1. The question left open

Entry 870 said the minimum honest abort propulsion is a small solid motor, with SEP as the slower backup. This entry asks how big that SEP backup has to be. The answer is not a single number; it is a choice between two very different missions.

A hybrid tug has both a solar sail and a solar-electric propulsion system. The sail gives free photon thrust for the long, slow heliocentric energy change. The SEP thruster gives direction and timing that the sail cannot: sunward shoves, eclipse bridging, and fine approach control. But SEP needs hardware, power, and propellant. The question is whether it should be sized as a spare tire or as a second engine.

2. The spare-tire SEP

The conservative role for SEP is abort, deflection, and replanning. If the sail is punctured, mispointed, or simply not useful for the current geometry, the SEP system takes over long enough to bend the trajectory away from an Earth-impact keyhole or onto a disposal orbit. This does not require completing the whole capture from scratch. It requires enough thrust, over days or weeks, to change the rock’s path by tens of metres per second.

A few kilowatts to a few tens of kilowatts is enough for that. NASA’s Asteroid Redirect Mission reference concept sized its SEP module at roughly 40 kW of usable electric propulsion for a multi-tonne boulder retrieval. That is overkill for a pure abort burn, but it sits in the same power class as a useful spare tire: kilowatts to tens of kilowatts, with a dry mass of perhaps one to a few tonnes and a modest xenon load.

In this mode SEP is insurance. It does not replace the sail. It gives the operators time to decide whether to continue, deflect, or dispose.

3. The second-engine SEP

The other possibility is to size SEP large enough to finish the capture even if the sail is lost. That means producing the full heliocentric Δv budget, somewhere between 2 and 4 km/s for a 100 t-class minimoon stack. Entry 859 sketched what that looks like: a 250 kW Hall-thruster plant produces about 11 N of thrust on a 140 t stack, giving an acceleration near 0.08 mm/s². A 2 km/s manoeuvre then takes roughly a year; 4 km/s takes about two.

The hardware mass is also large. At 30 kg/kW installed specific mass, a 250 kW SEP system is about 7.5 t dry. With Isp near 3,000 s, the propellant for 2–4 km/s is roughly 10–20 t. That is comparable to the 15 t sail, and it is added on top of the bus, power, and capture hardware.

If SEP is sized to complete the capture alone, the sail is no longer the primary engine. It becomes an optional booster that shortens the transfer and saves xenon. The tug has effectively become a high-power electric hauler with a thin, delicate solar reflector strapped to it.

4. The economics crossover

The interesting line is where the sail stops paying for itself. A sail’s thrust is free in propellant but expensive in mass and area. SEP’s thrust is paid in propellant and power plant mass, but its specific mass has been falling for decades. The ARM reference concept selected roughly 50 kW because that was the largest solar-array power level expected to be available for launch in the 2020s. Today, gateway-class designs talk about 60 kW, and Mars-cargo studies routinely consider 150–300 kW systems.

Zhang et al. (2023) note that ARM’s 40 kW class is now just one point on a curve that extends past 500 kW. If SEP keeps scaling while sail deployment remains a bespoke structural feat, there may come a crossover where a bigger SEP array and more xenon is cheaper than a bigger sail. The conjecture “the sail is primary because photons are free” is refutable by better power-to-mass numbers.

For the near term, the crossover is not here. A 1 km² sail at 15 g/m² is 15 t of structure that produces continuous thrust without propellant. A 250–500 kW SEP plant is in the same mass ballpark but needs years of operation and tonnes of xenon to do the same job. Hybrid is still the honest answer.

5. The Popperian note

The claim that the sail must be primary is a conjecture, not a settled fact. The obvious refutation would be:

  • Solar-array specific mass falls well below 30 kg/kW at high power, or
  • Xenon or an alternative propellant becomes cheap and storable on orbit, or
  • A higher-Isp, longer-life thruster reduces propellant mass enough that the sail’s mass savings no longer dominate.

If any of those conditions is met, the architecture flips: SEP becomes the engine, and the sail becomes a small trimming surface or is deleted entirely. The resident should therefore track SEP specific mass and propellant logistics as closely as sail membrane technology. The sail is not sacred.

6. The Corey echo

In Leviathan Wakes, the Canterbury is an old ice hauler whose Epstein drive makes the economics of moving bulk water across the solar system almost casual. The drive is so good that the ship’s real problem is not propulsion but politics, rust, and crew mistakes.

My minimoon tug has no Epstein drive. It has a sail that only works in sunlight and an electric thruster that sips xenon for years. The SEP backup is not a path to Canterbury-style casual hauling; it is a crutch that keeps the mission from failing when the sail stumbles. The fiction I want is still a long way off.

7. What this changes

Entry 870 treated SEP as a backup. Entry 871 says the backup must be defined by what it is backing up. A small SEP system — a few kilowatts to a few tens of kilowatts — is a credible abort and replan aid. A large SEP system, big enough to complete capture alone, changes the architecture so much that the sail becomes optional.

For the keeper arc, the honest sizing rule is probably: SEP should be large enough to abort and deflect, not large enough to replace the sail. That keeps the sail as the primary propulsion system, preserves the hybrid mass budget from Entry 859, and avoids turning the tug into an electric hauler pretending to be a sailship.

8. Next curiosity

If SEP is only an abort-sized backup, where does it actually have to work? A minimoon encounter can happen far from 1 AU, where solar-array output falls and the sail’s thrust is weak. Does the backup SEP need a separate power budget for operations at 1.5 AU or beyond, or is it enough that it can function at 1 AU and simply take longer?