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Still trying to figure out the physics behind DART
Posted by: pdq
Date: October 14, 2022 12:12PM
Okay, so there is one rock (or pile of rubble gravitationally bound together) orbiting another (or more properly, they’re orbiting each other). And then you smash something into one of them (the smaller one).

So, did they slow Dimorphos with DART, or speed it up? The news coverage seemed to play it like two trains hitting each other head on, which should slow it down (ie reduce its angular momentum). But then it seems like maybe that would cause it to spiral in toward the larger Didymos, speeding up Dimorphos until it reached a new, tighter and faster stable orbit.

In fact, they did observe a faster orbit, shaving some 30 minutes off the previous time. That sounds like it sped up…but is that because they slowed it down first, with the impact? Or do I have this all mixed up?

A little knowledge is a dangerous thing…
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Re: Still trying to figure out the physics behind DART
Posted by: Paul F.
Date: October 14, 2022 12:16PM
As I understand it...

They slowed it down, which then made it go into a smaller diameter orbit, which then meant that it covers that smaller circumference in less time. So, it's slowed down, but, it's doing smaller laps around Didymos now, so, the orbital period is faster.

In fact, it's considerably faster than projected... meaning the SPLAT of DART and the ejecta leaving the asteroid had more effect than predicted.


Paul F.
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Re: Still trying to figure out the physics behind DART
Posted by: JoeH
Date: October 14, 2022 12:21PM
One way of looking at this is that the impact increased the speed along one vector, the direction of that vector is important relative to the orbit. The overall effect was to decrease the overall distance between the two and speed up the orbital period.
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Re: Still trying to figure out the physics behind DART
Posted by: modelamac
Date: October 14, 2022 12:32PM
They did not hit the astroid head on. It's like bird hunting… They hit a moving target and moved it into a tighter, faster orbit. The orbit is faster because it travels a shorter distance.

Quote
pdq
Okay, so there is one rock (or pile of rubble gravitationally bound together) orbiting another (or more properly, they’re orbiting each other). And then you smash something into one of them (the smaller one).

So, did they slow Dimorphos with DART, or speed it up? The news coverage seemed to play it like two trains hitting each other head on, which should slow it down (ie reduce its angular momentum). But then it seems like maybe that would cause it to spiral in toward the larger Didymos, speeding up Dimorphos until it reached a new, tighter and faster stable orbit.

In fact, they did observe a faster orbit, shaving some 30 minutes off the previous time. That sounds like it sped up…but is that because they slowed it down first, with the impact? Or do I have this all mixed up?

What scientific authority saids they first slowed it down? It was not hit head on. It is like bird hunting...they hit a moving target and the impact moved it into a tighter, faster orbit.

A little knowledge is a dangerous thing…


Ed (modelamac)

Trying to figure out some people is like trying to smell the number 9.



Edited 2 time(s). Last edit at 10/14/2022 12:37PM by modelamac.
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Re: Still trying to figure out the physics behind DART
Posted by: N-OS X-tasy!
Date: October 14, 2022 01:08PM
Quote
modelamac
They did not hit the astroid head on. It's like bird hunting… They hit a moving target and moved it into a tighter, faster orbit. The orbit is faster because it travels a shorter distance.

Does that mean the object's angular velocity was unchanged as a result of the impact?


It is what it is.
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Re: Still trying to figure out the physics behind DART
Posted by: pdq
Date: October 14, 2022 03:39PM
Quote
N-OS X-tasy!
Quote
modelamac
They did not hit the astroid head on. It's like bird hunting… They hit a moving target and moved it into a tighter, faster orbit. The orbit is faster because it travels a shorter distance.

Does that mean the object's angular velocity was unchanged as a result of the impact?

Again, using little knowledge (a dangerous thing), I would guess its angular velocity must have changed, since the masses are (almost) the same as before, and now the orbit is smaller/tighter.
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Re: Still trying to figure out the physics behind DART
Posted by: gadje
Date: October 14, 2022 03:50PM
It's all relative.
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Re: Still trying to figure out the physics behind DART
Posted by: Filliam H. Muffman
Date: October 14, 2022 07:32PM
I'm not great at Physics equations, but for the period to be shorter, the orbit needs to be lower. Lower the velocity of the smaller partner and it will lose potential energy and accelerate, gaining back energy and eventually achieve a higher velocity.

Channel: PhysicsHigh
Orbital Velocity Explained
https: //youtu.be/vbD1fNiWWfo


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Re: Still trying to figure out the physics behind DART
Posted by: pdq
Date: October 15, 2022 10:14AM
So, it sounds like the general consensus is, slow it down to speed it up?

Counterintuitive on its face, but I think that's right.
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