Lead: Astronomers are closely studying interstellar object 3I/ATLAS after coordinated observations from four space telescopes in 2025 revealed a set of unexpected characteristics. First detected in early July as it traveled into the inner solar system from deep space, 3I/ATLAS is the third confirmed interstellar visitor after ʻOumuamua (2017) and Comet Borisov (2019). Data from Hubble, SPHEREx, TESS and JWST together show exceptionally high carbon dioxide relative to water, activity as far out as six astronomical units, and an unusual dust morphology lacking a clear tail. These findings complicate traditional comet classifications and create a short window for close study before the object departs the solar system.
Key Takeaways
- 3I/ATLAS was first identified in early July 2025 and is the third confirmed interstellar body observed passing through the solar system.
- Four space observatories—Hubble, SPHEREx, TESS and JWST—have collected multiwavelength data pointing to atypical composition and behavior.
- SPHEREx and JWST report an unusually high carbon dioxide–to–water ratio in the coma, described as the largest such ratio yet observed in a comet-like object.
- TESS archival imagery shows the object was already active at ~6 AU from the Sun, well beyond Jupiter’s orbit, indicating high volatility at large heliocentric distances.
- Hubble imaging captured a teardrop-shaped dust cocoon around the nucleus but no well-defined plasma or dust tail typical of solar-system comets.
- 3I/ATLAS will pass within roughly two million miles of Mars before later approaching Jupiter, creating rare opportunities for spacecraft and remote observations.
- Researchers propose two leading origin scenarios: heavy radiation processing or formation near a CO2 ice line in its parent protoplanetary disk.
Background
Interstellar visitors are extremely rare: before 2025 only two had been confirmed—ʻOumuamua in 2017 and Comet Borisov in 2019. Both objects challenged expectations in different ways, prompting rapid global follow-up and debate about their origins and compositions. The discovery of 3I/ATLAS in early July 2025 again mobilized the astronomical community because of its brightness, activity, and the availability of multiple operational space telescopes capable of rapid coordinated observations.
Cometary behavior in the solar system is dominated by sublimation of water ice and other volatiles as bodies approach the Sun. Typical comets become visibly active well inside Jupiter’s orbit; by contrast, activity beyond ~5 AU usually requires more volatile ices such as CO or CO2. The combination of instruments now applied to 3I/ATLAS—optical imaging from Hubble, infrared spectroscopy from JWST and SPHEREx, and time-series photometry from TESS—provides an unusually broad dataset to test models developed for solar-system comets.
Main Event
Initial detections in early July triggered rapid scheduling of observations across four missions. TESS archival frames showed increased brightness consistent with gas and dust release when 3I/ATLAS was about 6 AU from the Sun; that early activity was unexpected for an object arriving from interstellar space. SPHEREx and JWST then performed infrared spectral measurements that together indicate a coma chemistry markedly dominated by CO2 relative to H2O, a ratio described by investigators as the highest seen in any comet-like object to date.
Hubble imaging delivered the most visually striking result: a teardrop-shaped dust envelope surrounding the nucleus instead of a long, directed tail. That morphology suggests either very slow-moving dust grains or repeated localized jets that create a cocoon rather than a stream. Observers also report no clear ion tail caused by the solar wind, an absence that complicates straightforward comparison with familiar cometary physics.
Scientists offered two primary explanations in early analyses. One team argued 3I/ATLAS may have formed near a CO2-rich ice line in its parent system’s protoplanetary disk, producing a volatile inventory skewed toward carbon dioxide. Another group suggested prolonged exposure to cosmic radiation or stellar activity could have altered surface chemistry, releasing CO2 preferentially when warmed. Neither hypothesis is yet definitive; researchers emphasize that the combined multi-instrument dataset is necessary to discriminate among scenarios.
Analysis & Implications
The high CO2-to-water signal, if confirmed, has implications for planet-formation theory and the diversity of small bodies formed around other stars. Protoplanetary disks have radial temperature gradients that set volatile condensation zones; an object formed closer to a CO2 ice line may naturally retain more CO2 ice relative to water. If 3I/ATLAS originated from such a region, it would provide direct evidence of compositional differences among planetesimals formed at different disk radii around other stars.
Alternatively, strong radiation processing—either during a long interstellar voyage or in a harsh stellar environment—could chemically alter a body’s surface, driving off more volatile water near the surface and leaving CO2-rich layers to sublimate when warmed. That scenario would underscore the role of interstellar conditioning in shaping observable properties, meaning that interstellar visitors might not represent pristine samples of their birth environments.
Operationally, the object’s early activity at 6 AU expands the set of volatiles and physical mechanisms mission planners must consider when modeling cometary outgassing. For missions considering future intercepts of interstellar objects, 3I/ATLAS highlights the need for infrared capabilities to quantify CO and CO2 and for flexible planning to capture transient jets or morphological evolution.
Comparison & Data
| Object | Discovery | Detecting Survey/Instrument | Notable feature |
|---|---|---|---|
| 3I/ATLAS | Early July 2025 | ATLAS discovery; follow-up Hubble/JWST/SPHEREx/TESS | High CO2/H2O ratio; active at ~6 AU; teardrop dust cocoon |
| ʻOumuamua | 2017 | Pan-STARRS | Unusual shape and non-gravitational acceleration |
| Comet Borisov | 2019 | Discovered by G. Borisov | Chemistry more like solar-system comets, but different isotopic markers |
The table summarizes confirmed interstellar visitors and highlights how 3I/ATLAS adds a different chemical and morphological profile to the small sample. While the dataset remains limited, the contrast between these three cases already argues for broad diversity in interstellar small bodies.
Reactions & Quotes
Several scientists emphasized the importance of coordinated, rapid-response observations while the object remains accessible.
“The morphology and composition together make 3I/ATLAS a compelling puzzle—different enough from our comets to demand careful spectroscopic study,”
A senior scientist at a major space observatory (statement to press)
That remark was offered after initial JWST spectra were circulated to collaborating teams; the scientist stressed that spectroscopy across wavelengths is essential to separate dust and gas signals and to quantify volatile abundances. Independent commentators noted that early TESS detection of activity at 6 AU means archival planet-hunting data can be a valuable resource for identifying long-distance outgassing in other objects.
“If Mars-orbiting assets can capture higher-resolution images during the close pass, we could measure small-scale jets and constrain nucleus properties,”
A mission scientist connected with Mars Reconnaissance Orbiter (interview)
The mission scientist urged relevant teams to consider opportunistic observations from existing spacecraft, including orbiters around Mars and instruments that can point toward the incoming trajectory. Agencies are weighing requests to task assets for episodic observations while balancing primary mission priorities.
Unconfirmed
- Whether the measured CO2-to-water ratio represents the bulk composition of 3I/ATLAS or a surface-layer effect remains unconfirmed; deeper analysis and peer review are pending.
- Claims that the morphology implies artificial origin are unsupported; current interpretations favor natural processes such as localized jets or slow dust grains.
- Requests to re-task Mars and Jupiter orbiters for close imaging are under consideration but not yet formally approved.
Bottom Line
3I/ATLAS provides a rare, time-limited laboratory to study material from another star system. Its unusually high CO2 signal, early activity at ~6 AU, and atypical dust morphology expand the observable parameter space for interstellar small bodies, suggesting greater diversity than the two previously recorded cases.
Over the coming months, coordinated infrared spectroscopy and targeted spacecraft imaging—if secured—could clarify whether 3I/ATLAS is chemically distinct because of birthplace conditions or due to long-term exposure in interstellar space. Regardless of the final interpretation, this object strengthens the scientific case for flexible, rapid-response observation networks and for planning future missions that could intercept or rendezvous with interstellar visitors.