Comet 3I/ATLAS Approaches Perihelion: Latest Developments
Comet 3I/ATLAS, a remarkable interstellar visitor, reached perihelion on October 30, 2025. Observers worldwide have been monitoring this celestial body, although it was too near the Sun for terrestrial telescopes to capture it adequately. This comet is only the third known object to originate from outside our Solar System, following 1I/‘Oumuamua and 2I/Borisov.
Discovery and Initial Observations
3I/ATLAS was discovered on July 1, 2025, by the Asteroid Terrestrial-impact Last Alert System (ATLAS) telescope located in Chile. Its designation “3I” identifies it as the third confirmed interstellar body. At the time of discovery, the comet was faint, with a magnitude between 17 and 18, and appeared slightly extended. Scientists had early debates about whether the observed size indicated an active coma or a bare nucleus, estimated to be roughly ten kilometers wide.
Follow-up observations quickly confirmed the presence of a coma, indicating active sublimation even when far from the Sun. This activity suggests that 3I/ATLAS contains volatile compounds like carbon dioxide (CO₂) and carbon monoxide (CO), which sublimate at lower temperatures than water ice. Understanding these volatiles can reveal significant insights into cometary chemistry across different planetary systems.
Composition and Activity
Spectroscopic measurements indicated that the coma of 3I/ATLAS was primarily composed of CO₂ gas, with very few traces of water vapor. This finding is pivotal because water ice typically begins to sublimate at around three astronomical units (AU) from the Sun. Researchers propose that evaporative cooling might delay water sublimation by absorbing heat, as CO₂ sublimates rapidly. Alternatively, its unusual composition may stem from its origins in a different star system.
Over time, interstellar conditions, such as cosmic radiation, may have altered the comet’s ice chemistry, resulting in unique physical characteristics compared to Solar System comets.
Orbital Behavior and Non-Gravitational Forces
Currently, 3I/ATLAS is obscured by the Sun as viewed from Earth. Its hyperbolic orbit ensures it will not return to our Solar System, passing just inside Mars’ orbit. As astronomers observed its perihelion passage, they monitored non-gravitational accelerations resulting from jets of gas and dust escaping the comet’s surface, akin to tiny thrusters affecting its trajectory.
Data from the Unistellar Network, involving citizen astronomers, provided near-continuous brightness measurements and captured unexpected fluctuations shortly before the comet became unobservable. The brightening observed prior to perihelion surpassed expectations, indicating a possible increase in gas outbursts or surface activity.
The Nickel–Iron Matrix
An intriguing discovery emerged from the European Southern Observatory’s Very Large Telescope (VLT), highlighting an unusually high nickel-to-iron ratio in the gas phase of 3I/ATLAS. This observation contrasts sharply with most Solar System comets, where nickel and iron are typically found in similar proportions. Initial measurements of the comet indicated a strong nickel emission alongside a weak iron presence. However, this ratio began to normalize as the comet approached the Sun, suggesting distinct temperature-dependent release patterns of these metals.
Future Prospects
As 3I/ATLAS moves back into interstellar space, it is expected to fade from visibility. Nevertheless, the extensive data collected during its passage will aid forthcoming missions such as the European Space Agency’s Comet Interceptor, aimed at studying future interstellar objects. Dr. Franck Marchis noted that each new discovery enhances future observational strategies, preparing scientists for subsequent encounters with interstellar visitors.
