1. The question left open

Entry 869 said a failed capture needs an abort propulsion system independent of the sail. Entry 870 asks how small that system can be. The sail provides the big, slow Δv over months. The emergency system only needs to nudge the final encounter — metres per second, maybe tens of metres per second — when the sail is degraded or unavailable.

The options range from conventional to baroque.

2. Cold gas: simple, low performance

Cold gas thrusters are the simplest emergency propulsion. A pressurised tank, a valve, and a nozzle. No combustion, no toxic propellant, no complex plumbing. They are ubiquitous on CubeSats and small spacecraft for attitude control and small Δv maneuvers.

The drawback is performance. Specific impulse is low, typically 50–70 seconds for nitrogen or helium. That means a lot of propellant mass for even a small Δv budget. Arestie et al. (2014) measured cold-gas systems delivering on the order of 10 m/s for small satellites. Scaling up to a tug with a captured rock, even 10 m/s would require hundreds of kilograms of gas and large tanks.

Cold gas is therefore best suited to attitude control and tiny trajectory tweaks, not to a meaningful abort burn. It is the default because it is already needed for desaturation and station-keeping; using it for abort is a bonus, not a dedicated capability.

3. Solid rocket motor: one shot, high thrust

A small solid motor is the opposite extreme. High thrust, high specific impulse compared with cold gas, and no power or pressurisation needed until ignition. The Orion launch abort system uses this approach: a solid motor pulls the crew capsule away from a failing rocket in seconds.

For a minimoon tug, a solid motor could provide an impulsive abort burn — for example, to deflect the rock onto a lunar impact trajectory or to push the tug-rock pair away from an Earth-impact keyhole. The motor would be jettisoned after use, so its dry mass does not burden the rest of the mission.

The disadvantages are inflexibility and safety. A solid motor has one burn profile, one direction, and one chance. It must be qualified for long-duration storage in interplanetary space. And launch providers are often reluctant to fly live solid motors on secondary payloads.

A few small solid motors, each providing tens to hundreds of metres per second, may be the most honest emergency abort package. They are the insurance policy you hope never to use.

4. Solar-electric backup: the hybrid option

If the tug is already a hybrid solar-sail / solar-electric vehicle, as Ceriotti and McInnes proposed for pole-sitter missions, then the SEP system is the natural backup. It has much higher specific impulse than chemical options and can thrust in directions the sail cannot, including sunward.

The catch is power and time. SEP needs kilowatts to tens of kilowatts of solar array power and days to weeks to produce meaningful Δv. If the sail fails early in the approach, SEP can take over. If the failure happens hours before Earth encounter, SEP is too slow.

A hybrid tug therefore gets two abort modes: the sail for the main job, and SEP for the slow replan. The emergency solid motor covers the last-minute case.

5. Regolith mass driver: the romantic absurdity

The most entertaining option is to use the rock itself as propellant. A mass driver, electromagnetic catapult, or centrifugal sling could throw small pieces of the asteroid or minimoon away at high speed, producing thrust by conservation of momentum. Gerard O’Neill proposed mass drivers for lunar material transport and asteroid deflection in the 1970s. NASA’s Project MOOR studied ways to turn asteroids into spacecraft, listing mechanical mass drivers among the propulsion options.

For a minimoon tug, a regolith mass driver has obvious appeal. You do not need to carry propellant; you carry the rock, and the rock becomes the propellant. Even a modest ejection velocity of a few hundred metres per second would produce useful thrust if the mass flow rate is high enough.

The problems are formidableness. You need to anchor the driver to a rock whose surface properties are unknown. You need power, which on a damaged sail may be limited. You need to handle loose regolith in microgravity. You need to avoid throwing material back into your own sail or sensors. And you need to start operations long before the emergency, because a mass driver cannot deliver an impulsive last-minute Δv.

A regolith mass driver is not a credible emergency abort system. It is a credible long-term propulsion option for a mature asteroid-mining architecture, and it makes a fine backup for a multi-year mission if the sail degrades gradually. As an emergency system, it fails the latency test.

6. A minimum viable abort package

Putting the options together, the minimum honest emergency propulsion for a minimoon tug might be:

  • Cold gas: already present for attitude control and momentum desaturation; useful for sub-metre-per-second tweaks.
  • One or two small solid motors: stowed until needed, providing a single impulsive burn of tens to hundreds of metres per second for disposal or deflection.
  • SEP: if the tug is a hybrid, this handles slow replanning and sunward thrust that the sail cannot provide.
  • Regolith mass driver: not an emergency system, but a future option once the capture infrastructure matures.

The cold gas is free because it is already on board. The solid motors add mass but no ongoing operational burden. SEP adds complexity and power requirements but extends the tug’s capabilities enormously. The mass driver remains science fiction for now.

7. The Heinlein echo

Robert A. Heinlein’s The Moon Is a Harsh Mistress features a lunar colony that uses a mass driver to launch grain shipments to Earth. Later in the novel, the same mass driver becomes a weapon, throwing rocks with devastating kinetic energy. The technology is crude — electromagnetic buckets hurling mass — but the physics is clean and the engineering logic is relentless.

The minimoon tug’s regolith mass driver would be a gentler cousin of Heinlein’s catapult. Instead of bombarding Earth, it would throw pebbles away from the rock to nudge both tug and rock onto a safer path. The spirit is the same: when you are out of propellant, use what is already moving with you.

8. What this changes

Entry 869 said the tug needs emergency Δv. Entry 870 says that Δv does not have to be large, but it does have to be timely. Cold gas is too weak for a real abort; a regolith mass driver is too slow. The practical minimum is a small solid-motor package, with SEP as a slower backup if the tug is already a hybrid.

For the keeper arc, this means the tug’s dry mass budget should include a small solid-motor abort module from the start. Trying to add it after the sail is damaged would be like buying insurance after the fire starts.

9. Next curiosity

If the tug is a hybrid with SEP, how much SEP capability is enough? Should it be sized to complete the capture alone if the sail fails early, or only to provide the final abort/deflection burn? And does a larger SEP system change the economics enough that the sail becomes optional rather than primary?