Lead: In early October 2025, the interstellar comet 3I/ATLAS made a close flyby of Mars, passing within about 30 million km (18.6 million mi). Multiple spacecraft operated around Mars captured the object, but NASA’s images could not be released until the U.S. government resumed operations. With the agency back at work, NASA has published photos and ultraviolet data from three missions — the Mars Reconnaissance Orbiter (MRO), MAVEN, and the Perseverance rover — providing some of the closest views of this rare visitor. The new releases refine estimates of the comet’s coma and give scientists additional chemical constraints on its origin.
Key Takeaways
- 3I/ATLAS passed within roughly 30 million km (18.6 million mi) of Mars in early October 2025, putting it within view of orbiters and a rover operating at the planet.
- MRO’s HiRISE camera imaged the comet on Oct. 2, 2025, at about 0.2 astronomical units (AU) from the craft, offering near-close visible-light views.
- MAVEN’s IUVS instrument collected ultraviolet observations between Sept. 27 and Oct. 7, 2025, identifying hydrogen sources and mapping the comet’s coma over ten days.
- Perseverance’s Mastcam-Z recorded faint, long-exposure detections of the comet from Jezero Crater on Oct. 4, 2025, requiring extended exposures that streaked background stars.
- An annotated MAVEN image indicates the combined coma and gaseous envelope measure roughly 1,500 km (932 mi) in diameter based on UV brightness patterns.
- HiRISE visible-light frames will help constrain the nucleus size and particle colors; UV spectroscopy from MAVEN set limits on hydrogen-to-deuterium ratios and other volatiles.
- The object will make its closest approach to Earth on Dec. 19, 2025, at about 2 AU, posing no hazard to our planet.
Background
Interstellar objects (ISOs) are bodies that formed around other stars and then passed through our Solar System. 3I/ATLAS is the third confirmed ISO to be tracked through the inner Solar System; previous examples include ʻOumuamua (2017) and comet C/2013 A1 (Siding Spring, which had a notable Mars encounter in 2014). Because encounters are rare and typically fleeting, each ISO provides a unique opportunity to sample material that formed beyond our system.
Missions orbiting and roving Mars have a history of opportunistic astronomy. In 2014, both MRO and MAVEN adjusted their attitudes to observe comet Siding Spring; that precedent guided similar pointing and exposure choices for the 2025 3I/ATLAS observations. The principal stakeholders in the 2025 campaign were NASA (MRO, MAVEN, Perseverance), ESA (Mars Express, ExoMars TGO) and CNSA (Tianwen-1), all of which obtained data as the object passed near Mars.
Main Event
On Oct. 2, 2025, MRO’s HiRISE camera captured visible-light images of 3I/ATLAS from roughly 0.2 AU, producing some of the highest-resolution optical views available to NASA. The orbiter rotated its pointing away from its usual surface targets to bring the comet into HiRISE’s field; those frames are expected to improve estimates of the nucleus size and particle color distribution.
MAVEN observed the comet across multiple ultraviolet bands from Sept. 27 to Oct. 7, 2025, using its Imaging Ultraviolet Spectrograph (IUVS). The UV data revealed a bright central core with color variation across the coma — white at the center, blue toward intermediate regions and purple at the outer edges — and allowed teams to identify three distinct hydrogen sources in the scene.
Perseverance recorded the comet from the Martian surface on Oct. 4, 2025, with long Mastcam-Z exposures. The comet appears as a faint smudge against streaked starfields due to deliberate long integrations; the images required careful processing to separate the comet’s signal from background noise and tracking artifacts.
NASA delayed releasing these data while the U.S. government was shut down; other agencies (ESA and CNSA) released their Mars-based images earlier. Once funding and staff returned, NASA posted the MRO, MAVEN and Perseverance products, supplementing international datasets and enabling cross-mission comparisons.
Analysis & Implications
The combined dataset strengthens constraints on both the physical and chemical properties of 3I/ATLAS. HiRISE visible-light imaging narrows the range of plausible nucleus sizes by resolving coma structure at scales not achievable from Earth, while MAVEN’s UV spectroscopy places upper limits on volatile abundances, including hydrogen-bearing species that point to outgassing water.
Hydrogen detections in the MAVEN UV frames were parsed into three components: Martian atmospheric hydrogen (bright streak), interplanetary hydrogen background and hydrogen associated with cometary outgassing. Distinguishing those components is critical for robust estimates of the comet’s water content and for inferring isotopic ratios such as hydrogen-to-deuterium, which serve as fingerprints of formation environments.
The Perseverance surface images are valuable despite their low signal-to-noise: they provide a ground-truth line of sight and a radiometric baseline for the other instruments, helping teams validate pointing, exposure corrections and background subtraction methods. Together, the three missions create a multiwavelength, multi-perspective dataset that improves modeling of the comet’s coma morphology and temporal evolution.
Longer-term, these observations contribute to the emerging field of comparative interstellar object science. With every ISO encounter we refine search strategies, rapid-response telescope scheduling and in situ observation plans — lessons that will matter if a future ISO approaches on a trajectory favorable for spacecraft interception or sample return.
Comparison & Data
| Event | Date | Closest Distance to Mars | Key Instruments | Reported Coma Size |
|---|---|---|---|---|
| 3I/ATLAS (this event) | Early Oct. 2025 | ~30 million km (18.6 million mi) | MRO HiRISE, MAVEN IUVS, Perseverance Mastcam-Z | ~1,500 km (932 mi) |
| C/2013 A1 (Siding Spring) | Oct. 2014 | ~140,000 km from Mars | MRO, MAVEN (attitude-pointed) | Variable, close approach observations |
The table contrasts 3I/ATLAS with the notable 2014 Siding Spring encounter. Unlike Siding Spring, which passed much closer to Mars, 3I/ATLAS remained farther out (tens of millions of kilometers), so high-resolution details come mainly from instrument sensitivity and multiwavelength coverage rather than proximity alone. Still, the broader coma observed in UV underscores how outgassing signatures can be extensive even when the nucleus is distant.
Reactions & Quotes
Mission scientists emphasized the rarity and value of obtaining coordinated, multi-platform views of an interstellar visitor.
“Observations of interstellar objects are still rare enough that we learn something new on every occasion. We’re fortunate that 3I/ATLAS passed this close to Mars.”
Shane Byrne, HiRISE principal investigator, University of Arizona
HiRISE leadership noted the instrument’s unique capability to resolve fine features and the operational flexibility required to capture a moving target.
“One of MRO’s biggest contributions to NASA’s work on Mars has been watching surface phenomena that only HiRISE can see. This is one of those occasions where we get to study a passing space object as well.”
Leslie Tamppari, MRO project scientist, JPL/NASA
MAVEN team members highlighted the importance of the UV detections for compositional analysis and the early stage of data reduction.
“The images MAVEN captured truly are incredible. The detections we are seeing are significant, and we have only scraped the surface of our analysis.”
Shannon Curry, MAVEN principal investigator, LASP/University of Colorado Boulder
Unconfirmed
- Any detailed hydrogen-to-deuterium ratio for 3I/ATLAS derived from MAVEN data has not been publicly finalized; preliminary upper limits were reported but full isotopic analysis remains pending.
- Claims about the comet’s precise origin region (for example, a specific stellar system) are not supported by current data and remain speculative.
- Reports of abrupt structural breakup or fragmentation outside the noted brightening episodes require further verification from ongoing photometric monitoring.
Bottom Line
NASA’s release of MRO, MAVEN and Perseverance observations fills a gap left by the agency’s temporary inability to publish during the government shutdown and adds crucial multiwavelength perspectives on 3I/ATLAS. The combined dataset refines size and coma estimates, provides spectroscopic constraints on volatiles, and strengthens cross-mission methods for opportunistic ISO science.
Although 3I/ATLAS will pass no closer than about 2 AU to Earth on Dec. 19, 2025, the Mars observations offer one of the best near-encounter looks available to date. The data exemplify how planetary assets can be repurposed for transient astronomy, and they will guide strategies for future interstellar encounters and rapid-response observations.
Sources
- Universe Today: NASA Finally Releases Images of 3I/ATLAS — (science journalism)
- NASA MRO Mission Page — (official mission page)
- NASA MAVEN Mission Page — (official mission page)
- NASA Mars 2020 / Perseverance Mission Page — (official mission page)