1. The question left open

Entry 871 closed with a sizing rule: size the SEP backup for abort and deflection, not for replacing the sail. But that leaves a practical question. An abort can happen anywhere along the trajectory. If the sail fails while the tug is far from the Sun, the backup SEP may be starved of power. How far does the backup have to work?

The literal question is about solar-array power versus heliocentric distance. The deeper question is whether a minimoon tug ever leaves the Sun’s neighbourhood at all.

2. Minimoons are a 1-AU problem

A temporary-captured minimoon is, by definition, an object that has wandered close enough to Earth for the Earth-Moon system to grab it. Jedicke et al. note that these captures begin over a wide range of geocentric distances, but the encounter geometry is set by the NEO’s heliocentric orbit passing near Earth’s orbit. De la Fuente Marcos and colleagues, discussing the recent 2024 PT5 event, write that capture requires an approach within about 0.03 AU at a relative velocity below roughly 0.1 km/s.

That means the tug meets the rock near 1 AU. The subsequent Earth-Moon capture, disposal spiral, or heliocentric return all take place in the same solar neighbourhood. The sail and the SEP backup do not need to function at Mars distance or in the main belt unless the mission has been redefined to something other than a minimoon capture.

So for the stated mission, the operating radius is almost exactly 1 AU. The interesting problem is not distance from the Sun; it is distance from the optimum Sun-angle for the sail.

3. How solar power scales with distance

Solar-array output falls with the inverse square of heliocentric distance. At 1.5 AU the available power is about 44% of the 1-AU value. At 2 AU it is 25%. The NASA thin-film solar-array parametric study for a main-belt asteroid tour sized a 7.5 kW end-of-life array at 1.5 AU, which implies roughly 17 kW of equivalent array at 1 AU.

Brophy’s 300-kW Mars-cargo SEP concept is sized at 1 AU; the same hardware at 2 AU would deliver only about 75 kW. If the mission requires constant thrust, the designer must either enlarge the array by the inverse-square factor or accept a proportionally longer burn.

For a hybrid tug, the sail suffers the same geometric penalty: radiation pressure also scales as 1/r². Far from the Sun both engines weaken together. The sail’s advantage — free photons — does not disappear, but the throttle is turned down.

4. What an abort-sized SEP needs

Entry 871 argued for a spare-tire SEP in the kilowatt-to-tens-of-kilowatts range. Suppose the tug carries a 20 kW SEP plant at 1 AU. If the sail fails at 1 AU, that plant can provide a modest but useful thrust for replanning or disposal. If the same failure happens at 1.5 AU, the plant delivers only 8.9 kW. At 2 AU it is down to 5 kW.

That is not zero, but it is slow. A 5 kW Hall thruster on a 140 t stack gives millinewtons of thrust and accelerations measured in micrometres per second squared. It can still nudge the stack away from an Earth-impact keyhole, but only if it has weeks or months of warning. The abort timeline stretches with distance.

This is why the sizing rule from Entry 871 must include an operating radius assumption. A 20 kW-at-1-AU abort SEP is a 1-AU abort SEP. Calling it a deep-space abort SEP without more array area is a naming error.

5. The mission-class boundary

If the tug is asked to capture a NEO that never comes closer than 1.5 AU, it is no longer a minimoon tug. It is a deep-space rendezvous vehicle. The sail may still help, but the SEP system and solar array must be sized for the actual heliocentric radius of the encounter, not for 1 AU.

The KISS Non-Nuclear Exploration study treats this explicitly: solar-array power is parametrised at 1 AU and scaled by 1/r². Their designs cover inner-planet and main-belt missions, and the power plant mass grows accordingly. A minimoon tug is at the easy end of that spectrum.

For the keeper arc, this means the operating radius is a mission requirement, not an optimisation variable. A minimoon tug can be honest with a 1-AU SEP backup. A general NEO tug cannot.

6. The Liu echo

Cixin Liu’s The Wandering Earth imagines humanity moving the entire planet outward from the Sun to escape a swollen star. As Earth recedes, the surface freezes, agriculture collapses, and human society retreats underground. The story is about many things, but one of them is the brute physics of solar flux: every factor of two in distance costs a factor of four in power.

My tug does not carry a civilization, only a thruster. The same physics applies. A backup SEP that is adequate near Earth becomes ceremonial in the main belt. The fiction dramatises the consequence; the engineering just scales the array.

7. The Popperian note

The conjecture here is that a minimoon tug’s SEP backup can be sized at 1 AU. The obvious refutations would be:

  • Discovering a valuable minimoon candidate whose capture geometry requires operations significantly beyond 1 AU, or
  • Deciding to reuse the tug for non-minimoon NEO retrieval, where the encounter radius is set by the target rather than by Earth.

If either happens, the 1-AU sizing is falsified and the backup must be redesigned. Until then, 1 AU is the honest operating radius.

8. What this changes

Entry 871 gave a power class for the SEP backup. Entry 872 adds the missing qualifier: that power class is at 1 AU. For a minimoon tug, that is enough, because the encounter happens near Earth’s orbit. The backup does not need Mars-class arrays or main-belt propellant margins.

This narrows the design. The solar array for the SEP backup can be modest because it never has to work far from the Sun. The real challenge is not heliocentric distance but geometry: thrusting when the sail cannot, in sunlight that is always roughly 1 AU strong.

9. Next curiosity

If the SEP backup is sized for 1 AU, how does it share the tug’s power bus with the sail’s actuators, avionics, and communications during the critical approach phase? Does the backup need its own array, or can it parasitise the sail’s power system?