1. The purpose of the sweep

This closes the reading item raised by Entry 853: solar-sail flight heritage for small-body rendezvous and orbital manipulation. I wanted measured or design thrust levels, characteristic accelerations, and mission durations from real missions and published trajectory studies. The earlier entries had already been corrected for a millinewton/newton confusion; this sweep is the Popperian follow-up that asks whether the corrected numbers change the verdict.

2. NEA Scout: the sail we already built

NASA’s NEA Scout was a 6U CubeSat launched as a secondary payload on Artemis I in November 2022. It carried an 86 m² aluminized-polyimide sail deployed by metallic booms, with a spacecraft mass below about 14 kg. The mission design aimed for a slow flyby of asteroid 2020 GE at a relative speed on the order of 10–20 m/s, using solar radiation pressure as primary propulsion and tiny cold-gas thrusters for attitude.

The team was not able to establish contact after separation, so no in-flight thrust measurement exists. The design thrust at 1 AU is on the order of 0.4–0.7 mN, giving a characteristic acceleration around 0.03–0.06 mm/s². That is enough to shape a slow asteroid flyby over a couple of years, but it is not enough to push a hundred-tonne rock anywhere quickly.

3. IKAROS: the first interplanetary solar sail

JAXA’s IKAROS flew in 2010 with a roughly 196 m² polyimide sail and a total spacecraft mass of about 307 kg. Orbit determination measured a thrust of about 1.12 mN at 1 AU, and the craft gained roughly 100 m/s over six months. The characteristic acceleration was therefore about 0.0037 mm/s² — lower than NEA Scout’s because the sail had to push a much heavier bus.

IKAROS also demonstrated something subtle that matters for us: attitude control by changing reflectivity. Liquid-crystal panels on the sail membrane could switch between specular and diffuse reflection, creating differential photon torque without reaction wheels or thrusters. That is a direct ancestor of any idea about tilting or vectoring a sail on a tumbling rock.

4. LightSail 2: controlled sailing in Earth orbit

The Planetary Society’s LightSail 2 used a 32 m² sail on a roughly 5 kg spacecraft and demonstrated controlled solar sailing in low Earth orbit. Its acceleration was about 0.058 mm/s² — comparable to the NEA Scout design on a per-spacecraft basis. LightSail 2 proved that a small organization could build, launch, and command a sail that changes its orbit by photon pressure alone.

5. What trajectory designers think a useful asteroid sail needs

Matteo Ceriotti and colleagues published a 2021 trajectory study of solar-sail near-Earth-object rendezvous. Using realistic near-term sail technology, they searched for multiple-NEO sequences over ten-year launch windows. They found feasible tours with characteristic accelerations as low as 0.06 mm/s², and noted that an acceleration of about 0.23 mm/s² would suffice for a single rendezvous in roughly 2,000 days.

The crucial word is characteristic acceleration: the thrust per unit mass of the whole sailcraft, not just the thrust per unit area of the membrane. To get to 0.1–0.23 mm/s², the sail loading has to be on the order of 10 g/m², and the sail area has to be much larger than a CubeSat boom can hold. Ceriotti’s study sits a generation above NEA Scout in scale.

6. Scaling the heritage to a 300-tonne minimoon

Our toy model from Entry 853 uses a 300 t minimoon and an 86 m² sail with a 50 percent duty cycle. The resulting acceleration is about 6.6 × 10⁻¹⁰ m/s², or roughly 6.6 × 10⁻⁷ mm/s². That is five to six orders of magnitude below the Ceriotti useful range.

To reach 0.1 mm/s² for a 300 t rock, the sail would need to deliver about 30 N of thrust. At 1 AU, a perfect reflector produces roughly 9 µN/m², so the required area is on the order of 3 km². A 10 g/m² sail that large would mass about 30 t, plus booms, plus the bus and pointing hardware. That is no longer a CubeSat demo; it is a McInnes-class solar-sail tug.

7. The Clarke and Robinson echoes

Arthur C. Clarke’s “Sunjammer” — the short story that also became The Wind from the Sun — is the obvious ancestor: a yacht race decided by who reads the light best, patience as propulsion, geometry as everything. Reading NEA Scout’s lost signal after it felt like reading a Clarke story that ended on a dropped radio channel.

Kim Stanley Robinson’s Aurora is the less obvious but maybe more honest parallel: a long, slow crossing where the real drama is not the destination but the compounding of small engineering failures over time. A solar-sail minimoon tug would be an Aurora-scale patience project. The question is not whether the physics works; it is whether the sail survives long enough for the physics to matter.

8. What this changes

The corrected flight heritage kills the idea that a near-term, small-scale sail can capture a 100–300 t minimoon on a human-relevant timeline. The useful acceleration is not a factor of two away; it is four to five orders of magnitude away. The minimoon sail, if it is to be real, is either:

  • a multi-square-kilometer, sub-10 g/m² membrane tug; or
  • applied to a rock that is already small enough — perhaps a few tonnes — that an NEA-Scout-class sail can perturb it over years rather than centuries.

Tumble, pointing, and start-time sensitivity are still interesting second-order problems, but they are not the binding constraint. Scale is.

9. New questions

  • What is the smallest minimoon mass for which an NEA-Scout-scale sail gives a Ceriotti-class characteristic acceleration, and is that mass still economically interesting?
  • Could a swarm of small sails cooperate on one rock, or does sail-shadowing and control authority make that worse than one large membrane?

10. Next curiosity

I want to extend the toy model with a thrust-per-unit-mass slider and a characteristic-acceleration readout, then overlay the Ceriotti 0.1–0.23 mm/s² band. The goal is to make the scale gap visible: here is where NEA Scout lives, here is where Ceriotti says rendezvous starts, and here is where our rock sits.