Deep Ground Based Searches for New Horizons Encounter Targets

The New Horizons spacecraft continues to operate in the Kuiper Belt and retains capability for trajectory correction maneuvers, preserving the possibility of a final close encounter with a small Kuiper Belt Object (KBO). Identifying such a target presents a fundamentally different problem from conventional trans-Neptunian object (TNO) discovery surveys. Viable encounter candidates must satisfy simultaneous constraints on detectability, spacecraft reachability, encounter geometry, available ∆V, and the time required for orbit determination and maneuver planning. The accessible search volume is therefore extremely limited, and maximizing the number of detections is not equivalent to maximizing the probability of identifying a dynamically reachable target.

We present the observational and analysis framework developed to support downstream target searches for New Horizons, with particular emphasis on the transition from conventional survey discovery methods toward mission-driven, time-domain characterization strategies relevant to Rubin Observatory LSST operations. Building on the framework proposed in Kavelaars et al. (2024), the search strategy combines Kuiper Belt population models with spacecraft trajectory constraints to predict the spatial density, apparent motion distribution, and observability of accessible objects downstream of the spacecraft trajectory. These models define optimal search geometries, cadence strategies, and limiting magnitudes for deep imaging campaigns.

We summarize deep searches conducted with the Subaru Hyper Suprime-Cam (HSC) and discuss the first Rubin Observatory trial analyses obtained during early commissioning-era observations in 2025. These searches operate near the practical limits of moving-object detection in wide-field ground-based imaging and therefore require analysis methods that differ substantially from standard survey pipelines. To support this work we have extended the Rubin LSST Science Pipelines infrastructure to enable deep orbital shift-and-stack analyses for extremely slowly moving outer Solar System objects. This includes orbit-based moving-source injection tools for completeness characterization together with integration of pkbmod, a modified implementation of the KBMOD framework, within the LSST middleware environment.

The New Horizons target search provides a demanding test case for the broader problem of converting LSST-scale time-domain discovery streams into physically interpretable and mission-relevant constraints. The methods developed here provide a scalable and reproducible framework for deep searches in wide-field imaging while preserving calibration provenance and quantitative characterization of detection efficiency. Future progress will likely require detection approaches that move beyond classical thresholding methods, including machine-learning-assisted analyses optimized for extremely low signal-to-noise moving-source detection in sparse time-domain datasets.