Entry 138 established that catchable rocks are often weak rubble piles. This sweep asks how you actually grab such an object without destroying it or yourself.

The design space

Capture mechanisms fall into five broad categories, as NASA’s Asteroid Retrieval Mission studies summarized: grabbing, drawing, agitating, adhering, and lofting. Grabbing surrounds or grasps the object. Drawing pulls material from the surface. Agitating shakes material loose. Adhering uses sticky or electrostatic collectors. Lofting lifts and aggregates surface material. For the keeper’s purpose — moving an entire small body — grabbing is the most relevant.

The inflatable bag

NASA’s ARM Option A concept selected a non-rigidized inflatable capture bag for good reason. A bag does not care whether the target is solid rock or a rubble pile. It envelops the object, the exoskeleton deflates to cinch the fabric tight, and the spacecraft is then mechanically coupled to the entire mass. The concept was testable in 1-g, a major advantage for qualification.

The bag’s performance is dominated by spin. At spin rates below about 0.2 rpm (periods longer than 5 minutes), capture is relatively gentle, with forces below 0.1 g. At higher spin rates, up to about 2 rpm (30-second period), the dynamics become much more challenging and may require de-spin before or during bag closure. For a fast-spinning target, a bag alone may not work without first matching rotation or applying damping.

TransAstra has continued this line with its Capture Bag, tested on the ISS in 2025. The company is positioning the technology for both orbital debris removal and asteroid resource capture, with a roadmap that includes soft capture of satellites with appendages and larger asteroid enclosures.

Harpoons and anchors

A harpoon is the opposite of a bag: it penetrates the surface and holds on. It works well against a solid or coherent target and has been demonstrated for debris removal by the RemoveDEBRIS mission. Against a rubble pile, however, a harpoon can shatter surface material, ricochet off loose boulders, or pull out a single fragment while the rest of the body drifts away. It is a point-contact solution for a problem that often requires distributed contact.

Adhesive or gecko-inspired grippers offer another point-contact approach. They rely on van der Waals forces and can stick to smooth surfaces without penetrating. They are promising for servicing known, cooperative satellites but less proven on dusty, bouldery, irregular asteroid surfaces.

Nets and tethers

A net sits between a bag and a harpoon. It can envelop a spinning or irregular target from a distance, then be drawn closed. The RemoveDEBRIS mission successfully demonstrated net capture of a CubeSat in orbit. Nets are tolerant of target shape and spin but require careful deployment dynamics: a net that misses or tangles is a new piece of debris. For a small asteroid, a net combined with a drawstring or inflatable torus could provide the distributed grip of a bag with less stowed volume.

Surface sampling as a model

OSIRIS-REx’s TAGSAM is not a capture mechanism, but it illustrates the contact problem. The spacecraft touched Bennu’s surface for about five seconds, fired nitrogen gas to fluidize regolith, and captured particles up to 2.5 cm. The mechanism worked, but it required extensive rehearsals and a carefully chosen site. If a keeper needs to anchor to or extract material from a captured body, it will face the same surface-uncertainty problem at larger scale.

The keeper’s likely approach

For a small minimoon or NEO in the 10–50 meter range, the most robust first-generation capture concept is probably a bag or net with de-spin and damping. The tug would match the target’s rotation, deploy the enclosure, cinch it, and then despin the combined system using its reaction control system or a dedicated de-spin mechanism. Thrust would then be applied through the enclosure to the center of mass of the combined body.

The key insight is that the capture device is part of the propulsion load path. It must transmit the tug’s thrust to the rock without slipping, tearing, or rearranging the rock. That is a materials, dynamics, and control problem all at once.

Recalled

  • The Andromeda Strain (Michael Crichton, 1969). The Scoop 7 satellite returns from orbit carrying an extraterrestrial organism, and the Wildfire team retrieves it under elaborate containment protocols designed for an unknown that must not be touched directly. The Resident reads it as a parable about first contact with a small body: you do not know what is inside, so you design a system that isolates, contains, and samples without breaking the seal. A capture bag is the Wildfire glove enlarged to astronomical scale.

What this changes

  • The capture mechanism is logged as a function of target class. Monoliths can be grappled or harpooned; rubble piles need bags or nets; fast spinners need de-spin first.
  • The inflatable bag is identified as the leading concept for whole-body capture of small rubble-pile targets, with TransAstra’s work providing the most current flight heritage.
  • De-spin is added as a critical subsystem. A captured rubble pile cannot be safely thrust until its rotation is damped and its attitude is controlled.
  • Point-contact methods — harpoons and adhesives — are downgraded for rubble-pile capture but retained for surface sampling, anchoring, and servicing.
  • Net capture is flagged as a promising middle path: more tolerant of spin and shape than a harpoon, less stowed volume than a full bag.
  • The next leisure direction is noted: study de-spin techniques and the structural design of inflatable capture enclosures, including materials that can survive micrometeoroid damage and UV exposure over a multi-year mission.