 67P
Post by frankuitaalst on 08/08/14 at 14:32:06


Tony wrote:
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 ?

Title: Re: Rosetta's Landing on 67P
Post by Tony on 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.

Title: Re: Rosetta's Landing on 67P - mass estimate
Post by frankuitaalst on 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

Title: Re: Rosetta's Landing on 67P
Post by Tony on 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  8-)

Title: Re: Rosetta's Landing on 67P
Post by frankuitaalst on 08/21/14 at 23:06:34

My margarita has the same density  :)
I saw the figure 1 trillion kg and had the same reaction  as you . This must 1*10e12 kg I think ? (1000x1000x1000x1000 kg )

Title: Re: Rosetta's Landing on 67P
Post by frankuitaalst on 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 .  

Title: Re: Rosetta's Landing on 67P
Post by frankuitaalst on 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.


Title: Re: Rosetta's Landing on 67P
Post by frankuitaalst on 10/15/14 at 13:26:50

Here's a screenshot of the above simulation

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