A comet from another star system is doing something astronomers did not expect. As interstellar comet 3I/ATLAS swept through the Jupiter region of our solar system in early 2026, the European Space Agency’s Juice spacecraft captured an image showing a strange spike of material jutting out from the comet’s dusty envelope, pointed directly at the sun.
That sunward protrusion defies the textbook shape of a comet’s coma, the hazy cloud of gas and dust that normally surrounds a comet’s nucleus in a roughly symmetric halo or streams behind it in a tail pushed by solar radiation. Instead, 3I/ATLAS appears to be throwing material toward the sun, and no one is entirely sure why.
What Hubble and Juice have revealed
The observational record for 3I/ATLAS stretches back to mid-2025. The Hubble Space Telescope captured its first detailed image of the comet on July 21, 2025, showing a compact coma and a developing tail but nothing resembling the sunward spike that would appear months later.
Between December 2025 and January 2026, a follow-up Hubble campaign used a nucleus extraction technique to separate the comet’s core from its surrounding dust cloud. That analysis, led by astronomer Man-To Hui of the Macau University of Science and Technology and accepted for publication in The Astrophysical Journal Letters (available as an arXiv preprint), estimated the nucleus at roughly 1.3 kilometers in radius, assuming a standard geometric albedo of 0.04. The study also detected photometric variability and an opposition surge in the comet’s brightness behavior, both hints that the surface may be rough or porous.
Then came the Juice observations. The spacecraft’s JANUS camera produced an image in February 2026 that clearly showed the sunward protrusion. More striking still, the onboard ultraviolet spectrometer (UVS) detected water vapor streaming preferentially toward the sun, the opposite of what solar radiation pressure would normally dictate. The UVS also picked up elements and dust extending more than 5 million kilometers from the nucleus, according to ESA’s published summary of the findings.
“The water vapour was released preferentially in the sunward direction,” noted Juice project scientist Olivier Witasse in ESA’s summary of the results, calling the asymmetry an unexpected feature that sets 3I/ATLAS apart from comets typically observed in our solar system.
That directional bias in outgassing is unusual. Most comets shed water vapor and dust more or less evenly across their sunlit hemisphere, or trail it into a tail blown away from the sun by the solar wind. For 3I/ATLAS, the strongest water signal points sunward, suggesting that activity may be concentrated on a specific patch of the nucleus rather than distributed across the whole surface.
A fleet of instruments, one comet
Juice and Hubble were not working alone. NASA’s Mars Reconnaissance Orbiter observed 3I/ATLAS in early October 2025, when the comet was favorably positioned, and mapped hydrogen and hydroxyl in the coma from Mars orbit. Those detections confirmed that water ice sublimation was driving the comet’s activity, consistent with the Juice ultraviolet results months later.
Pre-discovery data from NASA’s TESS mission provided a longer baseline of brightness measurements through co-added imaging, while the NSF-DOE Vera C. Rubin Observatory contributed discovery-era photometry that quantified how much of the comet’s visible light came from dust versus the nucleus itself. Rubin’s early observations confirmed that telescopes were seeing mostly coma, not the solid body underneath, a distinction that matters when interpreting any apparent structural feature like the protrusion.
Together, these datasets paint a picture of an intrinsically active body whose visible appearance is shaped overwhelmingly by its envelope of gas and dust, not by the tiny nucleus at its center.
The protrusion nobody can explain yet
For all the data collected so far, the sunward spike remains a puzzle. No published study has confirmed whether it is a physical jet of dust and gas erupting from a localized vent, a trick of viewing geometry, or something else entirely. The Juice JANUS image shows the feature clearly enough to draw scientific attention, but without spectral analysis of the protrusion itself, its composition is unknown.
One hypothesis involves Jupiter’s gravity. The comet’s passage through the Jupiter region raises the possibility that tidal stresses could amplify pre-existing asymmetries in the nucleus, driving focused dust ejection in a preferred direction. That idea aligns loosely with the observed sunward vapor bias and the photometric variability detected by Hubble, which together suggest activity concentrated in specific surface regions. But no detailed gravitational modeling of the encounter has appeared in the peer-reviewed literature. Without that work, any link between Jupiter’s gravity and the protrusion remains informed speculation, not established science.
The timeline of the feature’s development is also unclear. TESS and Rubin captured the comet’s brightness behavior before the Jupiter flyby, but neither instrument was designed to resolve spatial features at the scale of the protrusion. Whether the structure existed before 3I/ATLAS entered Jupiter’s gravitational neighborhood, or appeared only during or after closest approach, is a question current data cannot answer.
Another possibility that scientists have not ruled out: the nucleus could be fragmenting. Cometary breakups are not uncommon when solar heating intensifies or gravitational stresses increase, and a fragment trailing sunward could mimic the appearance of a jet or protrusion in imaging data. No evidence of fragmentation has been reported, but the scenario illustrates how many explanations remain on the table.
Why interstellar comets matter
Before 3I/ATLAS, only two interstellar objects had ever been confirmed passing through our solar system: 1I/’Oumuamua, detected in 2017, and 2I/Borisov, spotted in 2019. ‘Oumuamua was small, inactive, and enigmatic, offering almost no compositional data before it sped away. Borisov looked more like a conventional comet, with a coma and tail driven by carbon monoxide and water ice, but it was observed at relatively large distances and with limited instrument diversity.
3I/ATLAS is the first interstellar comet observed by a dedicated planetary science spacecraft (Juice), the first studied simultaneously from Earth orbit, Mars orbit, and deep space, and the first to display a feature as structurally odd as the sunward protrusion. Each interstellar visitor carries disproportionate scientific weight because these objects formed around other stars and carry material that has never been processed by our sun. Studying their composition and behavior offers a direct window into the building blocks of planetary systems beyond our own.
The strong sunward bias in water vapor production, the possible jet-like protrusion, and the opposition surge behavior could indicate that 3I/ATLAS has an unusual surface texture, volatile inventory, or rotational state compared with typical solar system comets. Or these traits might be common among interstellar visitors and have simply gone unnoticed because such objects are so rare and so difficult to observe.
A narrowing window before 3I/ATLAS vanishes forever
3I/ATLAS is not coming back. It travels on a hyperbolic trajectory that will carry it out of the solar system and back into interstellar space, as NASA’s overview of the object emphasizes. Every observation collected now is the only chance scientists will ever have to study this particular body from another star.
The observation window is narrowing through mid-2026 as the comet recedes from the sun and fades. Additional Hubble imaging, continued monitoring by Juice as orbital geometry allows, and any remaining opportunities for ground-based spectroscopy could help clarify whether the protrusion is a transient jet, a persistent structural feature in the coma, or an artifact of viewing conditions. Researchers including Hui’s team are also hoping to refine estimates of the nucleus’s rotation rate, which could reveal whether a spinning vent is responsible for the sunward spike.
Whatever the final explanation turns out to be, 3I/ATLAS has already reshaped expectations about what interstellar comets can look like and how complex their behavior may be as they plunge through our solar system once and then vanish into the dark between the stars.
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*This article was researched with the help of AI, with human editors creating the final content.
