Future of Mid-IR colors of sub-km asteroids from serendipitous JWST observations

The James Webb Space Telescope (JWST) offers, for the first time, routine mid-infrared imaging capable of detecting the thermal glow of kilometre- and sub-kilometre-scale asteroids that are invisible to ground-based optical surveys. We present the first systematic data-mining campaign targeting publicly released MIRI images acquired between March 2022 and October 2023. We analyzed data from 29 programmes that targeted pointings that fell within ±10° of the ecliptic, and grouped them into 171 fields that could be searched for moving objects, spanning a cumulative area of about 0.1 deg². A GPU-accelerated linear-motion pipeline, tuned to JWST’s dither strategy and resolution, recovered 17 moving sources consistent with main-belt orbits, 14 of which have no prior catalogue entry. Most objects were detected in several MIRI filters, spanning F770W–F2100W.

We considered a model spectrum using the Planetary Spectrum Generator (PSG) for a canonical asteroid at 2.9AU and pV=0.13, to obtain typical colors of an asteroid, and the expected visible magnitudes in the range g′ ≈ 24–29, and diameters of ~0.3–5 km assuming moderate albedos. When comparing with multi-filter photometry we obtain a systematically bluer F770W–F1500W slopes than those predicted for the comparison asteroid.

We estimate our detection efficiency by injecting synthetic PSFs, and recovering them in each field and filter in our dataset. These translated to visible g’ magnitudes, and combined with the surveyed area, and asteroid luminosity function predict an expected yield of 27_{-5}^{+70} serendipitous asteroids, only ∼2 σ above our observed yield. The faint-end turnover we detect is therefore genuine and, if confirmed, will tighten constraints on collisional models and the size–frequency distribution below D ≈ 1 km. Extrapolating our cadence to the ∼2 deg² of MIRI and NIRCam imaging already in the archive suggests JWST is on course to deliver ≥100 main-belt discoveries by Cycle 5 and >500 over a ten-year mission, probing down to D ≃ 0.2 km at 2–3 AU and to D ≃ 1 km in the outer belt.

Our pilot survey demonstrates that the JWST archive harbours a rich and largely untapped reservoir of small bodies. By exploiting this resource we will construct an infrared census of the main belt and transitional populations, refining models of planetesimal accretion, and collisional evolution.