The Tale of Interstellar Comet 3I/ATLAS

A rare celestial traveler from interstellar space is racing toward the inner Solar System. Comet 3I/ATLAS is on a one-way trip past the Sun, defying expectations with its unusual behavior and mysterious origin. Its arrival has ignited global fascination, captivating astronomers, space agencies, and curious minds worldwide.

On July 1, 2025, astronomers using the ATLAS (Asteroid Terrestrial-impact Last Alert System) survey telescope in Chile first spotted this interloper, earning it the name 3I/ATLAS. The "3I" stands for third interstellar, marking it as only the third observed object from beyond our Solar System to ever pass through.

Its arrival follows two other confirmed interstellar visitors: the mysterious 1I/ʻOumuamua, discovered in 2017, which baffled scientists with its elongated, cigar-like shape, lack of a visible tail, and unexplained acceleration; and 2I/Borisov in 2019, which behaved like a typical comet, displaying a bright coma and a tail of gas and dust. Now comes 3I/ATLAS, an even more puzzling object. Its unusual size, extreme speed, and chemical composition set it apart, offering scientists a rare chance to study ancient material from beyond our Solar System as it begins to shed light and volatile gases while approaching the Sun. Interstellar comets like these are cosmic anomalies, rather than the norm.

A Visitor from Beyond

Traveling at an eye-popping 137,000 miles per hour(!), Comet 3I/ATLAS is moving fast enough to escape the Sun's gravitational influence completely. This extraordinary velocity places it on a hyperbolic trajectory, an open-ended orbital path unlike the elliptical orbits of most comets that return periodically. This trajectory means that Comet 3I/ATLAS will pass through our Solar System only once before continuing its journey into interstellar space, never to return.

Composition and Size

What makes Comet 3I/ATLAS so compelling is its unusual chemical composition. Observations from the James Webb Space Telescope (JWST) revealed a coma dominated by carbon dioxide, with a CO₂-to-water vapor ratio of roughly 8:1, the highest ever recorded in a comet and about 16 times greater than expected at its current distance from the Sun. This suggests it may have formed in a region of its parent star system where CO₂ ice was more abundant than water ice.

Estimates of its nucleus size vary significantly. Hubble data indicate a diameter between 320 meters and 5.6 kilometers, while interpretations from JWST and Gemini South suggest it could be even larger, possibly the biggest interstellar object ever observed. Although the nucleus hasn't been directly imaged, the surrounding coma and gas plume provide valuable clues. The plume contains carbon dioxide, carbon monoxide, water vapor, traces of cyanide, and possibly nickel without iron—an unusual feature in cometary atmospheres.

Behavior

New comet images from Chile's Gemini South telescope reveal that the comet is becoming more active as it travels through the inner Solar System. Its tail and coma are expanding, shaped by solar wind into a teardrop-like plume, though it lacks a prominent dust tail. Recent photos suggest the comet is transforming into a green hue, a surprising shift that may be linked to its closer proximity to the Sun. As solar radiation intensifies, the nucleus expels rare gases, possibly triggering the color change.

How to See Comet 3I/ATLAS

Comet 3I/ATLAS slipped into the Sun’s glare in late September 2025. In October and November, the comet makes its close approach to the Sun, reaching its closest point, called perihelion, on October 29. During this window, it is impossible to view the comet from ground-based telescopes. But after it swings around the Sun, the comet is expected to reemerge in the night sky in early December.

On Friday, December 19, 3I/ATLAS makes its closest approach to Earth, safely passing us by at approximately 270 million kilometers (168 million miles). Despite this relatively close encounter, the comet will remain invisible to the naked eye due to its faint brightness and distance. Telescopes with moderate (6 inches) to large apertures (>8 inches) will likely be able to detect it, offering astronomers and skywatchers an excellent opportunity to observe this interstellar visitor. Please check back for precise dates and times as we get closer to December. 

Unique Vantage Points

Although observers on Earth have lost sight of Comet 3I/ATLAS until early December, spacecraft offer us a different way to monitor it.

On October 3, 3I/ATLAS makes its closest approach to Mars, just 30 million kilometers away. Orbiters are poised to track its movement, tail development, and chemistry. Key spacecraft expected to contribute include:

• Mars Express (ESA): High Resolution Stereo Camera (HRSC) for imaging
• ExoMars Trace Gas Orbiter (ESA): CaSSIS for color imaging; NOMAD for gas analysis
• Mars Reconnaissance Orbiter (NASA): HiRISE camera for detailed visuals
• MAVEN (NASA): Atmospheric studies with potential comet data
• Tianwen-1 Orbiter (China): High-resolution camera that may be repurposed for observation

Then on March 16, 2026, Comet 3I/ATLAS will pass within 53.6 million kilometers of Jupiter, where NASA’s Juno spacecraft may be maneuvered to capture groundbreaking data. From these unique vantage points, spacecraft extend the comet’s observation window long after it disappears from Earth’s skies.

Final Thoughts

As Comet 3I/ATLAS glides through our Solar System, we're reminded of the vast and mysterious universe. This cosmic traveler from a distant star system shaped by forces we may never fully understand invites us to ponder the unknown. And by sheer luck, we are here at the right time to witness it. Soon, 3I/ATLAS will vanish into the cosmic void, never to return. So keep your eyes on the skies, as we witness history in motion. And who knows? The discovery of Comet 4I might be just around the corner.