Asteroid Bennu nearly ate NASA's sampling spacecraft

These articles are making Bennu sound like it could be easily blasted into a dust storm before it gets anywhere near Earth.

But, what if this very loose material is just the outer layer? Do we have density info on Bennu derived from the probe orbiting it? If so, is it consistent with a fluff ball, or a hard ball with fluff on its surface?
 

Catastrophe

"There never was a good war, or a bad peace."
More on Bennu composition:


Imgur: The magic of the Internet


Bennu is what scientists call a rubble-pile asteroid: Rather than a solid block of rock, the little world is essentially a clump of boulders, pebbles and sand, all produced in earlier collisions, that are held together only by gravity. This structure, Bierhaus said, works like "the crumple zone in a car," absorbing many impacts, especially less energetic ones, nearly without a trace.

Cat :)
 
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101955 Bennu - Wikipedia

Mean density1.190±0.013 g/cm3


Cat :)
That is about the density of solid coal. It is about half the density of typical rocks here on earth. See https://www.thoughtco.com/densities-of-common-rocks-and-minerals-1439119 .
So, I am guessing that it is something like 50% voids.

But, at 1,640 feet wide it might contain some sizeable boulders that are hard rock, or maybe even metallic. Because it doesn't have the structural integrity to shove as one piece to change a trajectory toward Earth, I guess the plan would be to blast it into small pieces. But, if there is some dense nugget inside, maybe the original nucleus for the other space debris to have been attracted to, then it might be hard to be sure that was fractured into small enough pieces to spare whatever its trajectory brought it to here on Earth.

And, if most asteroids are rubble piles like this, it might be much harder to deal with ones that are even bigger than Bennu.
 
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Catastrophe

"There never was a good war, or a bad peace."
That is precisely the problem I have been looking at. If it is 50% voids then 'nudging' is not not going to work, and blasting could produce a rain of smaller, but still dangerous, fragments. And it is not only Bennu we might have to worry about. Loosely fragments may even be the majority of objects, since they might not have the 'gravity' to compact them.

Don't forget:
Bennu was discovered on 11 September 1999 during a Near-Earth asteroid survey by the Lincoln Near-Earth Asteroid Research (LINEAR).[3] The asteroid was given the provisional designation 1999 RQ36 and classified as a near-Earth asteroid. Bennu was observed extensively by the Arecibo Observatory and the Goldstone Deep Space Network using radar imaging as Bennu closely approached Earth on 23 September 1999.[25][13]
Source: Wiki. My emphasis.

It is not inappropriate to think about possible, but larger, Bennu 'look alikes'.

Cat :)
 

Catastrophe

"There never was a good war, or a bad peace."
Here are a few references for a start:

(216) Kleopatra, a low density critically rotating M-type asteroid
https://www.aanda.org › articles › full_html › 2021/09


by F Marchis · 2021 · Cited by 11 — Both mass and volume estimates of (216) Kleopatra imply a low density of (3.38 ± 0.50) g cm−3. Such low density for a metallic asteroid suggests the presence ...
NOTE: Kleopatra has two moons which, to me, seems to go with low density - i.e., loosely attracted bodies, not yet incorporated into the loosely aggregated asteroid.

Rubble pile - Wikipedia
https://en.wikipedia.org › wiki › Rubble_pile


Many asteroids with low densities are thought to be rubble piles, for example 253 Mathilde. The mass of Mathilde, as determined by the NEAR Shoemaker mission, ...
The asteroids Bennu and Ryugu have a measured bulk density which suggests a rubble pile internal structure.[1][2] Many comets and most smaller minor planets (<10 km in diameter) are thought to be composed of coalesced rubble.[3][4]
Asteroid Density, Porosity, and Structure
https://www.lpi.usra.edu › AsteroidsIII › pdf

PDF
by DT Britt · 1987 · Cited by 535 — The bulk densities of C-type meteorites, however, fit into two ranges: CI and CM meteorites, characterized by signifi- cant water content and low Fe, have bulk ...
16 pages
. Most asteroids appear to have bulk densities that are well below the grain density of their likely meteorite analogs. This indicates that many asteroids have significant porosity. High porosity attenuates shock propagation, strongly affecting the nature of cratering and greatly lengthening the collisional lifetimes of porous asteroids. Analysis of density trends suggests that asteroids are divided into three general groups: (1) asteroids that are essentially solid objects, (2) asteroids with macroporosities ~20% that are probably heavily fractured, and (3) asteroids with macroporosities >30% that are loosely consolidated “rubble pile” st
Asteroid 21 Lutetia: Low Mass, High Density - Science
https://www.science.org › doi › science.1209389


by M Pätzold · 2011 · Cited by 101 — Unless Lutetia has anomalously low porosity compared with other asteroids in its size range, its high density likely indicates a nonchondritic bulk composition ...
Using the volume model of Lutetia determined by the Rosetta Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) camera, the bulk density, an important parameter for clues to its composition and interior, is (3.4 ± 0.3) × 103 kilograms per cubic meter.
A Low-Density M-type Asteroid in the Main Belt - Science
https://www.science.org › doi › science.1085844


by JL Margot · 2003 · Cited by 70 — The orbital parameters of a satellite revolving around 22 Kalliope indicate that the bulk density of this main-belt asteroid is 2.37 ± 0.4 ...


Comet 67P is fluffy and very low density.
https://slate.com › technology › 2016/02 › comet-67p-is-f...


16 Feb 2016 — Comets are weird. They're similar to asteroids—chunks of very old material orbiting the Sun, rough-hewn and in many ways primordial—but ...
This makes them much less dense than your typical asteroid.
Explosions could likely melt the water, producing something akin to a low density asteroid?

A low-density M-type asteroid in the main belt - PubMed
https://pubmed.ncbi.nlm.nih.gov › ...


by JL Margot · 2003 · Cited by 70 — The orbital parameters of a satellite revolving around 22 Kalliope indicate that the bulk density of this main-belt asteroid is 2.37 +/- 0.4 grams per cubic ...

Density of asteroids - NASA/ADS
http://ui.adsabs.harvard.edu › abs › abstract


by B Carry · 2012 · Cited by 498 — Comets and TNOs have high macroporosity and low density, supporting the current models of internal structures made of icy aggregates. Although


Cat :)
 
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Note the densities of meteorites in this link: https://sites.wustl.edu/meteoritesite/items/density-specific-gravity/ .

Those are the space rocks that make it through our atmosphere and land on the ground in solid form. Clearly, some make craters when they are large, but most small ones end up just thudding down on the ground like free-falling rocks that have lost their cosmic velocities to atmospheric drag on their way to the ground. Those seem to be pretty dense, compare to common earth rocks like granites and shales.

On the other hand, the air blasts at Tunguska and Chelyabinsk are meteors that never made it to the ground and still did major damage. That sounds more like the Bennu situation.

BTW, I did recently see an article about a possible crater at Tunguska. See https://explorersweb.com/siberia-crater-tunguska-meteor/ .

And, on the other hand, there is this: https://astronomy.com/news/2020/10/tunguska-explosion-in-1908-caused-by-asteroid-grazing-earth , which hypothesizes that the Tunguska event was a huge metallic asteroid that skipped back out of the atmosphere, and would have been a much more damaging event at a slightly steeper trajectory.

Seems to me that an actual large "space rock" might be a conglomeration of metallic chunks, carbonaceous chondrites and ices, rather than just one composition such as the alternatives modeled in that last link.
 
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Wiki’s dimensions and mass yield an average density of only ~ 0.5 g/cm3. This seems unusual.

It might be easy to shoot a bomb inside it if it needed to get destroyed.

The next landers should use floaties. ;)
 

Catastrophe

"There never was a good war, or a bad peace."
Helio,
It might be easy to shoot a bomb inside it if it needed to get destroyed.
Yep, that might work, depending on size and composition. Possible danger from getting the explosion the wrong side from Earth, and shooting yourself in the foot, so to speak.

Of course, this method cannot be used for pushing the asteroid aside. This means you have to know the composition and cohesivity well in advance. There are still the problems of guidance to target and placement, whatever its composition.

Cat :)
 
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Helio,

Yep, that might work, depending on size and composition. Possible danger from getting the explosion the wrong side from Earth, and shooting yourself in the foot, so to speak.

Of course, this method cannot be used for pushing the asteroid aside. This means you have to know the composition and cohesivity well in advance. There are still the problems of guidance to target and placement, whatever its composition.
Yes. Having just watched Space Cowboys, a small missile launch pad could send them in as needed.
 
This reminds me of the old joke about the statistician who died while trying to win a bet that he could walk across a river in lead shoes because it had an average depth of only 2 feet - 7' deep the channel got him.

Same with a fluffy rubble pile asteroid. If it is not composed entirely of fine stuff, but instead includes one or more very hard lumps of substantial size, then that/those lumps could still make serious impact trouble for Earth if not sufficiently deflected by a blast designed to disperse the fluffy stuff.

So, it seems to me that what we really want to know is more about the internal structure of every individual asteroid that might eventually strike Earth. That is probably doable for those that we can watch for extended periods after discovery. But, there are sure to be some that are going to hit Earth on the first pass since discovery - and discovery might be not much before impact for outward-bound objects like the one at Chelyabinsk, which seemed to "come out of the sun" like a WWII fighter plane attacking an enemy bomber. It basically announced itself by its impact before we saw it coming.

So, do we need an "overkill" defense that is ready to go with a nuclear warhead guaranteed to bust even the most solid asteroid up to some large size?
 
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Catastrophe

"There never was a good war, or a bad peace."
Good to see that some of what I have been saying here for ages is finally being taken seriously. The Laplacian view of determinism is like the average depth of the river. We are not able to predict, and deal with, every occurrence which might coincide with the orbit of planet Earth. Given the location and speed of every observable asteroid, is not going to cut it.

My old "out of the Sun" argument is being partially ameliorated by the placement of observation sites away from Earth, so prior notice is being improved, but there are some threats totally beyond our control. There are rogue planets, or even rogue stars, not to mention mobile black holes, against which we are totally unable to defend. But without worrying against such extraordinary occurrences, there are much more mundane threats to be overcome.

For one thing, we are likely to be faced with an approaching asteroid (or other object) travelling at high speed, say 100 km per second. We have to send an interceptor to meet it, presumably also at high speed - and then stop our missile by reversing the relative motion to match the incomer. Not only that, but we have to position our vessel prior to the required action - not trivial pursuits. And, these actions are also necessary against stray objects from the Kuiper Belt, or even an Oumuamua.

OK, I freely admit that these occurrences may be highly unlikely, but, if we are faced with such eventualities, there will be no second chances. You may say that tornados or floods are unlikely to affect you, but does that stop you insuring your homes?

Cat :)
 
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