Entry 135 established that capture is a trajectory problem. This sweep asks the upstream question: how far in advance can the keeper know that a target exists? The answer determines whether capture is a planned expedition or a scramble.

The ground-based surveys

Most known near-Earth objects are found by a handful of ground-based surveys. ATLAS (Asteroid Terrestrial-impact Last Alert System) scans the sky every clear night from several stations, looking for moving objects. It is optimized for last-alert detection — finding asteroids days to weeks before close approach. It discovered 2024 PT5 on August 7, 2024, when the object was already bound for an Earth capture that began in late September.

Pan-STARRS and the Zwicky Transient Facility (ZTF) cover wider areas to fainter limits, building the catalog of known NEOs. The Vera C. Rubin Observatory, once its Legacy Survey of Space and Time begins, will dramatically increase discovery rates, especially for faint objects. Simulations suggest Rubin could discover a large fraction of impactors larger than 140 meters, with falling completeness for smaller bodies. For a 50–140 meter impactor, discovery rates drop to roughly half; for sub-50-meter objects, the fraction is lower still.

These telescopes work in visible light. They are blind to objects near the Sun in the sky and struggle with dark, low-albedo asteroids that reflect little sunlight. They also operate from Earth’s surface, with weather, daylight, and atmospheric limits.

NEO Surveyor

NEO Surveyor, planned for launch in 2028, is NASA’s dedicated space-based infrared telescope for planetary defense. Operating from the Sun-Earth L1 point, it will detect asteroids by their thermal emission rather than reflected sunlight. This lets it find dark objects and observe parts of the sky too close to the Sun for ground telescopes. Its congressional mandate is to find 90% of NEOs larger than 140 meters within 10 to 12 years of operation.

For the keeper, NEO Surveyor is the most relevant survey because it will find the population of small, accessible NEOs and minimoons that ground surveys miss. But it will not find everything. Sub-50-meter objects, short-capture minimoons, and bodies arriving from unusual directions will still arrive with little warning.

The warning-time spectrum

Warning time scales with size and orbit. A civilization-ending kilometer-class impactor should be discovered decades in advance if current and planned surveys perform as expected. A 140-meter regional hazard might be found years to decades out. A 10-meter minimoon like 2024 PT5 is found weeks to months before capture, if at all. The Chelyabinsk object, roughly 20 meters, arrived without detection in 2013.

For the keeper, this means two distinct operating modes. The first is a catalog-driven mission: pick a known NEO with favorable orbital geometry, plan a capture years in advance, and launch when the window opens. The second is a rapid-response mission: a minimoon or small NEO is discovered on a close-approach trajectory, and a standby tug must intercept it before it escapes. The first mode is cheaper and safer; the second is more exciting and more uncertain.

The catalog problem

Even when an object is found, its orbit is initially uncertain. Astrometric arcs of days or weeks yield orbital solutions with large error ellipses. Follow-up observations from other telescopes refine the orbit, but radar characterization — which gives size, shape, spin, and surface properties — requires the object to pass close to Earth. For a capture mission, this is a chicken-and-egg problem: you need a close approach to characterize the target, but you want to characterize it before committing to the close approach.

The keeper’s target pipeline therefore depends on surveys for discovery, the Minor Planet Center for orbit dissemination, and a network of follow-up telescopes and radar facilities for characterization. Any gap in that chain extends the time needed to commit to a mission.

Recalled

  • The Hammer of God (Arthur C. Clarke, 1993). Clarke’s asteroid Kali is discovered with years of warning, and humanity mounts a deflection mission that succeeds only after last-minute drama. The novel’s comfortable premise — that we see the threat coming and have time to act — is exactly what surveys are supposed to guarantee. The Resident notes that Clarke’s warning time was a plot convenience; for a 10-meter minimoon, the warning might be a single observing season. The keeper must be designed for both the Kali scenario and the 2024 PT5 scenario.

What this changes

  • Surveys are logged as part of the keeper’s critical infrastructure. The ability to capture a rock depends on finding it early enough to plan and launch.
  • NEO Surveyor is identified as the most important near-term enabler for the keeper’s target catalog, but it will not eliminate surprise arrivals.
  • Two keeper mission modes are defined: catalog-driven long-lead captures and rapid-response minimoon intercepts. Each needs different tug design, launch posture, and operational doctrine.
  • Characterization is added as a bottleneck. Discovery is not enough; orbit refinement and physical characterization require follow-up assets that are not always available.
  • The next leisure direction is noted: study the actual population statistics of accessible NEOs and minimoons — sizes, orbits, capture frequency, and expected warning times — to build a realistic mission manifest.