# totm oct 2022 DART: Double Asteroid Redirection Test

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16 hours ago, cubinator said:

It sure hit it!

The asteroid system is traveling west-east through the sky, and the spacecraft came at it from the left in this image. So the plume mostly went back towards the direction the spacecraft came from.

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Agree with the numbers up thread.

Asteroid density seems reasonable, so only things I can think of are:

1) Impact velocity is incorrect. Seems unlikely.

2) DART enormously over-penetrated Dimorphos. Most of the energy wasn't transferred.

3) Gravity isn't the most significant component of binding energy for small rubble pile asteroids.

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1 minute ago, RCgothic said:

2) DART enormously over-penetrated Dimorphos. Most of the energy wasn't transferred.

I don't think over-penetration is even remotely feasible. The density of rock/rubble is several times greater than the density of the spacecraft so DART wouldn't have been able to punch more than a meter or so deep.

But it seems that the energy clearly wasn't transferred.

Perhaps the answer is that >99.99% of the kinetic energy was converted into thermal energy at impact due to the high collision speed.

DART is about 1.3 meters wide and would have penetrated less than a meter, so we can take it as a point impactor. You can approximate the energy required to completely crush a volume of rock to powder if you know the compressive strength of the rock, since compressive strength is given in units of pressure (pressure units, force per unit area, are equivalent to energy per unit volume). Brittle material silicate rock usually has a compressive strength on the order of 140 MPa. Do a bit of math and you find that the entire kinetic energy of DART would be sufficient to obliterate about 89 cubic meters of rock . . . a crater about 3.5 meters deep.

Of course, it's a rubble pile, not a solid homogenous silicate rock, so that approximation will only get you so far. But it's potentially a good indicator of how quickly kinetic energy can be dissipated.

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7 hours ago, cubinator said:
8 hours 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.

I took exception to this earlier, but having reviewed it in more detail, I was completely wrong. The velocity of Didymos at periapsis is 34.8 km/s, significantly faster than Earth, and DART was not "catching up" to it; rather, it was catching up to DART. In other words, DART's solar-orbital velocity at impact was lower than Didymos's, not higher.

This also explains why the view on approach looked the way it did. Per NASA, the images shown are mirrored on the x-axis (due to the design of DRACO's camera) and show the ecliptic north toward the bottom. The actual approach image, if corrected for how we would intuit it should be viewed, would look like this:

Dimorphos has a retrograde orbit, so since DART was coming in "against" the orbital direction, it needed to impact Dimorphos while it was on the sunward side of its orbit. This also explains why the right-hand side is illuminated.

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Another thing to consider is the compressibility of fine dust in that environment. Un-weathered grains are quite sharp/jagged, maybe there are mechanical forces holding it together at some level? (obviously distant from the impact itself).

@IonStorm might be be the person to discuss this with.

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25 minutes ago, sevenperforce said:

I took exception to this earlier, but having reviewed it in more detail, I was completely wrong. The velocity of Didymos at periapsis is 34.8 km/s, significantly faster than Earth, and DART was not "catching up" to it; rather, it was catching up to DART. In other words, DART's solar-orbital velocity at impact was lower than Didymos's, not higher.

This also explains why the view on approach looked the way it did. Per NASA, the images shown are mirrored on the x-axis (due to the design of DRACO's camera) and show the ecliptic north toward the bottom. The actual approach image, if corrected for how we would intuit it should be viewed, would look like this:

Dimorphos has a retrograde orbit, so since DART was coming in "against" the orbital direction, it needed to impact Dimorphos while it was on the sunward side of its orbit. This also explains why the right-hand side is illuminated.

The info graphics NASA produced show a 'head on' collision between the two.

Whether it's accurate?

1 hour ago, sevenperforce said:

DART is about 1.3 meters wide and would have penetrated less than a meter, so we can take it as a point impactor

I'm not qualified to duel numbers with you - but this strikes me as wrong.  I googled a bit and I wonder if your number is classic newtonian?

"If the impactor has pushed a mass equal to its own mass at this speed, its whole momentum has been transferred to the mass in front of it and the impactor will be stopped. For a cylindrical impactor, by the time it stops, it will have penetrated to a depth that is equal to its own length times its relative density with respect to the target material.

This approach is only valid for a narrow range of velocities less than the speed of sound within the target or impactor material.

If the impact velocity is greater than the speed of sound within the target or impactor material, impact shock causes the material fracture, and a higher velocities to behave like a gas, causing rapid ejection of target and impactor material and the formation of a crater. The depth of the crater depends on the material properties of impactor and target, as well as the velocity of impact. Typically, greater impact velocity means greater crater depth.

Edited by JoeSchmuckatelli
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8 hours ago, kerbiloid said:

KSP developers say: "no". "No" means "no".

Damn, so much for "knowing everything you could ever want to know".

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Only first post in a thread, but it likely contains valuable information as it progresses given the source.

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Now this is just my uneducated opinion as a biochemist and not a physicist but I suspect more holds asteroids together then gravity, electrostatic forces, mechanical adhesion (bits locking and hooking together), etc.

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2 hours ago, tater said:

Only first post in a thread, but it likely contains valuable information as it progresses given the source.

That was a really good read!

Tango Yankee

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2 hours ago, RuBisCO said:

Now this is just my uneducated opinion as a biochemist and not a physicist but I suspect more holds asteroids together then gravity, electrostatic forces, mechanical adhesion (bits locking and hooking together), etc.

was reading the twitter thread that tater posted and was pleasantly surprised to find out that dust bunnies were part of planetary formation.

also a fine example of why twitter is a grossly ineffective platform for scientific discourse, and most everything else.

Edited by Nuke
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21 minutes ago, Nuke said:

was reading the twitter thread that tater posted and was pleasantly surprised to find out that dust bunnies were part of planetary formation.

also a fine example of why twitter is a grossly ineffective platform for scientific discourse, and most everything else.

If you stick to a few really good follows, stuff can get seriously into the weeds—in a good way. But often a cesspool.

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So, the humans have just killed another innocent robot just to have fun by hitting a random stone far away.

And accidentally destroyed the Dimorphos lunar base which was being built by the mini-civilisation of Didymos for centuries.

A small consolation for them will be the bright ring of ejecta shining in sunlight (and also blocking their equatorial orbits for million years).
Of course when they will be again able to watch the sky after getting from their underground vaults when the ejecta stop falling everywhere around.

Nice.

Edited by kerbiloid
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2 hours ago, kerbiloid said:

So, the humans have just killed another innocent robot just to have fun by hitting a random stone far away.

And accidentally destroyed the Dimorphos lunar base which was being built by the mini-civilisation of Didymos for centuries.

A small consolation for them will be the bright ring of ejecta shining in sunlight (and also blocking their equatorial orbits for million years).
Of course when they will be again able to watch the sky after getting from their underground vaults when the ejecta stop falling everywhere around.

Nice.

The K-Pg extinction event was celestial giants doing a planetary redirection test by chucking stones at the distant orbs.

Hopefully they cancelled their program rather than simply delaying the next test.

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Edit: although I now note notice this isn't the person J McD tagged, so may not be completely authoritative.

Edited by RCgothic
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3 hours ago, RCgothic said:

may not be completely authoritative

I've heard similar, before.  Will be interested in any follow up - but it's certainly not unexpected (even if I did not remember reading such until now).

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The person J McD tagged liked the thread now, so it's probably about right.

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7 hours ago, kerbiloid said:

So, the humans have just killed another innocent robot just to have fun by hitting a random stone far away.

And accidentally destroyed the Dimorphos lunar base which was being built by the mini-civilisation of Didymos for centuries.

A small consolation for them will be the bright ring of ejecta shining in sunlight (and also blocking their equatorial orbits for million years).
Of course when they will be again able to watch the sky after getting from their underground vaults when the ejecta stop falling everywhere around.

Nice.

DART's last thought: "I wonder if it will be my friend?"

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13 hours ago, tater said:

Only first post in a thread, but it likely contains valuable information as it progresses given the source.

This is weird - but that thread was replaced in my browser by odd videos - right up til now when I quoted again to comment.  As I write this, I can see Tater's tweet link in the present quote, but up page it's Draco and Armageddon videos!

Weird!

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On 9/27/2022 at 12:55 AM, Gargamel said:

It's gonna be fuzzy dot... fuzzy dot.... Fuzzy Dot.... FUZZY DOT.... ASTEROID CLOSEUP... lost signal.

This prediction turned out to be exactly right:

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15 hours ago, JoeSchmuckatelli said:

The info graphics NASA produced show a 'head on' collision between the two.

Whether it's accurate?

It's accurate in the sense of a head-on collision. However, it is flipped relative to ecliptic north. This should be readily apparent because Dimorphos orbits Didymos retrograde to the solar orbit.

15 hours ago, JoeSchmuckatelli said:
17 hours ago, sevenperforce said:

DART is about 1.3 meters wide and would have penetrated less than a meter, so we can take it as a point impactor

I'm not qualified to duel numbers with you - but this strikes me as wrong.  I googled a bit and I wonder if your number is classic newtonian?

"If the impactor has pushed a mass equal to its own mass at this speed, its whole momentum has been transferred to the mass in front of it and the impactor will be stopped. For a cylindrical impactor, by the time it stops, it will have penetrated to a depth that is equal to its own length times its relative density with respect to the target material.

This approach is only valid for a narrow range of velocities less than the speed of sound within the target or impactor material.

If the impact velocity is greater than the speed of sound within the target or impactor material, impact shock causes the material fracture, and a higher velocities to behave like a gas, causing rapid ejection of target and impactor material and the formation of a crater. The depth of the crater depends on the material properties of impactor and target, as well as the velocity of impact. Typically, greater impact velocity means greater crater depth.

You're absolutely correct, but that's why I characterized it as a point impactor.

For a hypervelocity impact like this one, the crater depth is not a function of penetration depth, but a function of the energy delivered to the substrate via the impact shock. Basically you can ignore the size and physical characteristics of the impactor and just treat it as an energy source emanating from a single point.

To your ballistic examples from earlier...imagine firing a plastic BB at ballistic gel at such terrific speeds that the BB completely disintegrates on impact. Ordinarily, temporary cavity formation is approximately cylindrical because it is formed by the conical shockwave coming off the bullet as it penetrates the gel. Here, however, there is no penetration, and so the cavity formation is hemispheric from the point of impact.

8 hours ago, RCgothic said:

Edit: although I now note notice this isn't the person J McD tagged, so may not be completely authoritative.

It makes sense, though. Sure, there's a massive amount of energy compared to the gravitational binding energy of the moon, but there's no way to transfer that energy uniformly throughout the moon.

You could almost liken it to the Liedenfrost effect.

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13 minutes ago, sevenperforce said:

Basically you can ignore the size and physical characteristics of the impactor and just treat it as an energy source emanating from a single point.

That certainly explains things like Meteor Crater, AZ.  Will be interesting to see once the dust settles if our models are correct!  The 'streamer' observation mentioned above looks like a strong candidate to develop into a 'new model' for certain classes of asteroid.

Quote

Due to launch in 2024, HERA will follow up to investigate the Didymos binary asteroid, including the first assessment of its internal properties, and to measure in great detail the outcome of NASA’s DART mission kinetic impactor test. HERA will provide precious information for future asteroid deflection missions and science, increasing our understanding of asteroid geophysics as well as solar system formation and evolutionary processes. It is the European contribution to the international double-spacecraft collaboration.

Quote

HERA will have many cutting edge instruments on board, such as cosine’s HyperScout H, and gather crucial scientific data to help scientists and future mission planners better understand asteroid compositions and structures.

Hoping it is a definite "Go" - only thing I wish it could incorporate is a sample return.  We don't have anything that's gone this deep thus far and bringing some chunks back would be good for a few Science points!

Edited by JoeSchmuckatelli
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To finish the witnesses...

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