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 '20 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. $\endgroup$ – Cornelisinspace Sep 13 '20 at 16:24
  • $\begingroup$ Oh, I missed that completely, thanks! $\endgroup$ – uhoh Sep 13 '20 at 17:12

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.

  • $\begingroup$ @uhoh casualty of a last-minute edit, it did end up rather incoherent. $\endgroup$ – Christopher James Huff Sep 14 '20 at 0:32
  • $\begingroup$ Thanks, looks much better! Is it possible to say if this is mostly a "yes" or a "no" answer? $\endgroup$ – uhoh Sep 14 '20 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. $\endgroup$ – Christopher James Huff Sep 14 '20 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 ? $\endgroup$ – Cornelisinspace Sep 14 '20 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 '20 at 8:45

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. $\endgroup$ – Christopher James Huff Sep 14 '20 at 18:35

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