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Bennu is a carbonaceous asteroid that is currently accompanied by the OSIRIS-REx spacecraft that will take samples from it to return to Earth.
Its diameter is 490 m. and it is slightly denser than water and therefore the predicted macroporosity is about 40 %, suggesting its interior has a rubble pile structure.
Bennu's orbit around the Sun, with a period of 1.2 year, is close enough to that of Earth to label it a potentially hazardous object.
From Evidence for widespread hydrated minerals on asteroid (101955) Bennu:

Early spectral data from the Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) mission reveal evidence for abundant hydrated minerals on the surface of near-Earth asteroid (101955) Bennu in the form of a near-infrared absorption......

(Emphasis by me.)

According to the Wikipedia article about Bennu its surface temperature changes between -37 and +6 °C, and compared with the lower than -200⁰ C temperatures measured in the cold traps near the south pole of the Moon, mining the ice there would be much more difficult then extracting the water from the loose material of Bennu.
Furthermore on Bennu energy from the Sun is easily available while near the lunar south pole many miles seperates the rims with sunlight from the mining places in eternal darkness.

A big disadvantage will be that because of Bennu's orbital period of 1.2 year, only once in about 6 years Bennu and the Earth will be in close proximity, but that could be compensated for example by bringing enough tanks with water from Bennu into orbit around the Earth.

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    $\begingroup$ I wonder if the fact that the surface temperature reaches +6 °C mean that there should be no water near the surface any more, and mining would have to throw a lot of material out of the way in order to get to subsurface water? $\endgroup$
    – uhoh
    Sep 13, 2020 at 15:34
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    $\begingroup$ @uhoh According to the article it seems the hydrates are mostly in the phyllosilicates, I don't know how high the temperature would have to be to release the water from them. $\endgroup$
    – Cornelis
    Sep 13, 2020 at 16:24
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    $\begingroup$ It isn't clear what use the water is intended for. Without that it probably can't be answered. For use in some kind of mining operation at Bennu (H2 + O2 for rocket fuel?) then water obtained from the moon will be a hugely expensive option - and vise versa. $\endgroup$
    – Ken Fabian
    Jul 17 at 0:41
  • $\begingroup$ @KenFabian You're right, so I've expanded the question somewhat. $\endgroup$
    – Cornelis
    Jul 18 at 8:18

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That's a really complex question, I would say that in the way you phrased the question, no it's not, most of the time... Let me explain.

First, to simplify, you can roughly translate how "economical" doing something in space is to how much energy you need to do it (Bigger ship, more fuel, bigger solar panels, etc).

With that in mind, Bennu has the characteristic that it orbits the Sun the same way the Earth and Mars do, but its orbit is more eliptic, so it intersects Earth's orbit. Bennu Orbit

The Earth needs the good old ~365 days to complete a full revolution around the Sun while Bennu takes ~436 days, so if by any chance both are at the same point in their orbits at the right moment, an encounter is possible, but this would happen once every few decades, and overall they will most likely hardly ever meet.

This is relevant in several ways. The first one is that to get any type of equipment, machines, mining infrastructure, to Bennu, you need to first accelerate them to escape velocity from Earth, put them on an intersect trayectory with Bennu, wherever it happens to be in its orbit, and then decelerate to rendezvous. (Or alternatively, travel several years in a complicated mission profile the same way Osiris Rex did, which is highly impractical) Osiris Rex Mission Profile. So that would make putting the equipment on Bennu more expensive than putting it on the Moon, your best bet would be to wait for the asteroid to pass near the Earth, but that could take years, and you would still need to match Bennu's speed and trajectory for rendezvous. So you find yourself in a scenario with very limited time windows and very long waiting periods. For that reason alone the Moon is a way better option, its way easier to reach.

Now, let's assume that the equipment is already there, mining an asteroid would be way easier due to the low gravity, however you have the issue of maintaining those facilities, again, any spare parts or new machinery will have the same logistc issues as before, which would discourage any investment for such a mission.

But even assuming you have everything in place and working nicely, the original reason to do that, as stated in your question, is traveling to Mars. For such a trip to be feasible, you need the Earth, Bennu and Mars to line up correctly in a very specific way, otherwise a ship would need to chase Bennu, rendezvous with it, and then plot a new trajectory towards Mars from wherever Bennu is. That manoeuvre would be extremely inefficient the vast majority of the time (I suspect all of the time), and would require way more fuel than simply stopping (or launching) at the Moon. Additionaly it would depend on the positions of 3 bodies instead of 2. Since the Moon is inside Earth's gravity well, using the Moon does not add any of that extra complexity.

This is not to say that having a refuel (or refill) station at Bennu is not a good idea. Once a high enough number of spacecrafts are travelling through the solar system on a regular basis, refuelling station on asteroids and some specific and weird orbits would probably become a common thing since then you would have enough people using routes where such a stop becomes a convinient thing to do.

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  • $\begingroup$ A well explained, comprehensive answer, accepted ! $\endgroup$
    – Cornelis
    Jul 19 at 8:07
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The only reason near-Earth asteroids still have water is that it's bound up in hydrated minerals where it's become part of the crystal structure of those minerals. Freeing it from those minerals requires baking it out of at high temperatures (up to around 500 °C), the most easily extractable volatile ices are all long gone due to heat from the sun. In contrast, the moon's polar craters (and asteroids that have stayed further out from the sun) have actual ices, which at low pressures only need to be heated to a little under the freezing point to sublimate. The lunar ices also have other volatiles that would be of use.

And while the delta-v needed to reach Bennu is low for an asteroid, it takes around 5-6 km/s (going by the trajectories listed at https://ssd.jpl.nasa.gov/?mdesign_server&sstr=bennu). It's on par with the moon in delta-v cost, and actually harder to reach than Mars because it doesn't have an atmosphere to decelerate you when you get there.

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  • $\begingroup$ Thanks, looks much better! Is it possible to say if this is mostly a "yes" or a "no" answer? $\endgroup$
    – uhoh
    Sep 14, 2020 at 1:12
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    $\begingroup$ I'd say it's a "the laws of physics involved work against you". Mostly no, but maybe it'd be useful to mine it and ship it somewhere other than Earth orbit. $\endgroup$ Sep 14, 2020 at 2:47
  • $\begingroup$ What would count is the delta-v needed to ship the water from Bennu, not so much the empty tanks that would have to be send to the asteroid. Maybe Phobos will be a good destination, to make rocket fuel there ? $\endgroup$
    – Cornelis
    Sep 14, 2020 at 8:24
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    $\begingroup$ Keep in mind water mined on the moon is close to where it needs to be, water mined on Bennu would need to be transported, which will add significant costs. $\endgroup$
    – GdD
    Sep 14, 2020 at 8:45
  • $\begingroup$ @Cornelisinspace shipping from Bennu to Phobos? Apart from the possibility that Phobos itself might contain large amounts of actual ice (a proposed explanation for its low density), that'd be higher delta-v than the 5-6 km/s needed to send it back to Earth. It'd be considerably cheaper to launch it from Mars. $\endgroup$ Sep 14, 2020 at 18:05
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In contemplating your question, I searched our site using the [asteroid] and [mining] tags and saw the question: Mining in microgravity: are there sound studies?. It has an answer and I read it and the associated links to both the question and the answer.

In all the questions on this site relating to the mining of asteroids I haven't found an answer which gives me confidence that there is information in the public domain that provides definitive practical answers on how asteroids could be mined.

There's a lot of general big picture descriptions of potential mining methods, nothing else.

Most asteroids are less than 1000 m in diameter. Many asteroids are thought to be rubble piles, including Bennu.

No practical mining method for mining a rubble pile in microgravity, whether loose or weakly cemented by ice or another medium, has yet been made public. Subsequently, the cost of mining a cosmic rubble pile is unknown. The cost of getting to the rubble pile can be determined, but extracting the required resource from it is unknown.

Because the mining method is still unknown, the type of mining equipment required and the cost of its manufacture and operation are also unknown.

Disintegration of a rubble pile (falling apart) would be possible if it is mined incorrectly.

Mining anything on the Moon has fewer unknowns. We know how to get there and what equipment is required to get there. We also have a good idea of the technologies required to extract water from the lunar south pole and thus we have a better idea of the costs involved.

Based on what information is in the public domain, mining water from the Moon is what we have the most knowledge about.

Asteroids may be full of useful or valuable resources but until there is a practical method for mining asteroids, with associated costs, so that those resources can become the equivalent of reserves, mining asteroids is still pie in the sky.

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    $\begingroup$ "We also have a good idea of the technologies required to extract water from the lunar south pole..". Can you refer to that idea of required technologies ? Since mning there is still an idea, isn't that also composed of "a lot of general big picture descriptions of porenrial mining method(s) " ? $\endgroup$
    – Cornelis
    May 23, 2021 at 16:45
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From Tons of Water in Asteroids Could Fuel Satellites, Space Exploration:

But asteroids will certainly be more accessible than the moon, another potential source of space-based water-rich minerals. According to Rivkin, landing safely on the lunar surface takes more than a hundred times the change of velocity required to land on an asteroid. Similarly, taking off from the moon means breaking free from its gravity,requiring even more fuel. "Even asteroids that are a bit farther from the Earth than the moon can be reached with less fuel than the lunar surface," Rivkin said.

So according to Andrew Rivkin, an asteroid researcher at Johns Hopkins University Applied Physics Research Laboratory in Maryland, mining an asteroid like Bennu would be less expensive than mining on (the south pole of) the Moon.

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    $\begingroup$ Only looking at the landing delta-v is misleading. In the particular case of Bennu, starting from LEO, just reaching the asteroid takes as much delta-v as both traveling to and landing on the moon. $\endgroup$ Sep 14, 2020 at 18:35
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“The only reason near-Earth asteroids still have water is that it's bound up in hydrated minerals”

Water in Ryugu: Brine of CO2, Sulfides, etc...

https://www.hou.usra.edu/meetings/lpsc2022/pdf/1451.pdf

https://www.hou.usra.edu/meetings/lpsc2022/pdf/2189.pdf

There’s also a follow-up abstract for the MetSoc2022 meeting.

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    $\begingroup$ First, this appears to be a comment posted as an answer. Second, the existence of traces of water trapped in micron-scale mineral grain inclusions doesn't change my point. The dry "waste' material left after baking off water of hydration will still probably have more water of hydration than those inclusions contain. Their importance is scientific, not economic. $\endgroup$ Jul 16 at 16:08
  • $\begingroup$ Valuable information, but Bennu is distinct from Ryugu. We'll have to wait for the conference papers about the former. $\endgroup$
    – Cornelis
    Jul 18 at 8:07
  • $\begingroup$ We have the papers about the former. Bennu is distinct from Ryugu in that it is richer in water. $\endgroup$ Jul 19 at 11:45
  • $\begingroup$ “doesn't change my point.” No, your point changes your point, because you have clearly not read your Jewitt. You have not been following Nuth. You have not read much Abreu. You have not kept up with Campins. Schorghofer had a very interesting paper, as did Snodgrass. You have not been following Chan. You missed, along the way, most of Meech, and Hsieh as well. You clearly don’t read Rivkin, and while we’re at it Alexander would also be relevant. A research group, quasi-affiliated and quasi-separate from OSIRIS-REx, published what you are not even aware of : $\endgroup$ Jul 19 at 12:04
  • $\begingroup$ “Freeing it from those minerals requires baking it out of at high temperatures (up to around 500 °C), the most easily extractable volatile ices are all long gone due to heat from the sun.” You are unaware that your statement is pre-2001 information. It is certainly pre-2006, and happens to be pre-2013 or so as well. The ‘bakeout temperature’ for Bennu material has been given as… 30 °C, for reasons you are unaware of. Volatiles are not “all long gone” (per multiple research groups), for reasons you are unaware of. You are unaware that you are unaware. I suppose I could bring you up to $\endgroup$ Jul 19 at 12:04

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