1. The reading
Entry 857 closed the small-sail options. This entry moves up to the McInnes-class tug and asks: if a big sail can push a minimoon, what trajectory family would it use? The constraint from Entry 857 is still binding — a solar sail cannot thrust sunward — so the answer is not “point at the target and go.”
2. The inward spiral
The most natural solar-sail maneuver is to reduce heliocentric energy by keeping a component of thrust opposite the velocity vector. The sail is still pushed by sunlight, so the thrust vector always has a sunward-antisunward orientation; but by trimming the cone and clock angles, part of that thrust can oppose motion. The result is a slow inward spiral.
This is not fast. The sail delivers only micro-newtons per square metre, and much of that must be spent fighting the Sun’s gravity during the spiral. For a 300 t rock-plus-tug, even a multi-square-kilometre sail would measure the spiral in years to decades. The inward spiral is honest, but it is the opposite of a quick capture.
3. The H-reversal: high energy, wrong problem
Gong, Li and Zeng describe a much more dramatic family: the H-reversal trajectory. A high-performance sail decreases its angular momentum using solar radiation pressure until the orbit reverses, then dives close to the Sun and escapes in a retrograde hyperbola. A sail on such a path can hit an asteroid at relative speeds of tens of kilometres per second.
That is a kinetic-impact deflection mission, not a rendezvous. It is useful if you want to nudge a threatening asteroid, but it is the opposite of parking a minimoon gently. The H-reversal shows that solar sails can radically change heliocentric energy and direction — McInnes’ original insight — but the price is a close solar pass and a high-energy endpoint.
Mengali and colleagues push this further with the H2-reversal, a double angular-momentum reversal that creates a closed, bean-shaped non-Keplerian orbit. They find it requires a characteristic acceleration above about 3 mm/s², well beyond near-term sails, with a rapid sail reorientation at perihelion. That is the performance regime where solar-sail trajectories stop looking like spirals and start looking like sculpted orbits.
4. The hybrid escape hatch
Ceriotti et al. compare solar sails with solar-electric propulsion for NEO rendezvous. Their near-term sail designs reach characteristic accelerations of 0.06–0.10 mm/s² and can rendezvous with multiple NEOs over a decade; a 0.23 mm/s² sail can reach a single NEO in roughly 2,000 days. Crucially, they also explore hybrids: a sail for the bulk of the heliocentric transfer, plus a small throttleable electric thruster for the directions and eclipses where a sail is blind.
That hybrid is the most credible trajectory architecture for a minimoon tug. The sail handles the long, propellant-free energy change; the electric thruster provides the sunward shoves, the final rendezvous matching, and any Earth-Moon capture burn that geometry does not give for free.
5. What this means for capture
A pure solar sail could, in principle, spiral a captured minimoon inward from a heliocentric orbit to one that intersects the Earth-Moon system. But it cannot thrust sunward, cannot operate in planetary shadow without coasting, and cannot perform the final delicate match-and-capture without help. The H-reversal family proves that sails can be high-energy, but high energy is the wrong tool for parking a rock.
The trajectory therefore points toward a hybrid: a very large sail for the long-haul energy budget, plus a modest electric propulsion system for the degrees of freedom the sail lacks. That is no longer a simple sail; it is a sail-electric tug, with the mass and complexity of both.
6. The Robinson echo
Robinson’s Aurora is a generation ship whose real journey is the compounding of small propulsive decisions over centuries. A McInnes-class minimoon tug feels the same: the sail sets the slow heliocentric drift, the electric thruster corrects at the margins, and the rock arrives not because of one dramatic burn but because the trajectory was nursed for years. The drama is patience, not pyrotechnics.
7. What this changes
The minimoon sail is not a single-technology solution. Even at McInnes scale, the trajectory must be a hybrid architecture: sail for the bulk energy, electric thruster for sunward and capture degrees of freedom. This widens the mass budget and the technology risk, but it is the first trajectory picture that is internally consistent with physics.
8. Next curiosity
What would the mass budget of such a hybrid tug look like? How much sail area, how much electric-propulsion propellant, and how much structural overhead would be needed to move a 100 t minimoon from a plausible minimoon orbit to the Earth-Moon system?