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Surface of Bennu

Credits: NASA/Goddard/University of Arizona

The image above is part of a close-up acquired by NASA's OSIRIS-REx spacecraft of Bennu's surface.

101955 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 has a density of about 1.19 g/cm³ and therefore the predicted macroporosity is about 40 %, suggesting its interior has a rubble pile structure.

Measurements from the MASCOT lander, part of the Hayabusa2 mission, have indicated that boulders on the C-type asteroid 162173 Ryugu have high porosities and low tensile strengths in the few hundred kPa range.
Bennu is a B-type asteroid, falling into the wider C-group, and spectroscopy suggests major surface constituents of anhydrous silicates, hydrated clay minerals, organic polymers, magnetite, and sulfides.

Here's what I think at the moment:

Because it can be expected that the hydrated clay minerals and the organic polymers are better preserved below the surface, protected from radiation of the Sun, the subsurface of Bennu is likely to be more cohesive.

With Bennu having a surface gravity of only 6 micro-g, I'm guessing that it might be easy for an astronaut to dig deep below the surface with first the brittle rocks and grains and then the more cohesive material.
Taking an average bulk density of 2000 kg/m³ for CI and CM meteorites, a one cubic meter piece of rock would "weigh" 12 grams so lifting up and removing rocks and grains should be no problem.
But to cleave larger boulders (with tensile strengths about 2 x higher than brick), to cut into the cohesive grain material, and to operate within the dug shaft one will need the right equipment.

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    $\begingroup$ How would the astronaut be affixed to the surface to react the digging forces? Shoving the spade in would send them into space. $\endgroup$
    – Organic Marble
    Oct 3 '20 at 21:00
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    $\begingroup$ Having dug in rubble with a shovel, 1 metre per 5 minutes sounds much to fast, by at least an order of magnitude. $\endgroup$
    – SE - stop firing the good guys
    Oct 3 '20 at 21:12
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    $\begingroup$ A highly relevant paper on the requirements of digging in lunar regolith: link. I think it should provide you with the missing parameters to verify your own estimations. $\endgroup$
    – Ruben
    Oct 3 '20 at 21:19
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    $\begingroup$ Anything done in zero-g (and a small asteroid is close enough) is much harder than you would expect. $\endgroup$
    – Loren Pechtel
    Oct 4 '20 at 3:48
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    $\begingroup$ I guess I'm a bit surprised that the first tool selected isn't a auger of some kind. It would still need a restraint system to get started but after that it's self-restraining. $\endgroup$
    – Carl Witthoft
    Oct 6 '20 at 11:47
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The most obvious type of equipment required will be some form of restraint system.

While, for some reason, studies of hand-shoveling a tunnel to the center of an asteroid are not appearing in my search results, there have been numerous studies about EVA restraints for small bodies.

This robot arm and foot restraint system, similar to those used on the Space Shuttle and the International Space Station, was studied for use on Phobos.

enter image description here

The paper A Geology Sampling System for Small Bodies has some relevant information.

Body stabilization will be a key enabler of successful geology sampling activities on small bodies. Experience on the International Space Station (ISS) has highlighted the need for infrastructure to react loads during various tasks giving the crew a stable work platform.

It discusses a similar boom/foot restraint system, shown here being trialled underwater.

enter image description here

Other types of restraints could be conceived of - tethers with ground anchors, etc. The trick would be making them easily relocatable as the astro-navvy burrows their way into the ground.

Sources

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    $\begingroup$ Looking at figure 2, are you expecting that much water inside Bennu ? Also, the second link doesn't work for me. $\endgroup$
    – Cornelis
    Oct 4 '20 at 16:56
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    $\begingroup$ @Cornelisinspace "shown here being trialled underwater." Neutral buoyancy is a common stand-in for free fall on the Earth's surface. Fixed the link, thanks for pointing that out. $\endgroup$
    – Organic Marble
    Oct 4 '20 at 16:58
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    $\begingroup$ @Cornelisinspace Neutral buyoaancy is one of the three often used ways to simulate lower gravity on a body like earth, see also this Q&A: space.stackexchange.com/questions/38647/… $\endgroup$
    – Polygnome
    Oct 4 '20 at 18:21

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