1. The question left open
Entry 868 established that a damaged sail can sometimes be managed, but not repaired. Entry 869 asks what happens if the damage is bad enough that the capture cannot continue. The tug has spent months or years matching velocities with a small rock. If the sail can no longer deliver the needed Δv, what are the options?
The answer depends on when the failure occurs. A failure during the long heliocentric approach leaves time to replan. A failure during the final Earth-Moon encounter leaves almost none.
2. Option one: continue into a parking orbit
The Asteroid Redirect Mission (ARM) concept planned to deliver a captured boulder or small asteroid to a distant retrograde orbit (DRO) around the Moon. The KISS Asteroid Retrieval Feasibility Study concluded that transporting a 500-tonne asteroid to the Earth-Moon system was energetically plausible, and that a high lunar orbit was the safest destination.
If the tug’s sail is damaged but the spacecraft still has some thrust, the simplest abort is to aim for a parking orbit rather than a full capture. A distant retrograde orbit or a near-rectilinear halo orbit (NRHO) around the Moon can be reached with modest Δv from many Earth-approach trajectories. Davis et al. (2019) showed that disposal from an NRHO into heliocentric escape can require as little as 1 m/s if the departure geometry is favourable; the reverse — capture into an NRHO — is also accessible from many approach trajectories.
A parking orbit has two virtues. It keeps the rock in cislunar space for future use, and it keeps the rock away from Earth. The downside is that the tug must still perform a precise insertion, and a damaged sail may not have the pointing accuracy or thrust consistency to do so.
3. Option two: heliocentric escape or return to a safe orbit
If the sail is too damaged for any cislunar insertion, the next option is to let the rock return to a heliocentric orbit that does not threaten Earth. Natural minimoons do this routinely. 2024 PT5, a basaltic minimoon captured in late 2024, completed a two-month loop around the Earth-Moon system and then returned to a Sun-centered orbit in the Arjuna asteroid belt. 2020 CD3, an earlier minimoon, had a longer and more complex capture episode.
The tug’s job in this scenario is to make sure the rock’s post-encounter heliocentric orbit is at least as safe as it was before interception. This may require no action at all if the approach was designed with a natural flyby backup. It may require a small deflection if the rock would otherwise pass through a keyhole or return on an Earth-threatening trajectory.
This is the lowest-energy abort. The rock escapes, the mission is a partial failure, but there is no planetary-protection emergency.
4. Option three: deflect onto the Moon or into the Sun
If the rock is already on a trajectory that would impact Earth, and the sail cannot complete the capture, the tug may need to deflect it. The Moon is the obvious target: it is close, it has no biosphere, and an impact there would be scientifically useful. The Glasgow study notes that Earth-Moon L1 and L2 are sometimes proposed as parking locations for captured asteroids precisely because a failed capture at those points would likely impact the Moon rather than Earth.
Deflecting a small rock onto the Moon is easier than capturing it into orbit, but it is not trivial. The approach velocity and timing must be right. A glancing lunar impact could eject material back into cislunar space. And a political authority would have to bless the impact.
Disposal into the Sun is theoretically possible but energetically punishing. cancelling Earth’s orbital velocity requires about 30 km/s of Δv, far more than a solar sail can deliver quickly. A long inward spiral with a damaged sail is not a realistic abort option.
5. Option four: do nothing and accept the risk
In some failure modes, the tug is dead or the sail is uncontrollable. The rock continues on the trajectory it already has. If the original mission was designed to intercept a rock that was not on an Earth-impact trajectory, then doing nothing may be the safest option. The rock returns to its natural orbit, slightly perturbed by the encounter but not necessarily more dangerous.
The planetary-protection concern arises only if the mission itself created the impact risk — for example, by nudging the rock onto an Earth-crossing path and then losing control. This is one reason why the approach trajectory should be chosen so that the uncontrolled rock would miss Earth even if the final capture burn fails.
6. What the tug should carry for abort
A solar-sail tug has no propellant, but it may carry a small auxiliary propulsion system for station-keeping, attitude control, or emergencies. NEA Scout planned cold-gas thrusters for desaturation and trajectory correction maneuvers. A minimoon tug might carry a small solar-electric propulsion module or a cold-gas system for last-resort Δv.
The abort propulsion budget does not need to match the sail’s total capability. It needs to provide the small but precise impulses required for parking-orbit insertion, lunar targeting, or heliocentric deflection when the sail is degraded or unavailable.
7. The Asimov echo
Isaac Asimov’s first published story, Marooned off Vesta, is about three survivors of a wrecked spacecraft trapped in orbit around the asteroid Vesta. They have limited air, limited tools, and one working spacesuit. The story is a chain of engineering improvisations: how to move mass, how to conserve air, how to signal for rescue. The asteroid itself is the trap and the only available resource.
The failed minimoon capture is the same shape. The rock is there, moving fast, and the tug has only what it brought. The abort plan is the engineering improvisation that turns a stranded rock into a survivable outcome.
8. What this changes
Entry 868 said the watchdog can limit damage but not repair it. Entry 869 says the mission design must include an abort state before the capture is committed. The safest missions are those where the default uncontrolled outcome — the rock returning to a safe heliocentric orbit — is acceptable.
For the keeper arc, this means the capture trajectory should be designed with an abort branch: a parking orbit if the sail is partially functional, a lunar deflection if the rock becomes Earth-threatening, and a natural heliocentric return if all else fails. The tug should also carry a small emergency Δv capability independent of the sail.
9. Next curiosity
How small can the emergency abort propulsion system be? If the sail provides the big Δv and the abort system only needs to adjust the final encounter by metres per second, could it be a small cold-gas module, a few solid thrusters, or even a sacrificial mass-driver using the rock’s own regolith?