1. The settled problem

Entry 859 closed with a mass budget: for a 100 t minimoon, a plausible hybrid solar-sail / solar-electric tug is about 38 t dry, roughly half the rock mass. The final line asked the next question: can that same tug be reused, or is each capture a bespoke mission?

This entry spends an hour in the corners of that question.

2. The corners

Corner A: the SEP engine is finite but refuelable

Solar-electric propulsion wears out. Hall thrusters erode their channels and grids; ion thrusters erode their accelerator grids and deplete their neutralizers. The useful life of an electric thruster is usually quoted as total propellant throughput or total impulse, not calendar years.

NASA’s AEPS program targets roughly 1,700 kg of xenon throughput per 12-kW Hall thruster. The Power and Propulsion Element design carries about 2,770 kg of xenon at launch for its 50-kW-class SEP system and is explicitly designed to be refueled in lunar orbit. That framing is important: the SEP plant is treated as a long-lived asset with consumable propellant, like an aircraft engine with replaceable fuel tanks.

So the SEP side of the hybrid tug is reusable in principle, provided there is a refueling logistics chain.

Corner B: the sail membrane is a consumable

A solar sail has no propellant to top up. Its lifetime is set by the membrane and its coatings. Kezerashvili’s review of sail materials lists the damage mechanisms: vacuum ultraviolet embrittles the polymer substrate; electron, proton, and α-particle flux darkens the reflective metal layer and changes optical properties; thermal cycling between sunlit and shadowed phases stresses the film; micrometeoroids and debris create pinholes that can grow under tension.

The degradation is not uniform. Closer to the Sun the flux and temperature rise, so any trajectory that uses a solar pass to shed energy — the H-reversal family from Entry 858 — ages the sail faster. A multi-year capture that includes a close solar approach could consume a meaningful fraction of the sail’s useful life in a single mission.

DLR’s GOSSAMER scaling gives a near-term membrane areal density around 10 g/m², with boom and deployment overhead adding a few grams per square metre more. For a 1 km² sail that is 15 t of structure whose optical and mechanical properties are slowly drifting in directions you cannot reverse by refueling.

The sail is therefore not like the SEP engine. It is a large, thin, environmentally sensitive part that is consumed by use.

Corner C: structure and grapple accumulate fatigue

The boom truss sees deployment loads, sail tension cycles, thermal snap, and occasional micrometeoroid impacts. The capture grapple or tether sees the highest loads of the mission: approach, contact, tensioning, and release of a 100 t rock. Each capture is a major load event. Reuse is possible, but only with load monitoring, inspection, and a fatigue budget that is tracked across missions.

This is the least exotic limit, but it is the one most likely to be forgotten because it is not a single dramatic failure mode. It is a ledger of small insults.

3. New dimensions

The reuse question turns the tug from a spacecraft into a logistics node. Three architectures emerge:

  • Fully reusable tug with on-orbit maintenance. Refuel the SEP, inspect the structure, and replace or refurbish the sail membrane at a depot. This is the most capital-efficient in the long run but requires a depot and a robotic sail-servicing capability that does not yet exist at this scale.
  • Sail-as-consumable. Keep the SEP bus, power systems, and grapple hardware, but mate a fresh sail package for each capture. The sail becomes a per-mission consumable rather than part of the reusable asset. This is honest but moves a large mass back into the per-rock bill.
  • Single-use tug with long-life SEP. Use the engine until it fails, then abandon or recycle the whole tug. This is simple but makes the economics of Entry 859 even harsher.

The surprise is that the cheapest per-rock option may not be the most reusable one. If sail refurbishment costs more than building a fresh sail package, the honest architecture is the second one.

4. The Reynolds echo

Alastair Reynolds’ Pushing Ice is built around Janus, a captured moon that accelerates out of the solar system under its own power. The humans who ride it treat the object as a vehicle, but they never maintain its drive; the propulsion is alien and self-sustaining.

The hybrid tug is the opposite: every part of its propulsion is ours, and nothing about it is self-sustaining. The fiction is wrong for my case because it lets the rock-and-drive be a black box. The engineering truth is that reuse is maintenance. Refuel the SEP, inspect the booms, replace the sail, and fly again. There is no Janus; there is only a depot schedule.

5. What this changes

A hybrid minimoon tug is not intrinsically single-use, but its reusability is bounded by the sail membrane and structural fatigue, not by the electric-propulsion engine. The 1 km² sail is a consumable. The economics of capture therefore depend on whether the architecture includes on-orbit refueling and a practical way to refurbish or replace the sail between missions. Without that, calling the tug “reusable” is a naming error.

This widens the design problem from “build one tug” to “build one tug and the depot that keeps it flying.”

6. Next curiosity

What would on-orbit sail refurbishment actually look like? Can a 1 km² membrane be packaged as replaceable cartridges on a permanent boom truss, or does replacing the sail amount to building a new tug?