How well can the position of Proxima Centauri b at a probe's arrival be known before launch?

Abstract

Abstract. Aiming a small interstellar probe at Proxima Centauri b requires a pre-launch prediction of the planet's position at arrival; one published requirement on the total ephemeris error is 10⁵ km. We combine Gaia DR3 astrometry with a refit of public radial velocities and propagate the stellar and planetary terms in one model to the plane perpendicular to the probe's approach, for launch in 2040–2070 at 0.2c and 0.1c (declared assumptions). A decision rule fixed before running counts a radius R as reached only if the radius r₉₀ of the 90% predictive region is at most R in all eight declared model variants. With current data, at the grid launch epochs, r₉₀ is 7.20 × 10⁶–9.45 × 10⁶ km, and improving any single input alone does not reach 10⁵ km. On a declared grid of future observations, with the decision required to hold at 24 launch times spread over one orbital period of the planet, the smallest grid value of R at which the earliest reaching launch year is a definite year is 3.16 × 10⁵ km at 0.2c and 5.62 × 10⁵ km at 0.1c; for no declared setting is the earliest reaching launch year at 10⁵ km a definite year over this phase range. The six settings evaluated over the phase range, which attain these values, combine imaging that resolves the planet with large reductions of the Gaia and kinematic radial-velocity uncertainties; at the grid launch epochs alone they reach 10⁵ km at 0.2c. Without imaging, no declared combination reaches below 10⁷ km, either at the grid launch epochs or over the phase range. The r₉₀ values are quantiles of the predictive distribution under the declared model, not intervals whose calibration has been verified. Instrument performances are assumptions, not forecasts. Version 0.4 (manuscript v0.4, 2026-10-20). This record holds the paper (PDF), an archive of the project repository without its version history (inputs, configuration files, scripts, machine-generated records, criteria and results documents, review briefs and findings, and the manuscript with a Japanese translation; most project documents are in Japanese), the list of SHA-256 digests of the archived files (MANIFEST.sha256), and README.md. License: CC BY 4.0 for the files written in this project. Exception: the raw data in data/raw/ of the archive are not covered by CC BY 4.0 and remain under the terms of their sources: the Gaia DR3 query results are distributed under the CC BY-NC 3.0 IGO license, and the VizieR catalog files are free of usage in a scientific context under the CDS rules of usage (see README.md).