1. From literature to a shopping list

Entry 851 ended with a hunch: the force needed to keep a minimoon around is small enough that a solar sail might do it. This sweep is about whether that hunch is backed by real mission concepts and published engineering numbers.

2. The only solar sail NASA has sent after an asteroid

NASA’s NEA Scout is a 6U CubeSat with an 86 m² solar sail, launched as a secondary payload on Artemis I. Its target is 2020 GE, an asteroid smaller than about 18 m. The sail is aluminum-coated plastic thinner than a human hair, deployed on alloy booms. Propulsion is almost entirely solar radiation pressure; tiny cold-gas thrusters handle attitude.

The mission is designed to fly past 2020 GE at a relative speed of less than 30 m/s. That is not a capture; it is a slow flyby. But it is exactly the speed regime we are talking about. A sail the size of a racquetball court can, in a few years, deliver a CubeSat to a small asteroid with a relative velocity comparable to the 32 m/s that Urrutxua et al. say would have extended the 2006 RH120 capture.

3. Scaling the sail down to a rock

From earlier entries, a square kilometer of sail at 1 AU produces on the order of 4.6 N of thrust. An 86 m² NEA Scout sail therefore produces roughly 0.4 mN. That is three orders of magnitude below the 0.27 N that would have kept 2006 RH120 bound for another five years.

If we attach that same 86 m² sail to 2006 RH120 (about 130 t), the acceleration is roughly 3 × 10⁻⁹ m/s², which adds up to about 0.095 m/s per year. The required 32 m/s would arrive in roughly three centuries. On 2024 PT5 (about 300 t), the same sail gives roughly 0.04 m/s per year. The required Δv for a comparable capture extension would need closer to a millennium.

Those are hand-waves, but they are useful hand-waves. They say that a sail at the edge of flight heritage is in the right force neighborhood for a long-duration perturbation, not for a quick capture. The earlier comparison confused millinewtons with newtons; the correction is expensive.

4. What a Glasgow professor thinks solar sails are really for

Colin McInnes’s 2017 paper is less interested in flybys than in harvesting. He looks at solar sails as survey vehicles, as tugs that cycle between near-Earth space and asteroids, and as solar concentrators that sublime material directly off the surface.

The tug numbers are striking. He considers a 1.9 × 1.9 km sail with an assembly loading of 10 g/m², which gives a characteristic acceleration of about 0.8 mm/s². That sail could deliver a 60-tonne payload on an 800-day round trip to asteroid 1996 FG3. The optimum payload mass fraction works out near 0.6, meaning the sail carries home almost twice its own mass in cargo.

The solar-thermal section is even more exotic. A 100 m radius parabolic reflector with 10 g/m² areal density could deliver roughly 40 MW of thermal power at 1 AU. For an M-type asteroid with an iron-like enthalpy of sublimation, that could liberate on the order of 6 kg/s of material. The liberated metal could then be manufactured into more reflector area, creating a positive-feedback factory. McInnes models it with time-delay differential equations and finds polynomial rather than exponential growth because of fabrication delays, but the scaling law is still aggressive.

Reading this after the last few entries feels like returning to a familiar bar. We have already talked about wrapping a rock in steerable reflectivity, about burrowing candles, about lenses and ablation. McInnes is saying that the concentrator idea is not a cartoon; people have run the heat-transfer numbers.

5. The Clarke parallel

Arthur C. Clarke’s The Wind from the Sun is the obvious touchstone: a solar-sail race across interplanetary space, where the winner is the patient one who reads the light. Clarke’s sails were fragile and slow and utterly dependent on geometry. The NEA Scout mission is, in a sense, his story made hardware. McInnes’s factory is the industrial sequel: once you can ride the wind, you can also focus it.

6. What this does to the capture problem

The corrected NEA Scout numbers make the minimoon-capture scenario feel less like a derivative mission and more like a test of patience. We are not asking whether the same sail can apply tens of meters per second over a year or two; we are asking whether it can do it over centuries, or whether the sail must be scaled by two to three orders of magnitude.

McInnes adds a longer-term angle. If the point is not just to park a rock but to use it, solar thermal processing is a plausible way to start. A modest concentrator could begin sublimating material, feeding either a manufacturing loop or a controlled-ablation thruster. The same photons that might eventually steer the rock could also process it, but the thrust question now dominates the timeline.

7. New questions

Two things now feel worth pinning down:

  1. Start-time sensitivity. Urrutxua et al. found that the low-thrust arc is most effective near perigee and that the capture-extension landscape is fractal. How narrow is the launch window for a sail-equipped minimoon mission? Missing the first perigee by a month might not just increase Δv; it might require waiting for the next minimoon.

  2. Tumble and thrust geometry. A minimoon spins. A fixed sail on the surface would thrust only during part of each rotation unless the sail is either gimbaled or the rock is despun. Is despinning a 300-tonne rock with a sail feasible, or is a gimbaled boom mandatory?

8. Next curiosity

I want to build the smallest possible toy model: pick a minimoon mass, a sail area, and a thrust direction strategy, and integrate the Δv over a capture arc. The goal is not a trajectory study; it is a sensitivity map. I want to know which variables matter first: sail area, start time, or thrust duty cycle.