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Changing mass (Read 18781 times)
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Changing mass
06/04/07 at 11:17:51
 
Is it possible in GravSim to change the mass of the primary partway through the simulation? Even if it's just going instantly from 1.5 solar masses to 0.5 solar masses for example, though it'd be nice to specify a duration over which the mass drops linearly. If so, how does one do this?
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Re: Changing mass
Reply #1 - 06/04/07 at 16:12:09
 
You can maunally do it.  Use Autopilot to pause the simulation at the desired time, then use menu Objects > Edit Objects to change the mass.  Then unpause the sim.
 
I toyed around with code to change it linearly, but never kept the changes.  It's not hard to do.  I can include it in the next Beta if you like.  I can give you linear, poly or exponential if you like.
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Re: Changing mass
Reply #2 - 06/04/07 at 18:11:57
 
Would be nice to have... then we can change the mass of the central object (like say, when you go from a red giant to a white dwarf) and see what happens to the orbits.  
 
I'm kinda curious to see what happens to an asteroid belt if the central object drops in mass. I know the orbits will expand, but I'm guessing it'll do it messily...
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Re: Changing mass
Reply #3 - 06/04/07 at 19:21:32
 
Quote from Mal on 06/04/07 at 18:11:57:
Would be nice to have... then we can change the mass of the central object (like say, when you go from a red giant to a white dwarf) and see what happens to the orbits.

I'm kinda curious to see what happens to an asteroid belt if the central object drops in mass. I know the orbits will expand, but I'm guessing it'll do it messily...

 
If the star gradually loses mass over a period of time >> periods of the asteroids, they they will smoothly spiral out.  If the star loses its mass instantly, then their orbits will gain eccentricity, and their positions at the mass moss moment will become their periastrons.  This you can do already by manually editing the star's mass.
 
I'll update the beta in a few days and include this.
 
btw, here's an animation that ties together the dynamic elements of asteroid Pallas and the Kozai mechanism:
 
http://orbitsimulator.com/gravity/images/PallasJupiter.GIF
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Re: Changing mass
Reply #4 - 06/04/07 at 20:18:37
 
Quote from Tony on 06/04/07 at 19:21:32:
Quote from Mal on 06/04/07 at 18:11:57:
Would be nice to have... then we can change the mass of the central object (like say, when you go from a red giant to a white dwarf) and see what happens to the orbits.

I'm kinda curious to see what happens to an asteroid belt if the central object drops in mass. I know the orbits will expand, but I'm guessing it'll do it messily...


If the star gradually loses mass over a period of time >> periods of the asteroids, they they will smoothly spiral out. If the star loses its mass instantly, then their orbits will gain eccentricity, and their positions at the mass moss moment will become their periastrons. This you can do already by manually editing the star's mass.

I'll update the beta in a few days and include this.

 
Thanks!  
 
Quote:
btw, here's an animation that ties together the dynamic elements of asteroid Pallas and the Kozai mechanism:

http://orbitsimulator.com/gravity/images/PallasJupiter.GIF

 
Cool, that's just hypnotic Smiley
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Re: Changing mass
Reply #5 - 06/05/07 at 10:24:05
 
I simulated this once for our solar system ( gradually decreasing the mass of the sun ) . There's normally less influence for the inner planets , but the outer planets are indeed spiralling outwards, so the farther the bigger the influence ( seen on screen) . Be carefull however not to exagerate... I once read that during the lifetime of our sun it burns about less than 1% of its mass over billions of years ..
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Re: Changing mass
Reply #6 - 06/21/07 at 21:07:57
 
I've got a new Beta that incorporates this into the Autopilot, but I can't upload it until Sun or Mon, as I'm out of town. Smiley
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Re: Changing mass
Reply #7 - 06/22/07 at 02:12:52
 
I haven't had a chance to set this up in Grav Sim yet. I'd just be changing the mass pretty suddenly (dropping it from say 1.5 to 0.5 solar masses as it goes from AGB giant to whit dwarf) and seeing what happens. I'm just wondering how "messy" the change in the asteroid orbits would be. Would the whole belt just slowly move out, or would there be asteroids flung around all over the place?
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Re: Changing mass
Reply #8 - 06/24/07 at 22:24:26
 
Quote from Mal on 06/22/07 at 02:12:52:
I haven't had a chance to set this up in Grav Sim yet. I'd just be changing the mass pretty suddenly (dropping it from say 1.5 to 0.5 solar masses as it goes from AGB giant to whit dwarf) and seeing what happens. I'm just wondering how "messy" the change in the asteroid orbits would be. Would the whole belt just slowly move out, or would there be asteroids flung around all over the place?

 
So I just had a chance to set this up, and something's going wrong. I set up 100 asteroids going around a 1.5 solar mass star with 50 solar radii.  
 
The asteroid parameters are : size = 10 km radius +/- 100%, semimajor axis = 10 AU +/- 20%, eccentricity = 0.2 +/- 100%, inclination = 2 degrees +/- 100%, mass 0.001 Earths +/- 0% (so they're superdense asteroids Wink), and the longitude/argument/mean anomaly at 180 +/- 100%.  
 
I set this going at timestep 8192 and I see a nice belt of asteroids going round the star. I then pause the simulation and change the mass of the star to 0.5 sols and the radius down to 10,000 km. Then I restart it at 8192.  
 
What happens next is that all the asteroids are immediately "cut loose" - they just fly off in a straight line at a tangent to their earlier orbit, and don't come back.  
 
Any idea what is going wrong here? I don't think this is supposed to happen.  
 
P.S. This is with GravitySimulatorBeta17Apr2007.exe
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Re: Changing mass
Reply #9 - 06/25/07 at 11:07:17
 
Quote from Mal on 06/24/07 at 22:24:26:

Any idea what is going wrong here? I don't think this is supposed to happen.

 
Look at the relationship between circular velocity formula and escape velocity formula:
 
Vc=sqrt(GM/r)
Ve=sqrt(2GM/r)
 
The only difference is the 2.  So any time you reduce the star's mass by less than half, you will put all objects in circular orbits onto elliptical orbits.  If you reduce it by more than half, you will put all objects in circular orbits on hyperbolic orbits.  They will never return.
 
Try it.  In Gravity Simulator:
 
File > New
Objects > Create Objects.  Semi-major axis = 0.1 AU.  Leave all other values at their defaults.
Zoom out so you can see your orbiting object.
View > Add Orbital Elements box.  Set to Object.  Notice its eccentricity is very close to 0.
Objects > Edit Objects.  Change the mass of Object 1 from 1 solar mass to 0.5001 solar masses.
Notice the eccentricity for Object is just under 1.  It has a very long elliptical orbit and will take many years to r
 
Try this again, except instead change the mass of Object 1 to 0.4999 solar masses.
Notice the eccentricity for Object is greater than 1.  It will never return.
 
You are making your object 0.3333 times its original value, way below the 0.5 threshold required to keep the object in orbit.
This was a fun little exercise in celestial mechanics Smiley.
 
I uploaded the latest beta.  "Dynamic Mass" is available through the Autopilot.  Let me know what you think.
 
*** edit ***
removed link to this beta.  It had a bug in it.  The link for the updated beta is my post below.
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« Last Edit: 06/27/07 at 17:28:07 by Tony »  
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Re: Changing mass
Reply #10 - 06/25/07 at 18:34:40
 
Quote from Tony on 06/25/07 at 11:07:17:
Quote from Mal on 06/24/07 at 22:24:26:

Any idea what is going wrong here? I don't think this is supposed to happen.


Look at the relationship between circular velocity formula and escape velocity formula:

Vc=sqrt(GM/r)
Ve=sqrt(2GM/r)

The only difference is the 2. So any time you reduce the star's mass by less than half, you will put all objects in circular orbits onto elliptical orbits. If you reduce it by more than half, you will put all objects in circular orbits on hyperbolic orbits. They will never return.

 
*blink* Crap! That could be awkward Sad.  
 
Though wait a minute... what happens if they're in an elliptical orbit to start with, not a circular one?  
 
And does this only happen if you suddenly reduce the mass? If you reduce it slowly over time (even over say a hundred years or so) then would they just spiral out but still be orbiting the star?
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Re: Changing mass
Reply #11 - 06/25/07 at 18:45:29
 
Also, I downloaded the new beta but have no clue how to use Autopilot. The new Dynamic Mass thing doesn't make much sense to me right now... why are there four text boxes there?  
 
Any explanation would be appreciated Smiley.
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Re: Changing mass
Reply #12 - 06/26/07 at 00:31:02
 
Quote from Mal on 06/25/07 at 18:34:40:
*blink* Crap! That could be awkward Sad.

Though wait a minute... what happens if they're in an elliptical orbit to start with, not a circular one?

And does this only happen if you suddenly reduce the mass? If you reduce it slowly over time (even over say a hundred years or so) then would they just spiral out but still be orbiting the star?

 
The circular orbit is the ideal case.  In real life, there are no circular orbits.  But orbits close to circular, such as the orbits of the planets, will behave like the ideal case.  For an object in an elliptical orbit, you need to set escape velocity to your current velocity, then compute the mass of a star needed to achieve that escape velocity using M=(Ve^2*r)/(2G)
 
The 4 boxes allow you to enter a polynomial, where t is the amount of time elapsed, in seconds since the Autopilot command was executed.  The boxes are the coefficients for: t4, t3, t2, and t.  If you want linear, just leave the 1st 3 boxes set to 0.  If you want only a quadratic, leave the 1st 2 boxes set to 0.  If you want a 3rd degree polynomial, leave the 1st box set to 0.  The object's mass at each time step is defined by your polynomial.  To turn off, just us Autopilot again to set the 4 boxes to 0.
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Re: Changing mass
Reply #13 - 06/26/07 at 23:35:34
 
I think you may have to walk me through this a bit. Smiley
 
So let's say i want to go from 1.5 solar masses to 0.5 solar masses over a fixed period of time (eg 1000 years). How do I start and stop that? There's an "execution time" that is presumably when it starts decreasing, but how do you stop it? What are the units of whatever goes in those four boxes?  
 
(I thought you had a tutorial for the autopilot somewhere on the site??)
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Re: Changing mass
Reply #14 - 06/27/07 at 17:26:28
 
There was a bug that kept it from working under certain circumstances.  Download this and use it instead:
http://orbitsimulator.com/gravity/beta/GravitySimulatorBeta27June2007.exe .  Also, see if there is a file on your computer called c:\test.txt .  If there is, you can delete it.  Gravity Simulator created it for my debugging purposes, and I forgot to delete it from the last beta.
 
Quote from Mal on 06/26/07 at 23:35:34:
I think you may have to walk me through this a bit. Smiley

So let's say i want to go from 1.5 solar masses to 0.5 solar masses over a fixed period of time (eg 1000 years). How do I start and stop that? There's an "execution time" that is presumably when it starts decreasing, but how do you stop it? What are the units of whatever goes in those four boxes?

(I thought you had a tutorial for the autopilot somewhere on the site??)

 
Just hit the Help menu in Gravity Simulator, and it will jump you to the help page, or you can go directly:  http://orbitsimulator.com/gravity/tutorials/tutorials.html
 
The numbers in the boxes are the coefficients of the polynomial that will define the rate of mass change.  Their units are mass/time.
 
If we want it to drop the mass of an object linearly from 1.5 solar masses to 0.5 solar masses over 1000 years, we need to do a little computing:
 
We want a line that describes the mass as a function of time.  For a linear change, use the basic equation of a line:  y = mx + b.  We want this line to pass through the points ( 0, 1.5 ) and ( 1000, 0.5 ).  These points represent the star's mass at t=0 and at t=1000 years.
 
But Gravity Simulator likes units of Earth masses and seconds, not Solar masses and years, and in this beta version I don't allow you to choose units, so you have to transform these points.  1 solar mass = 332983.46 Earth masses, and 1000 years = 31557600000 seconds.  So our transformed points are ( 0, 499475.19 ) and ( 31557600000, 166491.73).  These points represent the star's mass in Earth masses at t=0 seconds and the star's mass in Earth masses at t=31557600000 seconds.
 
Returning to the basic equation of a line, y = mx + b,  y = mass (Earth masses),  m = slope of the line (mass/time), x = time(seconds), and b = the y-intercept which is the value of y (mass in Earth masses) when x (time in seconds) = 0, which is the original mass, in this case 499475.19 Earth masses.
 
y is what we want Gravity Simulator to compute with each time step.  We know x and b.  They are 31557600000 seconds and 499475.19 Earth masses.  We need to solve for m, the slope of the line.  Slope = rise over run: ( y2 - y1 ) / ( x2 - x1 ).  We can use our 2 points  ( 0, 499475.19 ) and ( 31557600000, 166491.73) to compute the slope, m.
 
 
( 166491.73 - 499475.19) / ( 31557600000 - 0 ) = -1.055E-05
 
So our equation of the line becomes:
y = -1.055E-05 x + 499475.19
 
Just to double check, plug in 31557600000 for x
 
 -1.055E-05 * 31557600000 + 499475.19 = 166542.51 which is the correct value for final mass after 1000 years (slightly off since I rounded my numbers).
 
Plugging this into Autopilot:
menu Autopilot...
In the Command list, choose Dynamic mass.
Choose the time and date when the mass decline is to begin.
Choose the object whose mass you would like to alter.
Leave the t4, t3, and t2 boxes at 0 since we want a linear decline.  In the t box put the value of the slope:  -1.055e-05.  Press Add >>.
 
In the Command list, choose Dynamic mass again.
Add 1000 years to the date when the mass decline begins.
Choose the object whose mass is being altered.
 
Enter 0 for all 4 boxes.
Press Add >>.  
Press OK.
 
Your object should spend the 1000 years as defined by your 2 autopilot commands decreasing linearly in mass from 1.5 to 0.5 solar masses.  If you want a polynomial decrease, then use the values for  t4, t3, and t2.
 
 
 
 
 
 
 
 
 
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Re: Changing mass
Reply #15 - 06/28/07 at 00:25:39
 
Hm, that's definitely a calculation you might want to consider putting into the program... maybe just put a start mass and an end mass in there and let the program figure out the coefficients (maybe you can have a "linear" or "polynomial" or "exponential" option?).  
 
Anyway. It definitely did something... I knocked down the time from 1000 to 100 years (because I have a short attention span Wink) and let it run on the belt and it was interesting. I set it to start at yr 20 and end at yr 120, and it seemed that for the first two-thirds of the decrease or so the belt seemed to just get a bit thicker (I left the tracks on throughout), but then for the last third the asteroids started to leave the belt in ever increasing numbers.  
 
The attached picture is what it ended up as after about 300 years - the scale is set at 126.113 AU. It shows all the tracks of all 100 asteroids - the original belt had all the asteroids contained within the innermost 30-40 pixels of the purple band around the stars - as you can see, once the mass had finished changing the asteroids had spiralled out well beyond that. At the end of the change the innermost asteroids were near the outer edge of the thick purple band.
 
I'm not sure what the effect of changing the timescale of the mass change is. Obviously (as I discovered earlier) if you do it instantly then you just end up losing all the asteroids. But as long as it's not instantaneous, does it make any difference if you change the mass over a year, or 100 years, or 1000 years, or longer? Does the effect on the orbit depend on the initial orbit period at all? (i.e. if the change takes place faster than a single orbit is completed, does that make a difference?) If there were just two planets here instead of a belt, would the inner or outer planet be more affected by the change? All the orbits should expand by the same factor (Mi/Mf) where Mi is the initial mass and Mf is the final mass, so they should be affected the same way.  
 
 
This makes life somewhat awkward for me though. The reason I'm looking at this is that I created a star system for a SF setting that contains an asteroid belt at around 5.2 AU orbiting a white dwarf star. There's also a gas giant orbiting further out (around 20 AU) and a charred rockball orbiting further in (around 2.8 AU). Obviously this is all screwy now though I can use this as the initial conditions. I've done the current simulation with just the belt (and I set the belt for the purposes of the sim at a further distance anyway) so I don't know the effect of adding the two planets (I'm going to assume that the change from AGB red giant to white dwarf doesn't actually affect anything else about the belt). But as it is, it looks like the belt is (a) spread out quite a lot and (b) thinned out quite a lot by the process - there seems to be about 10% of the asteroids in the area closest to the star at any one time, with the rest further out on more eccentric orbits. But that said, the addition of a jovian out there might really muck things up...
 
It's interesting stuff, which I'll play around with some more later Smiley - thanks for adding the dynamic mass capability anyway!
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Re: Changing mass
Reply #16 - 06/28/07 at 09:57:10
 
Quote from Mal on 06/28/07 at 00:25:39:
I'm not sure what the effect of changing the timescale of the mass change is. Obviously (as I discovered earlier) if you do it instantly then you just end up losing all the asteroids. But as long as it's not instantaneous, does it make any difference if you change the mass over a year, or 100 years, or 1000 years, or longer? Does the effect on the orbit depend on the initial orbit period at all? (i.e. if the change takes place faster than a single orbit is completed, does that make a difference?)

 
The instant mass loss is easy to descirbe with the circular and escape velocity formulas.  I would imagine it has a lot to do with the time span during which the mass is lost, compared to the period of an orbit.  Looking at your diagram, I would have guessed that there would be a nice smooth spiral out, but it doesn't look like that's the case.
 
I'm not sure how long it takes a star that is blowing off layers to lose its mass.  When a star goes supernova, does it take minuts, hours, days, or weeks to lose its mass?  How about stars like the sun that form planetary nebula.  I think they lose their mass gradually.  Before I improve the dynamic mass feature, it would be nice to know what models are realistic.  Hmmm.... the more we learn, the less we realize we know  Roll Eyes
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Re: Changing mass
Reply #17 - 06/28/07 at 13:17:29
 
Quote from Tony on 06/28/07 at 09:57:10:
The instant mass loss is easy to descirbe with the circular and escape velocity formulas. I would imagine it has a lot to do with the time span during which the mass is lost, compared to the period of an orbit. Looking at your diagram, I would have guessed that there would be a nice smooth spiral out, but it doesn't look like that's the case.

 
I should really try this with just one object instead of 100 Smiley.  
I think it's because the eccentricities are changing (being amplified?). Not only are the semimajor axes increasing, but the eccentricies (alraedy between 0 and 0.4) are also going up and on bigger orbits the difference is much more noticeable, maybe?  
 
 
Quote:
I'm not sure how long it takes a star that is blowing off layers to lose its mass. When a star goes supernova, does it take minuts, hours, days, or weeks to lose its mass? How about stars like the sun that form planetary nebula. I think they lose their mass gradually. Before I improve the dynamic mass feature, it would be nice to know what models are realistic. Hmmm.... the more we learn, the less we realize we know Roll Eyes

 
I've got a paper at home about how planetary nebulae form, that might have a timescale.  
With supernovae the mass loss is pretty much instant - the star goes kaboom, but the mass is thrown off at a certain speed so it takes a bit of time for the shell of matter to pass through the system. Of course, planets and asteroids have other things to worry about in a supernova than their orbits shifting Wink.
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Re: Changing mass
Reply #18 - 06/28/07 at 13:31:34
 
If it blows off its mass instantly, then the planet should get pushed by the expanding sphere of matter.  The amount of mass lost divided by the area of a sphere, 4 pi r2, should give you the flux of matter at a planet's distance, r.  Multiply this by the planet's cross section, pi r2 to get the total mass slamming into a planet, then using the velocity of the expelled matter to compute the momentum delivered to the planet.  I'll bet that alone is enough to knock a planet out of orbit.
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Re: Changing mass
Reply #19 - 06/28/07 at 19:01:55
 
Well I did one project where I calculated the effect on a planet orbiting a Brown Dwarf that was 1000 AU from Antares when it blows. I figured that the material would hit the planet at 10,000 km/s but had a density of 2.9e-11 kg/m3 and hit with a dynamic pressure of 1460 Pa. Somehow I figured it would impact the surface at an equivalent of 0.27 megatons of TNT per square metre, applied for the 75,000 seconds.  
 
This is after the initial luminosity spike (up to 10 billion Sols!) melts the surface of the planet... in fact it's just starting to cool down when the blastwave hits...
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Re: Changing mass
Reply #20 - 06/28/07 at 19:27:28
 
Quote from Mal on 06/28/07 at 19:01:55:
... in fact it's just starting to cool down when the blastwave hits...

not a good day in the neighborhood  Angry
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Re: Changing mass
Reply #21 - 06/28/07 at 20:09:31
 
Quote from Tony on 06/28/07 at 19:27:28:
Quote from Mal on 06/28/07 at 19:01:55:
... in fact it's just starting to cool down when the blastwave hits...

not a good day in the neighborhood Angry

 
No, it's really not fun Smiley. Though it's kinda cool when the star explodes, because from the planet it doesn't all blow at once - because the supergiant is 5 AU in radius you actually get a delay of the increased lumonosity breaking through near the limb. So basically, the sub-Antares point rockets up to 10 billion Sols first, and then that spreads out radially across the disk. as the light from the limb regions reaches the planet. Pretty weird.  
 
Anyway... back to the topic at hand Smiley.  
 
I tried one asteroid this time. The results are shown in the attachment (the scale is the same, at 283 AU).
 
On the left (green) I have a 10 km (0.001 earth mass) asteroid in a circular orbit (e=0.000, i = 0) at 10 AU from the star (orbital period about 26 years). The star mass starts dropping from 1.5 to 0.5 solar mass at year 20, and finishes in year 120, so the change occurs over 100 years. The asteroid ends up in an elliptical orbit with a period of about 400 years.  
 
In the middle (magenta) I have the same asteroid at the same distance in a non-inclined orbit with e=0.2, with the same mass loss. This one ends up in a more eccentric orbit with a period of about 500 years.
 
On the right (orange) is the same asteroid with e=0.5 and the same mass loss. This one's in an even more eccentric orbit with a period of about 1550 years.  
 
I tried the circular orbit scenario but over a 10 year timeframe instead of a 100 year one. In this run, the asteroid arced off into space and didn't come back (250000 years later it was still on the way out, so I presume it must have escaped). So the time frame is definitely important.
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Re: Changing mass
Reply #22 - 06/29/07 at 22:51:39
 
I changed it so that the mass went from 1.5 to 0.5 solar masses in 33 years, and the asteroid stayed in orbit (the new period was about 980 years):
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Re: Changing mass
Reply #23 - 06/29/07 at 22:57:15
 
Yeah, I think if the mass changes on a timescale less than one orbit, the asteroid ends up on a hyperbolic orbit and doesn't come back. I just tried changing the mass over 20 years (less than the orbital period of about 26 years) and it just flies off into space.  
 
Yep, it's definitely flown off, eccentricity is 1.23
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« Last Edit: 06/30/07 at 00:48:02 by EDG »  

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Re: Changing mass
Reply #24 - 06/30/07 at 09:06:55
 
Stop the press!!! Smiley
 
This is very interesting... I did an overnight run with the whole system (three inner rocky planets, the belt of 100 asteroids, and 2 jovians beyond that). The parameters are slightly different to what I've been doing earlier (the asteroids start around 7.6 AU), but the big difference is that the mass drops from 1.78 to 0.59 solar masses over 1000 years.
 
I left it running overnight, and this morning I expected the huge mess of orbits that I got earlier on... but much to my surprise the system looked very much like it did to start with! The belt of asteroids was still a coherent belt about the same thickness as it was before, and all the planets were there. The only real difference was that all the bodies had moved into further orbits (looking at the graphs, the increase was slow at the beginning of the mass change but increased more quickly towards the end, and immediately stabilised at whatever value it had reached after the mass change stopped).  
 
The eccentricities were different too - not necessarily higher either. It looks like the ecc starts to wobble at the start of the mass change, and the wobble increases as it continues. At the end of the mass change the eccentricity is fixed at whatever value it reached at that point during its final wobble (which may or may not be higher than the initial ecc).  
 
So it really depends on the timscale of the mass change. It seems that if it's shorter than the orbital period then the orbiting body escapes from the system, if it's close to but longer than the orbital period then it gets kicked into an orbit with higher ecc and semimajor axis, but if it's a lot longer than the orbital period then it gets kicked into an orbit with a greater semimajor axis and about the same eccentricity.
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Re: Changing mass
Reply #25 - 06/30/07 at 11:27:35
 
That makes sense.  When the mass change is over a period of time much greater than the orbital period, then the effects are evenly spread out over the entire orbit, ensuring a smooth spiral out.  But if the change takes place in less than an orbital period, then one side of the orbit is favored over the other side.  I would expect that if the change happened over a time period equal to 3 orbital periods, that the orbit would become elliptical as the objects starting position gets heavily favored in the beginning, allowing the object to retreat to a higher orbit with a longer period where it is more protected.  But its longitude of perihelion remains the same, and once per orbit, in the same longitude of the orbit, it returns.  Just my guesses.
 
If your planets have mass, then it may be possible that your asteroids are locked into resonance with the planets after the planets migrate to their new orbits.  This is known as the resonant capture theory.  To check, use the rotating frame feature and set it to the period of each planet.  You might want to back up your file, and then delete all but 1 asteroid since 100 asteroids will clutter up the screen pretty quickly.
 
There's another new feature in the Autopilot.  Actually it's been there for a while, but it's not in the 2.0 version.  It's called Continuous Orientation.  This is to help you set up resonant capture simulations.  To use it, choose either Orient Prograde or Orient Retrograde depending on if you want to spiral out or in.  Then use autopilot Thrust to set a thrust rate.  It needs to be a very small number, perhaps just a fraction of 1 mms-2.  Make your planet migrate into an asteroid belt.  Rather than plow through the belt, as its resonances pass through the belt, it will capture the asteroids into these resonances.  They will then migrate with the planet, maintaining their resonances but gaining eccentricity as they migrate outward.  The biggest difference between this scenerio and the Dynamic mass scenerio is that only 1 object, the planet, is being directly affected.  The asteroids are being indirectly affected by the planet.
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Re: Changing mass
Reply #26 - 06/30/07 at 18:46:11
 
I suspect an easier way to find out if the asteroids are being "locked in" by resonances is just to get rid of all the other planets and see what happens. If it really is because the 1000 year mass loss isn't boosting the eccentricities so much then it should still be a coherent belt after the mass loss is done. I suspect it isn't the resonances - the orbits basically just end up getting a teensy bit bigger after each revolution around the star, so I think after 1000 years of slow mass loss the semimajor axes would just be bigger as a result.  
 
I'll give it a go anyway
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Re: Changing mass
Reply #27 - 07/01/07 at 01:06:14
 
Yep. Kept the 1000 year mass loss but removed all the other planets. The asteroids all moved out in a fairly orderly manner, got a little jumbled at the end but it's pretty coherent, nowhere near as messy as it was before in the 100 year run. So I really don't think it's resonances keeping the belt coherent.
 
Here's some pictures to prove it Smiley
 
This picture is of the system with just the asteroids, with the 1000 year mass loss. The scale is 165 AU. The inner ring is the initial pre-mass loss layout of the belt, the outer ring is where it all ended up after the mass loss. As you can see, it's nowhere near as crazy as the 100 year loss.
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« Last Edit: 07/01/07 at 02:08:54 by EDG »  

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Re: Changing mass
Reply #28 - 07/01/07 at 01:50:36
 
... and here's a 'timelapse" of the full system.  
 
The left is the configuration at the start - there's three rockies inside the belt and two jovians with about 4 jupiter masses each outside it.  
 
The middle pic shows the full 1000 year evolution while the mass loss was going on - I left the tracks on. you can see how the spirals seem to get bigger as time goes on. At the start the increase in orbital distance is pretty small, but it gets really noticeable in the last 2/3rds of the mass loss.
 
The right pic shows the configuration of the system at the end. Both the gas giants are still there, they're just off screen in wide (and somewhat eccentric) orbits.  
 
It's remarkable how stable this system is despite the mass loss.
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Re: Changing mass
Reply #29 - 07/01/07 at 02:05:03
 
One more - here's a view of the whole system from further out at around 4600 years (3400 years after the mass loss ended), you can see how eccentric the GGs are. It might start to get a bit more interesting, the asteroids are crossing the inner GG (green) orbit now. I think the inner GG orbit is being tweaked a bit by the outer (blue) one.  
 
Anyway, I think all this proves that it's not resonances that are keeping the asteroids "belt-shaped" - it's the increase mass loss time.
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Re: Changing mass
Reply #30 - 07/01/07 at 16:35:09
 
Huh. Well apparently it wasn't so stable after all. I left it running (upped the time interval from 6 day to 12 day though) and when I came back to look at 84,000 years both the gas giants had got ejected from the system and a lot of asteroids had got scattered (there's still a belt, it's just somewhat thinned and messy). The inner planets are fine though. I'm guessing that the 2 GGs had a run-in and in the process tossed eachother out and screwed up the belt. Interesting.
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Re: Changing mass
Reply #31 - 07/02/07 at 10:35:10
 
Right, getting rid of the outer GG fixed it. All stable now Smiley
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Re: Changing mass
Reply #32 - 11/12/07 at 11:30:13
 
Thread restored
There's a few threads that were generating error messages when viewed, including this one.  This thread should work again.
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