On December 19, 2025, the interstellar comet 3I/ATLAS will make its closest approach to Earth, marking a significant event for astronomers and comet enthusiasts alike. This encounter presents a unique opportunity for ground-based observatories to capture and study the comet as it journeys back out of the Solar System. At its perigee, 3I/ATLAS will be approximately 270 million kilometers (about 168 million miles) away from our planet, which, while nearly twice the distance between Earth and the Sun, is still close enough for valuable observations.
Unusual Characteristics of 3I/ATLAS
Since its discovery on July 1, 2025, 3I/ATLAS has exhibited a range of unusual features that set it apart from traditional comets. As it approached the Sun and Mars, scientists noted early outgassing that produced significant amounts of carbon dioxide. Further observations revealed high levels of hydrogen cyanide (HCN) and methanol (CH3OH) in its coma, driven by the warming effects of solar radiation on the comet’s icy components.
In the initial months following its discovery, researchers also identified unusually elevated quantities of nickel and iron, suggesting a composition that diverges from typical cometary materials. In early October, 3I/ATLAS made a close pass to Mars, an event monitored by various Martian and solar observatories.
Observations of the comet faced challenges when its trajectory took it behind the Sun from Earth’s perspective during perihelion, the closest approach to the Sun, which occurred on October 29. Once it emerged from the Sun’s glare, various observatories resumed their observations. The XMM-Newton space telescope, operated by the European Space Agency (ESA), dedicated 20 hours to studying 3I/ATLAS, detecting X-radiation emitted as the solar wind interacted with the comet’s expanding coma.
Changing Colors and Ongoing Research
In late November, data from the Gemini North telescope, part of the National Optical-Infrared Astronomy Research Laboratory (NOIRLab), indicated a shift in the comet’s coloration, revealing a hint of green. This change is noteworthy, as earlier images had depicted a reddish hue consistent with organic compounds known as tholins that coat the comet’s surface. The greenish tint often seen in comets is produced by diatomic carbon (C2), which fluoresces when exposed to solar radiation.
Interestingly, observations prior to perihelion suggested that 3I/ATLAS was low in C2, typically not found in significant amounts within cometary ice. The new findings imply that the comet may have only begun forming C2 late in its journey through the Solar System, adding to its intrigue.
While the full implications of these characteristics remain unclear, some scientists speculate that 3I/ATLAS could be a metal-rich object with cryovolcanic activity, releasing gases into space. Others propose that it may be exhausting its icy reserves and transitioning into a post-cometary state akin to that of an asteroid. Despite these peculiarities, all evidence points to it being a comet.
According to Amit Kshatriya, Associate Administrator at NASA, “This object is a comet. It looks and behaves like a comet, and all evidence points to it being a comet. But this one came from outside the Solar System, which makes it fascinating, exciting, and scientifically very important.”
As 3I/ATLAS approaches its closest point to Earth, the upcoming observations are expected to yield further insights into the unique properties of this interstellar visitor and enhance our understanding of cometary science.
