Discovering Numerous Interstellar Objects with A Dedicated Space Telescope

Brief

Loeb (2025) calculates that the number density of 10-m interstellar objects, extrapolated from the 'Oumuamua-derived density at 100 m using a JWST-calibrated power-law index of q = −2.66 ± 0.6, reaches n(D > 10 m) ≈ 10^(1.7 ± 0.6) au⁻³. Gravitational focusing at a heliocentric distance of 0.35 au drives a passage rate of R ≈ 4.4 (× 10^±0.6) day⁻¹ through a 20° solar annulus. A meter-aperture sun-tolerant space telescope could detect such objects at V ~ 26 mag in ~3-hour exposures at SNR 10, and separate their ~600 K thermal blackbody emission (peaking at ~4.8 μm) from reflected sunlight to measure diameter and albedo. Spectroscopic follow-up of evaporated material would probe chemical composition and constrain birth environments, including whether non-gravitational accelerations resemble those of Solar system dark comets.

Metadata

Category

Search

Venue

preprint (arXiv astro-ph.IM)

Type

Preprint

Year

2025

Authors

Abraham Loeb

arXiv

2502.08478

Access

Open access

Length

97.6 K

Programs

Galileo Project

Instruments

meter-aperture dedicated space telescope (proposed), Hubble Space Telescope, Vera C. Rubin Observatory / LSST, JWST NIRCam (referenced for asteroid size distribution), Parker Solar Probe (cited as thermal engineering precedent), SOHO (cited as thermal engineering precedent), Inouye Solar Telescope (cited as thermal engineering precedent)

Data sources

JWST main asteroid belt survey (Burdanov et al. 2025), 'Oumuamua discovery photometry (Meech et al. 2017; Drahus et al. 2018), Do et al. 2018 number density estimate

Tags

interstellar objects, SETI-adjacent, technosignature-precursor, solar telescope, infrared astronomy, planetary science

Key points

• Extrapolating from the 'Oumuamua-derived number density at D > 100 m (~0.1 au⁻³) using JWST's power-law index q = −2.66 ± 0.6 yields n(D > 10 m) ≈ 10^(1.7 ± 0.6) au⁻³ for interstellar objects.p.2
• At a heliocentric distance of 0.35 au, the gravitational-focusing-dominated passage rate is R ≈ 4.4 (× 10^±0.6) per day, with objects crossing a 1° field of view in ~9 hours.p.3
• A 10-m interstellar object at d ~ 0.35 au with albedo comparable to 'Oumuamua reaches V ~ 26 mag, detectable at SNR 10 in ~3-hour exposures by a meter-aperture space telescope.p.3
• Surface temperature at 0.35 au is ~600 K, corresponding to a blackbody peak at ~4.8 μm; separating this from reflected sunlight (T☉ ~ 5,800 K) permits direct measurement of diameter and albedo.p.4
• Objects enter and exit the 20° solar annulus once per ~5.5 (× 10^±0.6) hours, but HST's 50° sun-avoidance exclusion zone makes it unusable for this search.p.3
• The work was supported by the Galileo Project at Harvard University.p.4

Verbatim

• “Assuming that smaller interstellar objects follow a distribution similar to that in equation (1) down to D ∼ 10 m, implies an interstellar density, n ( D > 10 m) ≈ 10 1 . 7 ± 0 . 6 au − 3 .”

  p.2

• “New interstellar objects of 10-m diameter are expected to enter and exit a 20 ◦ circle around the Sun once per ∼ 5 . 5( × 10 ± 0 . 6 ) hours.”

  p.3

Most interesting

• At 0.35 au, gravitational focusing from the Sun accelerates interstellar objects to 71–90 km/s, fast enough to cross a 1° telescope field of view in only ~9 hours, still allowing multiple exposures before the target exits.
• The 20° solar annulus proposed for the search extends to ~75 solar radii, well outside the corona, meaning thermal engineering (not coronagraphy) is the primary obstacle.
• The JWST-measured power-law index for main-belt asteroid sizes (q = −2.66 ± 0.6 for D < 100 m) is the key empirical lever: a shift of just ±0.6 in the exponent changes the predicted interstellar object density by a factor of ~10 in either direction.
• A cometary dust/gas tail like that observed on 2I/Borisov would scatter additional sunlight and further improve detectability of even smaller interstellar cores.
• The paper explicitly asks whether anomalous non-gravitational acceleration seen in 'Oumuamua resembles that of 'dark comets' recently identified within the Solar system, a testable hypothesis using close-perihelion spectroscopy.
• At 600 K surface temperature, some 10-m interstellar objects are expected to partially evaporate or disintegrate during their solar approach, making each passage a one-time destructive sampling event.