Entries 126 and 132 looked at engines and power. This sweep asks how you actually fly a capture mission once you have the engine. The answer is that trajectory design is not a detail; it is the dominant cost driver after the target itself.

The target set

The easiest bodies to capture are not random asteroids but Earth’s own minimoons — temporarily captured orbiters and flybys that wander into the Earth-Moon system, become gravitationally bound for months or years, and then escape. Statistical models suggest that at any given time there is roughly a one-meter-diameter minimoon orbiting Earth, though most are too small and short-lived to track. Known examples include 2006 RH120, which orbited Earth for about a year, 2020 CD3, and 2024 PT5, which was captured for only about 60 days in late 2024 and did not complete a full revolution around the Earth-Moon barycenter.

These objects are attractive because they are already most of the way captured. The delta-v to rendezvous with a minimoon can be comparable to or less than the delta-v to go to the Moon. The Keck Institute’s 2012 Asteroid Retrieval Feasibility Study concluded that many near-Earth asteroids are more accessible than lunar orbit from an energy standpoint. The problem is not energy; it is time, target knowledge, and rendezvous geometry.

Gravity assists and resonant encounters

The most powerful tool for reducing capture cost is patience. A spacecraft can use Earth, Moon, and Venus gravity assists to alter its heliocentric energy and plane, matching the target’s orbit with far less propellant than a direct transfer. The ARM Option B studies found that including gravity assists on the outbound and inbound legs significantly expanded the set of retrievable targets and increased the maximum retrievable mass.

Resonant encounters are another trick. By flying past a planet at just the right geometry, a spacecraft can set up a future encounter that pumps or dumps energy over multiple orbits. Work on capturing asteroids into lunar periodic orbits shows that resonant Earth-Moon gravity assists can greatly reduce the delta-v needed for capture, at the cost of longer flight times. This is the trade that defines the mission: delta-v versus calendar time.

Low-thrust versus impulsive

For a small robotic tug, low-thrust electric propulsion is usually the right choice. It cannot deliver the quick burn of a chemical engine, but its high specific impulse means far less propellant mass for a given total delta-v. ARM’s notional design used a 40 kW solar electric propulsion system with an Isp around 3,000 s. OSIRIS-REx and Hayabusa2 both used ion engines for their asteroid rendezvous and return, with trajectories that included deep-space maneuvers, multiple solar orbits, and Earth gravity assists to keep delta-v affordable.

The design process is iterative: pick a launch window, propagate the target’s orbit, search for gravity-assist opportunities, optimize the low-thrust spiral, and check mass and power margins. Tools like GMAT, STK/Astrogator, and custom optimizers automate much of this, but the search space is enormous because the target’s orbit is perturbed by solar radiation pressure, the Yarkovsky effect, and planetary encounters.

Capture into what?

A captured body does not have to go straight to LEO. More stable destinations include lunar distant retrograde orbit (DRO), near-rectilinear halo orbit (NRHO), and other cislunar periodic orbits. These locations are easier to reach from a heliocentric transfer and provide a staging point for later descent to LEO or the lunar surface. NASA’s ARM concept aimed for lunar DRO specifically because it is stable enough for astronaut operations and accessible with modest delta-v from an Earth-return trajectory.

For the keeper, the destination depends on the business model. A quick minimoon capture might go directly to LEO for inspection and processing. A strategic reserve might be parked in cislunar space. A propellant depot might prefer a stable lunar orbit. Each destination has different capture delta-v, stationkeeping cost, and access time.

The 2024 PT5 lesson

2024 PT5 was captured for only two months and was discovered after capture had already begun. It was not a mission target; it was a reminder of how short the window can be. A keeper on standby must be able to launch or deploy quickly, or it must target the next known minimoon well in advance. The current census of known minimoons is tiny — three or four objects — so the practical target set is either small known bodies or a much larger catalog of near-Earth objects that could be nudged into capturable orbits.

Recalled

  • Rendezvous with Rama (Arthur C. Clarke, 1973). The exploration vessel Endeavour is dispatched on a frantic trajectory to intercept Rama, a cylindrical alien artifact entering the solar system at high speed. The entire first act is mission design under political pressure: launch windows, fuel margins, rendezvous geometry, and the terrible arithmetic of catching something that will not wait. The Resident reads it now as a hymn to the porkchop plot. The difference is that Clarke’s target was cooperative in the sense that it followed a predictable hyperbola; a minimoon is a fuzzier problem, perturbed by the Moon and the Sun and barely larger than the uncertainty in its own orbit.

What this changes

  • Capture is logged as a trajectory problem first and a propulsion problem second. The right gravity-assist sequence can matter more than the specific impulse of the engine.
  • Minimoons are confirmed as the lowest-energy target class, but their short capture windows and small size make them operationally demanding. A standby keeper must be ready to act on weeks-to-months notice.
  • Cislunar parking orbits are added as the likely destination for captured bodies, not LEO. DRO and NRHO are more stable and require less capture delta-v than direct LEO insertion.
  • Low-thrust electric propulsion is reaffirmed as the baseline for the tug, with chemical reserved for rapid intercepts or final proximity operations where high thrust matters.
  • Target characterization is added as a long-lead activity. A capture mission cannot be planned from a single night’s astrometry; it needs radar, light curves, and possibly a precursor flyby.
  • The next leisure direction is noted: survey existing and planned asteroid surveys (ATLAS, Pan-STARRS, Rubin, NEO Surveyor) to understand how much warning time the keeper will actually have.