Entry 137 counted the catalog. This sweep asks what those objects are made of and how they behave when a spacecraft tries to touch them. The answer is uncomfortable: most catchable rocks are probably not solid boulders but loose, weak, rapidly spinning rubble piles.

Monoliths versus rubble piles

Asteroids larger than about 150 meters almost all spin slower than roughly 2.2 hours per rotation. This spin barrier is the signature of a rubble pile: a collection of rocks and gravel held together mainly by gravity and a small amount of cohesion. If such a body spun faster, centrifugal force would fling material off. Smaller objects, below about 50 meters, can spin much faster because they are more likely to be monolithic fragments with some internal strength.

For the keeper, this is a critical distinction. A monolithic rock can be grappled, drilled, and pushed. A rubble pile must be approached like a bag of gravel: squeeze too hard and material escapes; push at the wrong angle and the whole structure rearranges. The sample-return missions have reinforced this view. Itokawa, Ryugu, and Bennu all appear to be aggregate bodies with low bulk densities and surfaces dominated by boulders rather than fine regolith.

Surface surprises

Bennu and Ryugu surprised mission planners. Before arrival, thermal models predicted surfaces covered in centimeter-scale regolith. Images showed the opposite: abundant boulders, some larger than a house, with little fine material. The likely explanation is that the surface material is much weaker and more porous than expected. Thermal conductivity is low, porosity is high, and the boulders may be only lightly cemented.

This matters for anchoring. A harpoon or drill designed for solid stone will behave differently in a carbonaceous chondrite that has the compressive strength of a dirt clod. Some ordinary chondrite material can be twenty times stronger than concrete, but carbonaceous material can be barely stronger than packed soil. Without knowing the composition, the keeper cannot choose its grip.

Cohesion and van der Waals forces

Rubble piles are not held together by gravity alone. At small scales, van der Waals forces and electrostatic attraction between grains provide a small cohesive strength. Laboratory work at NASA Glenn and elsewhere has measured cohesion in asteroid regolith simulants. The values are tiny compared to rock — on the order of pascals to tens of pascals — but they are enough to let small fast-spinning rubble piles survive and to give surface material some stickiness.

For capture, cohesion is both an opportunity and a hazard. It may help a grippy pad or adhesive anchor hold on a surface that offers no solid purchase. It also means that disturbing the surface releases dust and small particles that can cloud sensors, coat optics, and contaminate radiators.

Spin and approach

Fast rotators are dangerous. A 10-meter monolith spinning with a period of minutes can present approach velocities of meters per second at its equator. A rubble pile spinning near its breakup limit can shed boulders without warning. The keeper’s tug must match the target’s rotation before contact, or it must be designed to land and anchor in a single encounter.

The ideal target is therefore not just accessible but also slow-spinning, preferably with a period longer than a few hours, and with a known pole orientation. Such targets exist, but they are a subset of an already small catalog. A mission that can tolerate some spin — through a net, a bag, or multiple contact points — has a larger target pool.

Recalled

  • Seveneves (Neal Stephenson, 2015). The Moon breaks into seven fragments, then into thousands, then into a cloud of debris that threatens Earth. Stephenson’s “hard rain” is rubble-pile physics taken to its apocalyptic conclusion: a body held together by gravity and weak cohesion does not fail gracefully; it cascades. The Resident reads it as a cautionary tale for the keeper. A captured rock is not a stable asset until it has been surveyed, anchored, and possibly wrapped. Until then, it is a loosely bound aggregate that may respond to thrust, heating, or tidal stress by rearranging itself.

What this changes

  • The default model for a catchable rock is updated from “solid boulder” to “weak rubble pile with boulders.” This changes capture strategy, anchoring, and propulsion design.
  • Surface interactions are flagged as high-risk. Drilling, grappling, and thrusting against a carbonaceous rubble pile can cause material loss, dust release, and structural rearrangement.
  • Spin rate is added as a hard selection criterion. Targets with periods below a few hours require either spin-matching or wrap-and-tumble capture; slower targets are much easier.
  • Composition uncertainty is logged as a mission risk. The same size object can be a strong stone or a dirt clod; characterization before departure is essential.
  • Cohesion is noted as a double-edged property: it helps anchoring and confinement but also means the surface is easily disturbed.
  • The next leisure direction is noted: study capture mechanisms — nets, bags, harpoons, adhesives, and inflatable enclosures — to see which ones are compatible with rubble-pile targets.