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DART: Double Asteroid Redirection Test


Ultimate Steve
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Also: CtTodAVeLtFseW8csHZSY8-1200-80.png

Looks like once the dust has settled (pun intended) we should get quite a lot of info 

Fun fact: if you Google NASA Dart - Google gets hit by a satellite and knocked off course! 

https://twitter.com/VirtualTelescop/status/1574551231267147776/photo/1

 

Edited by JoeSchmuckatelli
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11 hours ago, cubinator said:

I think this video should be rotated 180 degrees to show the proper path. The Didymos/Dimorphos system is orbiting the sun in the same direction as Earth, of course, but it is orbiting at a greater distance and thus its apparent motion relative to the fixed stars is from left to right, as viewed from the northern hemisphere. And the ejecta cloud, also, would have been moving from left to right since the impact was from right to left. 

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27 minutes ago, sevenperforce said:

I think this video should be rotated 180 degrees to show the proper path. The Didymos/Dimorphos system is orbiting the sun in the same direction as Earth, of course, but it is orbiting at a greater distance and thus its apparent motion relative to the fixed stars is from left to right, as viewed from the northern hemisphere. And the ejecta cloud, also, would have been moving from left to right since the impact was from right to left. 

But the system is near periapsis of a more elliptical orbit, so it could still be going faster than Earth.

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6 hours ago, mikegarrison said:
11 hours ago, tater said:

 

www.google.com/search?q=nasa+dart

I was all ready to put my moderator hat on and yell at you guys for being jerks to such a simple question further up the thread.   Well played Google.   That’s cool. 
 

But let’s do keep it on topic please. 

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25 minutes ago, cubinator said:
53 minutes ago, sevenperforce said:

I think this video should be rotated 180 degrees to show the proper path. The Didymos/Dimorphos system is orbiting the sun in the same direction as Earth, of course, but it is orbiting at a greater distance and thus its apparent motion relative to the fixed stars is from left to right, as viewed from the northern hemisphere. And the ejecta cloud, also, would have been moving from left to right since the impact was from right to left. 

But the system is near periapsis of a more elliptical orbit, so it could still be going faster than Earth.

Then we would see the the ejecta cloud moving from left to right in the video.

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I'm starting to think we shattered it.  Ringed asteroid at some point? 

Edit - perhaps not:

7GnpxdxC6eBjRQMjS2ZcxA-1200-80.png

This image indicates that there was likely a good bit of spalling from the impact - but the central mass is still mostly together 

Edited by JoeSchmuckatelli
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46 minutes ago, JoeSchmuckatelli said:

I'm starting to think we shattered it.  Ringed asteroid at some point? 

To a first-order approximation, the gravitational binding energy of Dimorphos is 1.18e7 J. 

At impact, DART had a mass of 570 kg and a relative impact speed of 6.6 km/s, giving it a total kinetic energy of 1.24e10 J.

Wait, what?

Here's another view from an Earth-based telescope:

 

 

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9 minutes ago, sevenperforce said:

the gravitational binding energy of Dimorphos is 1.18e7 J. 

At impact, DART had a mass of 570 kg and a relative impact speed of 6.6 km/s, giving it a total kinetic energy of 1.24e10 J.

Ah mite nawt be a maths guy, but one of those thangs looks bigger than t'other.

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5 minutes ago, JoeSchmuckatelli said:
15 minutes ago, sevenperforce said:

To a first-order approximation, the gravitational binding energy of Dimorphos is 1.18e7 J. 

At impact, DART had a mass of 570 kg and a relative impact speed of 6.6 km/s, giving it a total kinetic energy of 1.24e10 J.

Ah mite nawt be a maths guy, but one of those thangs looks bigger than t'other.

There's some variation in the impact mass of DART (I've seen a range from 500 kg to 570 kg) and the stated impact speed (I've seen 5.95 km/s to 6.6 km/s) but still, we are dealing with three orders of magnitude here.

Either my numbers are wrong, or Dimorphos got obliterated.

Here's a NASA tweet which seems to confirm these numbers.

To a first-order approximation, GBE = (3*G*M2)/(5*R).

3 * 6.67e-11 N*m2/kg2 * (4.8 billion kg)2 / (5 * 80 m) = 1.15e7 Joules.

Surely I'm missing something here.

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26 minutes ago, sevenperforce said:

Either my numbers are wrong, or Dimorphos got obliterated.

I'm guessing that might have to do with the actual density.   I do have experience shooting a lot of stuff... and if it's actually a clumpy gravel pile loosely held together by gravity, it's possible it got cored out.  You can look at images of people shooting snowmen or concrete blocks (both aggregates) and see what I'm describing.

In fact, my intuition tells me the greater the mis-match between the impact energy and the 'hold it together' energy, the more likely it is that the 'round' overpenetrated (leaving a hole).

It could have cracked it; much like a bullet will with an apple or something, but given that gravity is there to pull everything back together; if I'm right... the cored out spalling and other ejecta that has escape velocity will spread out, but the other stuff will just clump back together again, eventually.

Edited by JoeSchmuckatelli
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6 minutes ago, JoeSchmuckatelli said:

I'm guessing that might have to do with the actual density.   I do have experience shooting a lot of stuff... and if it's actually a clumpy gravel pile loosely held together by gravity, it's possible it got cored out.  You can look at images of people shooting snowmen or concrete blocks (both aggregates) and see what I'm describing.

In fact, my intuition tells me the greater the mis-match between the impact energy and the 'hold it together' energy, the more likely it is that the 'round' overpenetrated (leaving a hole).

I feel like that's more likely when you are using a projectile which is many times more dense than the target, like lead.

Per Newton's approximately for kinetic impactor penetration depth, the penetration of an impactor is independent of velocity and is a function of the impactor length and the relative densities of the impactor and the target.

DART's physical dimensions (box only, not including solar panels or other protrusions) are 1.2m * 1.3m * 1.3 m, giving it a total volume of 1.87 cubic meters and thus a density of approximately 300 kg/m3.

Using the numbers from NASA's tweet above, Dimorphos has an approximate/average density of 4.8 billion kg / 2.14e6 cubic meters or 2,243 kg/m3. That's about what we would expect for the density of a gravel pile. But this means we would not expect DART to penetrate significantly more than its own body length.

Is it under-penetration, rather than over-penetration, that would prevent Dimorphos from simply exploding? Does all of the impact energy get concentrated into a tiny region, blowing all that to atoms and flinging it out at terrific velocity but failing to deliver the energy to the object itself?

 

 

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30 minutes ago, JoeSchmuckatelli said:

I'm guessing that might have to do with the actual density.   I do have experience shooting a lot of stuff... and if it's actually a clumpy gravel pile loosely held together by gravity, it's possible it got cored out.  You can look at images of people shooting snowmen or concrete blocks (both aggregates) and see what I'm describing.

In fact, my intuition tells me the greater the mis-match between the impact energy and the 'hold it together' energy, the more likely it is that the 'round' overpenetrated (leaving a hole).

It could have cracked it; much like a bullet will with an apple or something, but given that gravity is there to pull everything back together; if I'm right... the cored out spalling and other ejecta that has escape velocity will spread out, but the other stuff will just clump back together again, eventually.

The impact created an detonation at the surface. The easiest past for the shock wave is into vacuum not into the rubble. Yes you will get some penetration because of momentum but not a lot of meters. Compare this to putting an stick of dynamite on top of an pile of gravel or at the bottom of it. 
Makes me wonder how an tungsten slug would behave hitting an gravel pile at 6 km/s. My guess that it would not penetrate very deep 

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1 minute ago, magnemoe said:

The impact created an detonation at the surface. The easiest past for the shock wave is into vacuum not into the rubble. Yes you will get some penetration because of momentum but not a lot of meters. Compare this to putting an stick of dynamite on top of an pile of gravel or at the bottom of it.

But with three orders of magnitude at play, even if only 0.11% of the kinetic energy actually ended up pointed into the rubble pile, you've still got full delivery of the gravitational binding energy.

1 minute ago, magnemoe said:

Makes me wonder how an tungsten slug would behave hitting an gravel pile at 6 km/s. My guess that it would not penetrate very deep 

My guess is that it would penetrate roughly nine times its length, since it's about 9 times as dense as a rubble pile asteroid.

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35 minutes ago, sevenperforce said:

Is it under-penetration, rather than over-penetration, that would prevent Dimorphos from simply exploding? Does all of the impact energy get concentrated into a tiny region, blowing all that to atoms and flinging it out at terrific velocity but failing to deliver the energy to the object itself?

I've seen that with pistol rounds into concrete blocks.  The round doesn't overpenetrate, but it can cause significant spalling out the back. 

We have a high likelihood from the last few photos that the craft hit some fairly big, solid boulders.  We can expect, at a minimum, that the individual boulders should absorb some energy and transfer some to their adjacent members of the aggregate; kind of like when you break the racked balls with the cue.  I'm going to guess that a lot of the energy will be transformed to heat, some to rotation and others to direct kinetic movement of adjacent members.  Given that the impact was highly directional and localized; it just seems likely to me that the craft's damage/penetration is highly analogous to a bullet - and while that's an enormous variable - given the pictures we've seen I think it's safe to say that there was both cratering ejecta and spalling ejecta.  Neither require the craft to overpenetrate, as you point out, but it could have given the density.

cNAxeIcQ=&risl=&pid=ImgRaw&r=0

Figure-9-4-300x300.jpg

This is the damage path of a bullet in meat and ballistic gel - but it also describes how I think the energy would dissipate through an aggregate. 

Edit: both of those show over-penetration.  You can analogize a bb into a window for the non-penetration spalling.

13 minutes ago, sevenperforce said:

you've still got full delivery of the gravitational binding energy.

Is that dependent on looking at the asteroid as a solid?  Is it possible to blow out the core and have the rest shift, then collapse back on itself / themselves, given the already considerable inertia of the whole?

Edited by JoeSchmuckatelli
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