3I/ATLAS is the third known interstellar object to pass through our solar system, after 'Oumuamua and 2I/Borisov. Unlike 'Oumuamua (anomalous acceleration, no visible coma) and 2I/Borisov (typical cometary activity), 3I/ATLAS offers a third data point — each interstellar visitor is a sample from the galaxy's population of planetesimal ejecta.
The authors (arXiv:2603.23240) observed 3I/ATLAS near perihelion with the IRAM 30-meter radio telescope at millimeter wavelengths. Radio observations probe different physics than optical: they detect molecular emission lines from outgassed species (HCN, CO, CH₃OH) and thermal emission from dust grains. The composition and outgassing rates constrain where and how the comet formed in its original stellar system.
The through-claim: each interstellar comet is a geological sample from another solar system delivered to our doorstep. We can't visit other planetary systems, but their debris visits us. Three objects in seven years suggests the Galaxy is full of wandering planetesimals, and each one's composition is a message about the disk chemistry where it formed. The sample size went from zero to three; the science went from speculation to measurement.