Interstellar comet 3I/ATLAS made its closest recorded approach to Earth on Dec. 19, 2025, passing about 168 million miles (270 million km) away at roughly 1 a.m. EST (0600 GMT). Observatories and spacecraft around the solar system — including NASA’s Europa Clipper, ESA’s XMM-Newton and ground-based telescopes — captured multiwavelength data before and during the flyby. The object, discovered July 1, 2025, by the ATLAS survey, will continue outward, making a distant pass of Jupiter in March 2026 and then leaving the solar system for good. Scientists are racing to analyze imaging, spectroscopy and X-ray detections to characterize the comet’s composition, rotation and activity.
Key takeaways
- Closest approach: Dec. 19, 2025 at ~1 a.m. EST (0600 GMT), distance ~168 million miles (270 million km) from Earth.
- Speed at flyby: measured on approach at roughly 144,100 mph (231,900 km/h) relative to the Sun; earlier tracking showed values near 148,600 mph (239,200 km/h) as it moved through the inner system.
- Discovery and pedigree: found July 1, 2025 by Larry Denneau via the ATLAS survey; 3I/ATLAS is the third confirmed interstellar visitor after 1I/ʻOumuamua (2017) and 2I/Borisov (2019).
- Spacecraft observations: Europa Clipper imaged 3I/ATLAS on Nov. 6 from ~103 million miles (164 million km); XMM-Newton and XRISM detected a 250,000-mile (400,000 km) X-ray glow.
- Rotation and activity: ground teams report a wobbling gas-and-dust jet that implies a rotation period of ~14–17 hours and episodes of strong outgassing after perihelion.
- Trajectory ahead: a distant Jupiter encounter is expected in March 2026 (~36 million miles / 58 million km from Jupiter); no impact risk for any planet.
- Public viewing: the Virtual Telescope Project hosted livestream coverage; weather forced a reschedule and some streams were delayed, but telescopes worldwide supplemented live observing.
Background
Objects that originate outside the solar system and pass through it are rare. Prior to 3I/ATLAS, astronomers had confirmed two interstellar visitors: 1I/ʻOumuamua in 2017 and 2I/Borisov in 2019. Each arrival offers a unique opportunity to sample material produced around other stars and to compare it with cometary populations born in our own system. The discovery on July 1, 2025 by ATLAS (the Asteroid Terrestrial-impact Last Alert System) triggered follow-up campaigns across professional observatories and in the international amateur community.
3I/ATLAS’ path through the inner solar system—its perihelion on Oct. 30, 2025, followed by a relatively close but safe passage by Earth on Dec. 19—made the object unusually accessible to space- and ground-based assets. Agencies coordinated observations in visible light, ultraviolet, X-ray and radio bands to measure the nucleus, coma, tail and any transient phenomena. The rapid mobilization reflects lessons learned from earlier interstellar encounters: such targets are visible for only a brief window and must be observed quickly to capture volatile-driven behavior.
Main event: the Earth flyby and coordinated observing
On the night of Dec. 18–19, observatories from Maunakea to Tenerife and spacecraft in transit captured the comet as it approached its closest Earth point. Ground-based teams used large optical and infrared telescopes to characterize dust and gas, while ultraviolet and X-ray instruments probed the interaction between cometary gases and the solar wind. The coordinated campaign produced time-tagged photometry and spectra that show heightened outgassing since perihelion in late October.
NASA’s Europa Clipper, en route to Jupiter, obtained imagery with its Ultraviolet Spectrograph (UVS) on Nov. 6 when the spacecraft was about 103 million miles (164 million km) from the comet. Those observations revealed a compact coma and twin-tail geometry when seen sunward, and UV spectral lines indicating oxygen and hydrogen consistent with volatile release. ESA’s XMM-Newton and the XRISM mission reported an extended X-ray emission roughly 250,000 miles (400,000 km) across, attributed to charge-exchange interactions between solar wind ions and neutral coma gases.
On the ground, the Teide Observatory team in Tenerife reported a faint, wobbling jet of gas and dust tied to the nucleus’ rotation. That feature allowed teams to infer a rotation period in the 14–17 hour range, the first time a rotation tied to active jets has been directly measured for an interstellar comet. Live public coverage from the Virtual Telescope Project was partially affected by local weather and was rescheduled in places, but many professional facilities filled coverage gaps with streamed feeds and near-real-time updates.
Analysis & implications
Compositionally, the data gathered so far reinforce that 3I/ATLAS behaves much like comets formed in our system: it shows sunlight-driven jets, common volatiles such as water-derived species, and dust that produces a visible coma and dual-tail structure. The UV and X-ray fingerprints indicate active interaction with the heliospheric environment, allowing researchers to study how pristine interstellar materials respond to a star’s radiation and wind for the first time at this level of detail.
The measured rotation and jet behavior suggest a relatively low-strength surface that responds to solar heating in predictable ways. That apparent ‘normalcy’ is scientifically valuable: it provides a baseline for comparing formation conditions in other stellar systems with those in the solar nebula. If interstellar comets are frequently similar to native comets, that has implications for models of planetesimal formation across the galaxy.
Dynamically, 3I/ATLAS’ path is hyperbolic with a future that takes it past Jupiter in March 2026 (a distant 36 million miles / 58 million km flyby) and then out of the solar system. The Jupiter passage will slightly alter its trajectory but presents no collision risk. For planetary defense and small-body dynamics, studying such high-velocity, hyperbolic visitors improves orbit-fitting techniques and helps refine rapid-response observation strategies for future interstellar arrivals.
Comparison & data
| Event | Date | Distance | Reported speed |
|---|---|---|---|
| Perihelion (closest to Sun) | Oct. 30, 2025 | — | — |
| Europa Clipper observation | Nov. 6, 2025 | ~103 million miles (164 million km) | — |
| Closest Earth approach | Dec. 19, 2025, 1 a.m. EST (0600 GMT) | ~168 million miles (270 million km) | ~144,100 mph (231,900 km/h) |
| Planned Jupiter passage | Mar. 2026 | ~36 million miles (58 million km) from Jupiter | — |
The table above summarizes the key observational milestones and distances reported by mission teams and tracking services. Small variations in reported speed reflect different reference frames (heliocentric vs. geocentric) and updates as tracking solutions improved. The X-ray and UV detections provide critical cross-checks for gas production rates derived from visible-light photometry.
Reactions & quotes
Researchers and mission teams highlighted both the routine and extraordinary aspects of the encounter.
“This unexpected view from Europa Clipper gave us a unique vantage point we hadn’t planned for.”
Kurt Retherford, SWRI (Europa-UVS PI)
Retherford’s group emphasized that spacecraft en route to other targets can opportunistically collect high-value data, extending the reach of planetary missions for comet science.
“All evidence indicates 3I/ATLAS is a comet that originated beyond our solar system.”
Amit Kshatriya, NASA Associate Administrator
NASA officials used the flyby to reiterate that extraordinary claims require extraordinary evidence; the agency framed 3I/ATLAS as scientifically important rather than anomalous. Outside the agency, planetary scientists noted the public enthusiasm and the rapid international coordination of observations.
Unconfirmed
- Claims on social media that 3I/ATLAS emitted artificial radio signals remain unverified and lack supporting telemetry from radio observatories.
- Reports of a major fragmentation event prior to perihelion have not been confirmed by spectroscopic teams or resolved imaging; observations show increased outgassing but not large-scale breakup.
- Some early speed estimates varied by several percent as different tracking centers refined orbit solutions; final published ephemerides will settle residual differences.
Bottom line
3I/ATLAS’ Dec. 19, 2025 close pass offered a rare, time-limited chance to study material from another star system with a full complement of modern instruments. Multiwavelength detections—UV, visible and X-ray—combined with ground-based photometry and imaging, are already providing a coherent picture: an active, rotating comet that in many respects resembles solar-system counterparts but carries the imprint of formation in a different stellar environment.
Data from this campaign will be mined for months and years to come, informing models of cometary chemistry, dust physics and interstellar object populations. While the visitor will continue outward and not return, the observations it left behind will help prepare astronomers for the next interstellar interloper and refine rapid-response strategies for future discoveries.
Sources
- Space.com live coverage of 3I/ATLAS flyby (news media)
- NASA Eyes on the Solar System (official NASA tracking tool)
- International Gemini Observatory / NOIRLab image release (observatory)
- Virtual Telescope Project livestreams (observatory / public outreach)
- ESA XMM-Newton mission releases (space agency / mission)
- Southwest Research Institute (Europa-UVS instrument briefings) (research institution)
- ATLAS project / University of Hawaii discovery notes (survey / academic)