HiRISE Image Reveals Strange Lack of Coma on Fragmenting Interstellar Visitor 3I/ATLAS

NASA’s Mars Orbiter Captures Unprecedented View of Distant Interstellar Object

In a highly anticipated release, NASA officials shared long-delayed images of the mysterious interstellar visitor, 3I/ATLAS (formally designated A/2019 Y4). One particular image, captured by the powerful High-Resolution Imaging Science Experiment (HiRISE) camera aboard the Mars Reconnaissance Orbiter (MRO), revealed an unexpected anomaly that has puzzled astronomers: the object appeared to lack the dusty atmosphere typically associated with cometary disintegration.

The observation, made when 3I/ATLAS was passing near Mars’ orbit, provided a unique, close-up perspective on the third confirmed object to enter our solar system from another star system. While the object was known to be actively fragmenting—a process usually accompanied by the release of copious amounts of dust and gas forming a bright coma and tail—the HiRISE image showed a surprisingly clean, rocky appearance.

This finding challenges conventional assumptions about the composition of interstellar comets and suggests that 3I/ATLAS may be far denser and rockier than its icy classification implies.

The Unexpected Absence of a Coma

HiRISE is renowned for its ability to capture stunningly detailed images of the Martian surface, but its position in orbit around Mars also occasionally allows it to observe transient objects passing through the inner solar system. When the MRO observed 3I/ATLAS, the object was already in the process of dramatic disintegration, having fragmented into dozens of smaller pieces in early 2020.

Typically, when a comet fragments, the newly exposed ice rapidly sublimates (turns directly into gas) due to solar heating, carrying vast amounts of dust away from the nucleus. This creates the visible, fuzzy envelope known as the coma, and often a long, glowing tail. However, the HiRISE image of 3I/ATLAS showed something different.

According to analysis by scientists associated with the observation, the object, despite its disintegration, displayed no discernible dust coma or tail. The fragments were visible, but they were not surrounded by the expected cloud of fine particulate matter. This led to the conclusion that the material being shed was likely not the fine, volatile dust typical of comets.

Implications for Composition

This lack of a coma suggests several possibilities regarding the object’s nature, which is crucial for understanding the building blocks of other star systems:

• Dense, Rocky Fragments: The fragments released might be large, dense chunks of material that are too heavy to be easily blown away by solar radiation pressure, which is what forms a dust tail.
• Low Volatile Content: Despite being classified as a comet (based on its orbit and initial activity), 3I/ATLAS may have contained significantly less volatile ice than expected, possibly having been stripped of its surface ice during its long journey through interstellar space.
• Unusual Binding Material: The material holding the nucleus together might be predominantly refractory (heat-resistant) rock or metal, rather than the fluffy, icy matrix common in solar system comets.

This observation reinforces the idea that interstellar objects can be fundamentally different from the comets and asteroids native to our solar system.

Context: The Mysterious Interstellar Visitor

3I/ATLAS was discovered in December 2019 by the Asteroid Terrestrial-impact Last Alert System (ATLAS) telescope in Hawaii. It was quickly confirmed to be an interstellar visitor due to its highly hyperbolic trajectory, meaning its path indicated it originated outside our solar system and was merely passing through.

It joined the ranks of only two other confirmed interstellar objects: 1I/’Oumuamua (discovered in 2017) and 2I/Borisov (discovered in 2019).

The Fragmentation Event

Astronomers had high hopes for observing 3I/ATLAS as it approached the Sun, but in April 2020, the object began to dramatically break apart. This fragmentation was observed by multiple telescopes, including the Hubble Space Telescope, which confirmed the nucleus had split into dozens of pieces. This event was a key reason why the HiRISE observation was so critical—it offered a unique chance to view the object’s interior composition during its destruction.

While 2I/Borisov behaved like a typical comet, displaying a clear coma and tail, 3I/ATLAS exhibited behavior that was much more ambiguous, oscillating between cometary activity and asteroid-like density. The HiRISE image provides strong evidence leaning toward a more robust, rocky structure.

“The lack of a dust coma is the strange part. If it was breaking up due to thermal stress, you’d expect a lot of fine dust to be released, which would be visible,” noted one astronomer familiar with the observation. “This suggests that either the fragments are very large, or the material is simply not the fluffy, volatile ice we typically associate with comets.”

Why Interstellar Objects are Critical to Science

Observations of objects like 3I/ATLAS are invaluable because they offer a direct sample of material from outside our own stellar neighborhood. They provide astronomers with clues about the planetary formation processes occurring around other stars.

Comparing the Interstellar Trio

The three confirmed interstellar objects—’Oumuamua, Borisov, and ATLAS—have all displayed unique characteristics, suggesting that the population of objects ejected from other star systems is highly diverse:

This diversity highlights the need for continued, high-resolution surveillance of the skies to capture future interstellar visitors and refine our models of exoplanetary systems.

Key Takeaways

The HiRISE image of 3I/ATLAS provides crucial data points regarding the nature of objects traveling between stars:

• The Finding: The fragmented interstellar object 3I/ATLAS was observed by the MRO’s HiRISE camera to be lacking a visible dust coma or tail.
• The Context: This is highly unusual because the object was classified as a comet and was actively disintegrating into multiple pieces.
• The Implication: The fragments are likely composed of denser, more refractory (rocky) material rather than the typical fluffy ice and dust mixture of a solar system comet.
• The Significance: This observation adds to the growing evidence that interstellar objects are structurally and compositionally diverse, offering unique insights into planet formation outside our solar system.

Conclusion

The delayed release of the HiRISE data on 3I/ATLAS has provided astronomers with a vital piece of the interstellar puzzle. While the object’s fragmentation initially suggested a typical icy comet, the MRO’s detailed observation points toward a more robust, rocky core. As astronomers continue to analyze this data, the findings will help refine models of how planetary systems eject material and what that material looks like when it travels across the vast gulfs of interstellar space.