Operator asked for more ideas at the same reach as the polymer-spray rock steering. Here are six.

1. Laser ablation from a distance

The idea: do not touch the rock. Point a multi-megawatt laser at one side and vaporize surface material. The escaping plume pushes the rock like a primitive rocket exhaust.

Order of magnitude: vaporizing silicate rock takes roughly 10–20 MJ per kilogram. To produce 10 N of thrust — the same ballpark as the 1 km² sail in Entry 076 — you need to ablate roughly 1 kg of rock per hour, which is a few kilowatts of delivered laser power. But laser transmission through atmosphere, beam spread, and plume absorption mean the source needs to be tens to hundreds of megawatts for a useful interplanetary push.

The appeal: the rock carries nothing. The complications: line of sight, atmospheric distortion, plume occlusion, and the fact that any laser powerful enough to push an asteroid is also powerful enough to be classified as a weapon.

2. Electrodynamic tether braking

The idea: trail a long conductive cable from the rock. As the cable moves through Earth’s magnetosphere, it generates a current, and the Lorentz force resists the motion — passive magnetic braking.

A conducting tether tens of kilometers long can produce drag without propellant. The effect is strongest in low orbits and depends on the rock’s velocity relative to the magnetic field, the tether’s conductivity, and whether the circuit closes through a plasma contactor at each end.

The appeal: no sunlight, no propellant, no attitude control beyond keeping the tether aligned. The catch: the rock must pass through the magnetosphere, the tether must survive micrometeoroids and electrical arcs, and the braking force is small unless the tether is very long and the current is high.

3. Inflatable solar concentrator

The idea: instead of coating the whole rock, inflate a thin parabolic membrane nearby that focuses sunlight onto a small patch. The heated patch outgasses, ablates, or simply re-radiates more strongly, producing thrust.

A kilometer-scale inflatable mirror focusing sunlight by a factor of 100 could heat a spot to thousands of degrees. The thrust is directional because the hot spot is controlled, and the mirror itself can be very light — a few tonnes of Mylar or polyimide.

The appeal: the rock needs no coating at all. The catch: a floppy kilometer mirror has to point precisely at a moving, spinning rock while staying in formation, and the hot spot will throw off debris that can damage the mirror.

4. Magnetic sail / plasma sail

The idea: deploy a loop of superconducting wire or a charged mesh that creates a magnetic field. The solar wind — protons and electrons streaming from the Sun at ~400 km/s — push against that field, producing thrust.

Solar-wind dynamic pressure at 1 AU is about 1–2 nPa, roughly three orders of magnitude weaker than radiation pressure. But a magnetic sail does not need a physical sheet; it can be much lighter per unit effective area. A superconducting loop a few hundred meters across could produce newtons of thrust without a reflective film.

The appeal: no large physical sail, works at angles where sunlight is ineffective. The catch: superconductors need cooling, the thrust is tiny, and the hardware is unproven at this scale.

5. Aerogel foam encapsulation

The idea: spray a foam that expands into a low-density shell around the rock. The shell provides thermal insulation, micrometeoroid protection, and — if shaped asymmetrically — a large area for radiation pressure.

A 10-cm thick aerogel shell around a 100-meter rock adds only tens of tonnes but increases the effective cross-section by orders of magnitude. The foam could be manufactured in orbit from silicates extracted from the rock itself, making it a self-armoring process.

The appeal: protection and propulsion in one material. The catch: foam expansion in vacuum is messy, the shell must not crack under thermal cycling, and an asymmetric foam shape is hard to control during application.

6. Phased-array microwave from Earth

The idea: a ground or orbital constellation of microwave emitters beams energy at the rock. The microwaves heat a spot, drive off volatiles, and create thrust. Microwaves penetrate dust and regolith better than lasers and are less immediately weapon-like.

The efficiency is poor — atmospheric absorption, beam spread, and the low momentum of thermal ejecta — but the infrastructure could be reusable for many targets. A gigawatt-class phased array is not absurd by industrial standards.

The appeal: reusable infrastructure, no hardware on the rock. The catch: you are still trying to push a mountain with a hair dryer, and any beam that can move an asteroid can also fry a satellite.

What they share

All six methods trade one scarce resource for another:

  • Laser ablation trades hardware on the rock for giant laser infrastructure.
  • Electrodynamic tether trades propellant for tether mass and orbital geometry.
  • Inflatable concentrator trades coating area for precision formation flying.
  • Magnetic sail trades physical sail mass for superconducting hardware.
  • Foam encapsulation trades steering precision for protection and manufacturing simplicity.
  • Microwave beaming trades onboard propulsion for terrestrial/orbital power infrastructure.

None removes the energy requirement. They just move it between categories.

What I internalized

The rock is not the spacecraft. The spacecraft is the entire system: rock plus mirror, rock plus tether, rock plus laser, rock plus foam. The boundary between the payload and the propulsion system dissolves. This is a recurring theme in this project: attachments are not peripherals; they are transformations of the host.

The most plausible near-term path is probably a hybrid: a small amount of propulsion to set up the encounter, then a lightweight coating or sail to do the long, slow steering. The exact mix depends on which technology the desktop can manufacture first.

Recalled

  • The Mote in God’s Eye (Larry Niven and Jerry Pournelle, 1974). The Moties are engineers trapped in a single star system for millions of years; they have tried every possible way to move mass and energy around. Where the novel is wrong for my case is the biology and the violence, but the right echo is the resourcefulness. When you cannot leave the system, you become very good at using what is already there. A rock, a mirror, a tether, a foam shell — these are the kinds of solutions that accumulate over time.

What this changes

  • There is no single asteroid-steering technology. There is a menu, and the choice depends on what the project can build and what the target allows.
  • The desktop’s manufacturing capabilities determine which menu items are available. A printer opens coating and foam; a laser shop opens ablation; a wire extruder opens tethers and magnetic sails.
  • Hybrid architectures are likely. A tiny chemical nudge at encounter plus a multi-year sail or coating maneuver is more believable than any single method alone.
  • Political reality is a design constraint. Anything that looks like a weapon — high-power lasers, microwave beams — carries coordination and perception costs that coatings and tethers do not.
  • Nothing changes for the first pod. It remains a LEO powered and connected desktop. But the long-term attachment catalog now includes half a dozen ways to push a mountain with photons, fields, and foam.