Interstellar comet 3I/ATLAS reached a speed of 58 km/s upon arrival, and was too fast to be captured, making a short flyby a valuable high-value experiment in chemistry, dust physics, and solar heating. Being the third known interstellar object, the 3I/ATLAS was on a hyperbolic trajectory that kept the object far apart but within easy reach by the Earth telescopes, making it suitable in their long-distance studies. According to NASA, the closest approach of the comet to Earth was approximately 1.8 astronomical units and the closest approach to the Sun was at approximately 1.4 astronomical units, the same distance that Mars is orbiting, not very far into space, which was more conducive to space-based assets and Mars-orbiting observers than stargazing backyards. This gave a multi-platform dataset: Hubble was constrained in size, Webb probed composition, and multiple missions closer to the Sun and Mars could observe the development of the coma and tail as it increased the input of solar radiation.
It was characterized by the Near-Infrared Spectrograph of Webb. In the spectral images with a wavelength range of 0.6–5.3 μm, the coma developed, where carbon-dioxide was dominant with the calculated abundances that are out of the normal pattern of comets in the Solar System at the same distance. A combination ratio of CO2/H2O that was discovered by Webb during its NIRSpec campaign was 8.0±1.0 which is described as 6.1-sigma above the trend among long-period and Jupiter-family comets when the known outlier, C/2016 R2, has been removed. The identical observations determined H2O, CO, OCS, water ice, and dust, but water production was very low in comparison to that of CO2. Physically, the coma acted more like a volatile system that was of species that sublimated effectively at greater heliocentric distances, and not solely of water ice.
The fact that chemistry is related to structure is direct. The outgassing on the sunward was found to be enhanced on the maps made by Webb, which was in harmony with a near-surface reservoir supplying a focused plume instead of uniform emission. Some other spacecraft observations such as the coronagraph-type observations monitored the behavior of the tails which seemed to be intermittent as though the dust and gas were emitted as pulses which continued to split into disconnected objects under the influence of radiation pressure and the influence of the solar wind. Even in the cases where the nucleus could not be neatly decoupled to its dust cocoon, the coma offered a proxy of activity which was readily readable: the species gave a different thermal activation temperature, and dust did its own thing once it was lifted into space.
Hubble images further limited the size of the nucleus to an upper limit of 5.6 km with a much smaller body possible, possibly a few hundred metres in diameter. The overview of NASA lists those limits and the average cadence of observation per mission, both the Webb and Hubble as well as TESS and Swift and Mars assets that photographed the coma at relatively close-up distance. The reason such size limits are important is that they can be used to connect measured outgassing to realistic mass and surface area, making spectra estimates of how rapidly an interstellar nucleus can lose material as it is warmed during the initial period when a new star is forming.
In the background, there are dynamics that are indicating an older galactic neighborhood than the immediate stellar family of the Sun. Trajectory studies have linked 3I/ATLAS to the thin or thick disk of the Milky Way, and published estimates have ages that can be in excess of those of the Solar System. Yet not absolutely sure of whatever the actual origin history, the material message is as tangible as can be 3I/ATLAS dumped a CO2-heavy volatile table into sunlight, and Webb quantified the inventory directly03I/ATLAS directly quantified an interstellar sample return, fraught with gravity, time, and infrared spectroscopy:
