Interstellar Object Encounter Rate

Three confirmed ISOs in eight years. What flux does that imply? What fraction could be artificial probes? And what does a null SETI result on 3I/ATLAS actually tell us?

🔬 Established (rate est.) ⚠ Theoretical (SETI update)
2017
1I/ʻOumuamua
Rocky/pancake-shaped
Anomalous non-gravitational acceleration; no outgassing detected
2019
2I/Borisov
Active comet
Clear cometary activity; consistent with natural CO-rich comet
2025
3I/ATLAS
Active comet
FAST, ATA, GBT radio searches: no technosignature detected (EIRP < 0.1 W)
Survey Parameters
0.30
Fraction of ISOs actually detectable in current surveys. Pre-LSST surveys detect ~10–50% of passing ISOs.
10⁻⁴
Your prior probability that a given ISO is an artificial probe. Null SETI results update this downward.
0.10 W
Minimum detectable EIRP at ISO distance. GBT on 3I/ATLAS achieved ~0.1 W (Dec 2025).
10 AU
Closest approach of the ISO. SETI sensitivity scales as d⁻². 3I/ATLAS closest approach: ~2 AU.
Inferred ISO flux
—
detectable ISOs / yr (Solar System)
Underlying population rate
—
true ISOs / yr passing within 5 AU
Expected artificial ISOs / century
—
given prior P(artificial)
⟳ Bayesian update — what null SETI results on 3I/ATLAS tell us
Prior P(artificial | ISO)
—
Your starting assumption before any SETI observation
Posterior P(artificial | null result)
—
Updated after null SETI at the selected sensitivity
Posterior P(artificial) vs. EIRP sensitivity limit
How much a null result updates P(artificial) depends on how powerful a transmitter we could have detected · vertical line = current sensitivity
↗ Radio SETI (EIRP limits) ↗ Optical SETI ↗ Great Filter (ISO rate and Fermi) ↗ Drake Monte Carlo ↗ Interstellar Link Budget

The observed ISO catalogue

Three interstellar objects have been confirmed as of mid-2026: 1I/ʻOumuamua (2017), 2I/Borisov (2019), and 3I/ATLAS (discovered 1 July 2025). The detection rate of ≈0.4 yr⁻¹ from surveys with efficiency η represents a lower bound on the true passage rate. Current survey completeness for ISOs passing within 5 AU of the Sun is estimated at 10–50%.

ISO population flux

The implied true rate is R = N_detected / (T_survey × η) where N_detected = 3, T_survey ≈ 8 yr, and η is your detection efficiency. The inferred per-century rate of "artificial ISOs" is simply R × 100 × P(artificial), where P(artificial) is your prior probability that a given ISO is an artificial probe.

Bayesian SETI update

A null SETI result (no signal detected from 3I/ATLAS) is evidence against the artificial hypothesis, but only to the extent that an artificial probe would have been detectable at the achieved sensitivity. The Bayesian update uses:

P(art | null) = P(null | nat) × P(art) / P(null)

where P(null | natural) ≈ 1 (natural objects don't transmit), and P(null | artificial) = P(EIRP < threshold | artificial), which approaches 1 for very faint transmitters. If the probe is silent at the achieved sensitivity, the update is modest. The GBT achieved EIRP < 0.1 W on 3I/ATLAS (Breakthrough Listen, arXiv:2512.18142) — the most sensitive SETI observation of an ISO to date.

Key references

Breakthrough Listen / Allen Telescope Array on 3I/ATLAS: arXiv:2512.18142 (Dec 2025). GBT <0.1 W EIRP limit.

FAST narrowband search toward 3I/ATLAS: arXiv:2603.19023 (Mar 2026).

ISO detection rates: Levine et al. (2021), arXiv:2103.07608.

Tool v1.0 · 2026-06-01 · Code: MIT · Prose: CC BY 4.0 · omegacentauri.me/tools/iso-encounter.html