Scenario — A Visitor from Outside

01

ISO Encounter Rate tool (new) · population flux

Three in eight years: what the rate implies

Three confirmed detections in ~8 years at a survey efficiency of 10–50% implies a true ISO passage rate of roughly 1–10 per year within 5 AU of the Sun. The pre-LSST detection rate was suppressed by survey incompleteness; Vera Rubin Observatory (LSST) is expected to detect dozens per year when operating at full depth.

The ISO Encounter Rate tool lets you set your own detection efficiency and a prior probability that any given ISO is artificial. For a prior of 10⁻⁴ (one-in-ten-thousand chance any ISO is a probe), the expected number of artificial ISOs per century is ≈0.001–0.01 — solidly consistent with "none detected so far." The Bayesian update from the 3I/ATLAS null result (no transmitter above 0.1 W) shifts that prior downward further, but only modestly.

Open ISO Encounter Rate tool → 🔬 Rate estimation ⚠ Artificial-fraction prior

Step payoff

Even an optimistic prior of P(artificial) = 10⁻² applied to 1 ISO/year gives an expected 0.01 artificial ISOs per year — consistent with zero detections in a decade. The null result is not surprising at any remotely realistic prior. What the tool quantifies is how much the null result should update us, given the 3I/ATLAS sensitivity floor.

02

Radio SETI Sensitivity · 3I/ATLAS campaign

The most sensitive SETI observation ever made of a passing object

3I/ATLAS triggered the most concentrated multi-telescope SETI campaign ever aimed at an interstellar object. The Green Bank Telescope achieved EIRP sensitivity below 0.1 W at closest approach (December 2025) — sensitive enough to detect a transmitter weaker than a hand-held radio from the distance of the object. FAST conducted four observation windows from October 2025 to January 2026 (arXiv:2603.19023). No narrowband signals were found in any search.

Load the Radio SETI tool and set the distance to 3I/ATLAS at closest approach (≈1.5 AU from Earth). The tool shows the EIRP floor achievable with the GBT and FAST at that distance, and how it compares to hypothetical transmitter types — from a Voyager-scale omnidirectional beacon to a narrowband directed beam.

Open Radio SETI tool → 🔬 Observational ⚠ Narrowband assumption

Step payoff

The 3I/ATLAS null result at <0.1 W EIRP is the most constraining single-object SETI observation in history. It rules out an actively transmitting probe at any detectable power level. It does not rule out a dormant probe, a passive observer, or a probe transmitting in a direction other than toward Earth — those require different observational strategies.

03

Optical SETI Sensitivity · laser and reflectivity constraints

Can we see it shine? Optical constraints on 3I/ATLAS

Radio is not the only channel for an artificial ISO to signal. An engineered object might use laser pulses, anomalous reflectivity, or periodic optical modulation. The Optical SETI tool applies the same sensitivity-curve logic to the optical channel: what pulse energy or continuous-wave power would be detectable at the object's distance with current surveys?

3I/ATLAS was bright (≈12th magnitude at closest approach), well within the range of every major optical SETI program. No anomalous pulsations were reported. The optical constraints are therefore complementary to the radio null: together they rule out omnidirectional optical and radio transmitters above energy thresholds set by current survey sensitivities.

Open Optical SETI tool → 🔬 Observational ⚠ Directed beam not ruled out

Step payoff

The combination of radio + optical null results covers most plausible communication channels for an intentionally signalling probe. The remaining uncertainty is directional: a beam pointed away from Earth, a laser pulse at a wavelength or time not covered, or a technology entirely outside the radio/optical paradigm.

04

Great Filter Localizer · natural ISOs and the filter

If they are natural: what that tells us

All three ISOs are almost certainly natural objects — cometary or rocky debris ejected from forming planetary systems in other stars. If the ISO population is entirely natural, and if the universe produces ISOs prolifically but no artificial ones, that is itself evidence about the Great Filter: the filter must be located somewhere on the path between "solar system forms" and "civilisation capable of launching interstellar probes."

The Great Filter Localizer makes this quantitative. If we set P(ISO is artificial) to near zero based on three null results, and we note that a galaxy with K-II civilisations would produce a flood of deliberate ISO-probes, then the absence of artificial ISOs in a catalogue of three is weak evidence that K-II civilisations are rare — consistent with the filter being real and operating before the K-II threshold.

Open Great Filter tool → ⚠ Theoretical (Hanson 1998)

Step payoff

Three ISOs is a tiny sample. With LSST detecting dozens per year, the ISO catalogue will grow rapidly. If none prove artificial over decades of searching, that will be a genuine probabilistic constraint on the density of K-II probes in the galaxy — and by extension, on where the Great Filter sits. The accumulating null result from ISOs is therefore one of the most concrete SETI observational programmes now underway.