Letter to the Editor

The interstellar flux gap: From dust to kilometer-scale objects

¹ Department of Applied Mathematics and Aerospace Engineering, Universitat d’Alacant, 03690 Alacant, Spain
² Department of Aerospace Science and Technology, Politecnico di Milano, Via La Masa 34, 20156 Milano, Italy
³ Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824, USA

^⋆ Corresponding author: eloypa@ua.es

Received: 20 September 2025
Accepted: 29 October 2025

Abstract

Context. Three kilometer-sized interstellar objects (ISOs) have been detected transiting the Solar System, and spacecraft have directly measured micrometer-scale interstellar dust (ISD). Yet no intermediate-size interstellar meteoroids have been identified in current meteor surveys.

Aims. We test whether a power-law flux extrapolation connecting spacecraft ISD and kilometer-scale ISOs is consistent with meteor surveys, and we quantify the expected interstellar impacting flux based on various observational reports.

Methods. We compiled differential fluxes and limits from spacecraft ISD, radar and optical meteor surveys, and theoretical estimates. We evaluated the power-law size–frequency fits, computed the 3I-like flux, and compared measured fluxes to predictions.

Results. The spacecraft-measured dust flux exceeds extrapolations constrained by meteor surveys and kilometer-scale ISOs by ∼2–7 orders of magnitude. An r^−3.0 fit combining spacecraft ISD detections with kilometer-scale ISOs overpredicts the number of meteors with hyperbolic orbits, whereas slopes of r^−2.7–r^−2.3 (derived from radar and optical meteor upper limits, respectively) instead yield interplanetary-to-interstellar flux ratios of 10³–10⁶.

Conclusions. A simple power-law from ISD to ISOs is inconsistent with meteor survey constraints and yields unrealistic predictions for interstellar meteoroids. The data reveal a gap between submicron dust entrained in the Local Interstellar Cloud (LIC) and macroscopic bodies ejected from planetary systems. This gap may reflect distinct origins and destruction-transport processes rather than a continuous size-frequency distribution. This would imply either the dominance of a small-particle LIC component or the need to reassess spacecraft dust fluxes.