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Message started by Mal on 08/20/08 at 23:26:19

Title: Accelerating an asteroid?
Post by Mal on 08/20/08 at 23:26:19

I want to take a 200,000kg rock (radius 0.05 km) orbiting at 40 AU, and apply a constant 1G acceleration on it toward the sun (don't ask why ;)). Is it possible to program this into GravSim? Or can I only apply a constant thrust to the rock?

Title: Re: Accelerating an asteroid?
Post by Tony on 08/21/08 at 19:12:38


Mal wrote:
...don't ask why ;)


Why?

Yes, you can do this.

menu > Autopilot
Choose "Continuous Orientation" and select "towards" and choose your object in the left dropdown list, and choose the Sun in the right dropdown list.  Click the "Current Time" button.  Then press "Add"
This adds an autopilot command that tells it to always aim the asteroid towards the Sun beginning now.

While still in Autopilot
Choose "Thrust", select your asteroid, place 9.8 in the "meters/second/second" box, and make sure that "initiate" is choosen.  Press the "After Last" button.  Press "Add".

This adds an autopilot command that tells it to begin thrusting at 9.8 m/s/s in 1 second from now.

If you want it to thrust indefinately, then you're finished.  Simply press "OK".  If you want it to thrust for a specified amount of time, then enter the time and date you want the thrust to terminate.  Then choose "Thrust" from the dropdown list.  Select the asteroid, and choose "Cut off".  Press "Add".  Then close the Autopilot by pressing "OK".

I just tried it.  This is a pretty cool simulation.

Title: Re: Accelerating an asteroid?
Post by frankuitaalst on 08/22/08 at 09:13:33

I guess applying this thrust comes as a surplus to the other gravitational forces without replacing them .
Is this correct ?

Title: Re: Accelerating an asteroid?
Post by Tony on 08/22/08 at 11:14:55


frankuitaalst wrote:
I guess applying this thrust comes as a surplus to the other gravitational forces without replacing them .
Is this correct ?

Yes.  It's as if the asteroid has a rocket engine on it, and it aims straight at the Sun, accelerating at a constant 1g.  Where it's going to get the required energy to do this is a mystery.  Mal has some explaining to do. ;D

It's initial orbital velocity is what keeps it from crashing into the sun.  And it can't leave the solar system either, since after it passes the sun, its as if it turns around to continue thrusting towards the sun.  It's actually more interesting if you start it at 1 AU instead of 40 AU.

Title: Re: Accelerating an asteroid?
Post by Mal on 08/23/08 at 18:00:27

Thanks, I'll give it a go later!

And it's in part because of an argument about using rocks as weapons in the Traveller RPG universe. The idea is that you stick a reactionless drive on an asteroid (which in Traveller don't appear to use any fuel. Go figure), then have it accelerate constantly at 1G to hit a planet closer to the star. Because there's no fuel limit, you can potentially just keep accelerating it (not necessarily at 1G, you can accelerate it up to 6G) until you reach a significant fraction of the speed of light. If that hits the planet, then that makes for a very big boom. :)

I just wanted to see how it'd look in practice - one thing I didn't get was how the orbital velocity affected the inward acceleration.

Title: Re: Accelerating an asteroid?
Post by Mal on 08/24/08 at 10:28:10

Heh, that's awesome - it keeps missing :) - this is what it looks like after 20 months - the pink trails are the asteroid that keeps coming back to try to hit Earth :).

Title: Re: Accelerating an asteroid?
Post by Mal on 08/24/08 at 10:36:49

And here it is 7 years from the start :)

Title: Re: Accelerating an asteroid?
Post by Mal on 08/24/08 at 10:57:05

Same thing, but the asteroid starts at 5 AU, not 40 AU. This is after 2 full years of simulation time - it's a very pretty pattern, but it's still missing the Earth :)

In all cases the rock's initial orbit around the sun is eccentricity 0.1, inclination 0 - would changing that make any difference at all?

Title: Re: Accelerating an asteroid?
Post by Mal on 08/24/08 at 11:00:18

by the way, Tony - is the thrust really acceleration? It just says "metres/second" on my version of Autopilot, not "metres/second/second" (I'm using the 27th June 2007 Beta here... is there a newer version?)

Title: Re: Accelerating an asteroid?
Post by Mal on 08/24/08 at 11:08:50

Rock starting at 5 AU, with 6G acceleration toward planet. Still missing...! (after 1 year of simulation time)

Title: Re: Accelerating an asteroid?
Post by frankuitaalst on 08/25/08 at 09:37:42

Amazing pictures Mal . Is it still missing after all that time ?
I wonder what speed the rock gets , because after several months at 1g it should be close to lightspeed !
In that case , at such a speed the rock passes just through another body without the programm even notices a collision . Reducing the timestep may help ?

Title: Re: Accelerating an asteroid?
Post by Mal on 08/25/08 at 10:22:40


frankuitaalst wrote:
Amazing pictures Mal . Is it still missing after all that time ?
I wonder what speed the rock gets , because after several months at 1g it should be close to lightspeed !
In that case , at such a speed the rock passes just through another body without the programm even notices a collision . Reducing the timestep may help ?


Well the 6G rock was on a timestep of 1...  [smiley=shocked.gif]

Title: Re: Accelerating an asteroid?
Post by Tony on 08/25/08 at 14:18:38

It will not approach the speed of light because the thrust is not continuously in the same direction.  After it passes the sun, it turns around to face the sun again, and slows back down to 0.

Title: Re: Accelerating an asteroid?
Post by frankuitaalst on 08/25/08 at 14:40:13

You're right . I did some calculation and it takes about one year to reach c at 1g
. Distance travelled is then more than 10^15 meters .
However starting from 40 AU the speed may already be enormeous.

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