estimates for 3i/atlas's nucleus diameter range from 320 meters to about 10 km, based on pre-perihelion observations, assuming minimal erosion during its interstellar journey from a thick disk star system billions of years ago. hubble data pins it around 1-5 km, while avi loeb suggests at least 10 km if it survived intact. size likely unchanged since leaving its origin, as comets don't erode much in cold interstellar space.
britannica
https://www.britannica.com/topic/comet-3i-atlas
avi loeb medium
https://avi-loeb.medium.com/the-remarkable-large-scale-structure-of-anti-tail-and-tail-jets-from-3i-atlas-242c534cf7b3
nasa science
https://science.nasa.gov/missions/hubble/as-nasa-missions-study-interstellar-comet-hubble-makes-size-estimate
