— A small comet from another star system, cataloged as 3I/Atlas, is making its nearest sweep past Earth this week, reaching roughly 167 million miles (269 million kilometers) away on Friday (Dec. 19, 2025). Discovered by the ATLAS survey in July, the ice-rich visitor is estimated at between 1,444 feet (440 meters) and 3.5 miles (5.6 kilometers) across and is already dimming as it departs the inner solar system. NASA and ground observatories are tracking the object with space telescopes and backyard observers are being urged to view it while it remains visible. Scientists say the comet will pass much closer to Jupiter in March and will exit to interstellar space again in the mid-2030s.
Key Takeaways
- 3I/Atlas will make its closest approach to Earth on Friday, Dec. 19, 2025, at about 167 million miles (269 million km), according to published tracking data.
- Size estimates range from roughly 1,444 feet (440 m) to 3.5 miles (5.6 km), making it substantially larger than many near-Earth asteroids cataloged by surveys.
- The comet was discovered in July 2025 by NASA’s ATLAS survey in Chile while scanning for hazardous asteroids.
- This is the third confirmed interstellar visitor recorded in the solar system after 1I/‘Oumuamua (2017) and 2I/Borisov (2019).
- Observers and NASA report the object is fading as it leaves the inner solar system, narrowing the viewing window for amateur telescopes and research instruments.
- Trajectory projections show a nearer approach to Jupiter in March (within about 33 million miles / 53 million km), which will further alter its outbound path.
- Experts estimate the comet will re-cross into interstellar space by the mid-2030s and is not expected to return.
Background
Interstellar objects are bodies that formed around other stars and later wandered into the solar neighborhood. The first confirmed interstellar object, 1I/‘Oumuamua, was discovered by the Pan-STARRS survey in Hawaii in 2017; 2I/Borisov followed in 2019 after discovery by an amateur astronomer in Crimea. These detections reshaped how astronomers search for and study transient visitors arriving from beyond our system.
Surveys such as ATLAS (Asteroid Terrestrial-impact Last Alert System) and Pan-STARRS are optimized to spot moving objects against the starry background. ATLAS reported spotting 3I/Atlas while scanning for potentially hazardous asteroids in July 2025. Because interstellar objects typically have hyperbolic orbits and high incoming velocities, they spend comparatively little time bright enough for detailed study, so early detection is crucial for follow-up observations.
Main Event
When ATLAS flagged the object in July, observers worldwide coordinated to refine its orbit and physical properties. Follow-up photometry and astrometry allowed teams to classify the body as an interstellar comet and to estimate its size range at roughly 440 meters to 5.6 kilometers. That wide range reflects uncertainties in the comet’s reflectivity (albedo) and the limit of visible-light measurements as the object fades.
As 3I/Atlas approaches and then recedes from the Sun, gas and dust released by warming ices create a visible coma and tail that telescopes can monitor. By mid-December 2025 the comet was already dimming as it moved away, prompting appeals from the amateur community to observe it while it remains accessible. NASA has scheduled observations with several space-based telescopes and continues to update ephemerides via its tracking centers.
Orbital solutions show that the comet’s trajectory will bring it much closer to Jupiter in March, with a closest approach on the order of 33 million miles (53 million km). That planetary encounter will impart gravitational perturbations that determine the object’s outbound path, ultimately placing it back on an escape trajectory to interstellar space in the years ahead.
Analysis & Implications
The arrival and study of 3I/Atlas add to a growing, though still tiny, sample of interstellar material available for direct observation. Each visitor provides a data point about the variety of small bodies formed around other stars: their sizes, compositions and dynamical histories. A measured size range from 440 m to 5.6 km suggests significant diversity compared with previously observed interstellar objects.
Because interstellar comets likely formed in environments with different stellar ages, metallicities and dynamical histories, spectroscopy and dust analysis can test models of planetesimal formation beyond the Sun. If 3I/Atlas shows compositional markers distinct from typical Oort Cloud comets, that would strengthen arguments for diverse formation pathways across the Milky Way.
Practical implications are modest: the object poses no impact hazard and will leave the solar system in the coming decade. But scientifically, each additional interstellar visitor improves orbital detection techniques and motivates investments in fast-response observing networks able to capture fleeting windows of opportunity. The likely Jupiter encounter in March also offers a natural laboratory for observing how giant-planet encounters alter the trajectories of interstellar interlopers.
Comparison & Data
| Object | Year | Estimated size | Closest Earth approach |
|---|---|---|---|
| 1I/‘Oumuamua | 2017 | ~100–1,000 m (uncertain) | Detected at ~85 million km from Earth |
| 2I/Borisov | 2019 | ~0.4–1.0 km | Observed from beyond Mars’ orbit |
| 3I/Atlas | 2025 | 440 m–5.6 km | ~167 million miles (269 million km) |
The table places 3I/Atlas in context: its estimated size range is larger than earlier interstellar visitors’ lower estimates, but uncertainties in albedo drive wide bounds. The relative distances of detection reflect detection limits and geometry; brighter or closer objects are easier to characterize. Continued photometry and spectroscopy will narrow physical estimates for 3I/Atlas.
Reactions & Quotes
“It will be the mid-2030s before it reaches interstellar space, never to return.”
Paul Chodas, Director, NASA Center for Near Earth Object Studies (official)
Chodas’ comment frames the comet’s long-term trajectory: although the object is making a close sweep of the inner system, gravitational interactions will carry it back to interstellar space over the coming decade.
“NASA continues to aim its space telescopes at the visiting ice ball as amateur observers try to catch it from their backyards.”
NASA mission update (official)
NASA’s observing campaign emphasizes both professional instruments and community science; as the comet dims, coordinated observations become more valuable to capture compositional and dust-release data before signals fall below detection thresholds.
Unconfirmed
- Origin star system: while models suggest 3I/Atlas came from an older star system, its exact origin cannot be determined from current trajectory and velocity data.
- Precise size and composition: the 440 m–5.6 km size range is provisional and depends on assumptions about surface reflectivity and ongoing outgassing that remain unmeasured.
- Effect of the March Jupiter encounter: detailed changes to the comet’s spin state and mass loss during that flyby are speculative until post-encounter observations are completed.
Bottom Line
3I/Atlas is a scientifically valuable but transient visitor from another star, offering a rare chance to study material formed beyond the solar system. At roughly 167 million miles on closest approach, it poses no hazard to Earth but is close enough for telescopes to extract meaningful data while it remains visible.
The object reinforces the importance of wide-field surveys and rapid follow-up networks: each interstellar detection improves orbital models, observation planning and our understanding of planetesimal populations across the galaxy. Observers have a narrowing window; continued professional and amateur monitoring through the coming weeks and the March Jupiter encounter will be critical to refine estimates and deepen the scientific payoff.
Sources
- ABC News / Associated Press (newswire)
- NASA Center for Near Earth Object Studies (official tracking & analysis)
- ATLAS (Asteroid Terrestrial-impact Last Alert System) project (observatory survey)
- Pan-STARRS (survey that discovered 1I/‘Oumuamua) (astronomical survey)
- Minor Planet Center (international small-body reporting center)