3I/ATLAS
3I/ATLAS
What is it?
3I/ATLAS, also designated C/2025 N1 (ATLAS) and previously A11pl3Z, is a comet-like object originating from outside our Solar System.
The prefix “3I” stands for “third interstellar” — meaning it is the third confirmed object of interstellar origin to pass through our Solar System (after 1I/ʻOumuamua and 2I/Borisov).
It’s on a hyperbolic trajectory: in other words, it is not gravitationally bound to the Sun, meaning it came in, will pass by, and will leave again.
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Discovery & Observations
It was discovered by the survey telescope network Asteroid Terrestrial‑impact Last Alert System (ATLAS) on 1 July 2025, at the Chilean El Sauce / Río Hurtado site.
Earlier archival/precovery images indicate it was being observed since around mid-June or even May, enabling a refined orbit.
Its perihelion (closest approach to Sun) was around late October 2025 at roughly 1.36 astronomical units (AU) (about 203 million km from Sun).
Physical & Chemical Characteristics
Size: Estimates vary considerably due to observational uncertainty, but the nucleus is estimated to have a diameter somewhere in the range of ~0.3 km to ~5.6 km (or even larger in some speculative reports), though the exact size is still under refinement.
Coma and outgassing: Spectroscopy (including using the James Webb Space Telescope) shows that the coma (the gas/dust envelope around the nucleus) is rich in carbon dioxide (CO₂) compared to water (H₂O). A CO₂/H₂O mixing ratio of ~8 ± 1 has been reported — among the highest observed in a comet.
Additionally, emission of H₂O, CO, OCS and dust/ice grains has been detected.
Origin & age: Based on its trajectory, it is believed to come from the thick (or old) disk of the Milky Way galaxy (i.e., from an older stellar environment) — this suggests it could be significantly older than our Solar System (which is ~4.6 billion years). Some estimates suggest ~7 billion years or more.
Why it’s scientifically important
It provides a rare opportunity to study material formed outside our Solar System — i.e., from another star system / galactic environment — giving us direct sampling of ‘exogenous’ material.
The unusual chemical composition (CO₂-rich) may indicate either different formation conditions (e.g., near a CO₂ ice-line in a protoplanetary disk) or prolonged exposure to interstellar radiation that modified its volatile inventory.
Its entrance into our Solar System offers observational windows for many telescopes (space and ground) to characterise its orbit, composition, dust environment, etc., which can improve our understanding of both comet formation and interstellar small-body populations.
Designation: 3I/ATLAS (C/2025 N1 (ATLAS))
Discovery date: 1 July 2025 by ATLAS in Chile.
Trajectory: Hyperbolic, not bound to Sun.
Closest approach to Sun (perihelion): ~1.36 AU (≈203 million km) late October 2025.
Speed: Incoming speeds ~ 58 km/s (or higher) relative to Sun.
Size estimate: Very uncertain; nucleus possibly <1 km to a few km in diameter.
Coma composition: CO₂-dominated, water present but relatively low; evidence for water ice grains.
Origin: Likely from outside our Solar System, possibly from the Milky Way’s thick disk.
What we don’t yet know / open questions
The exact nucleus size and mass: Because of the surrounding dust/gas coma, separation of the nucleus contribution is challenging.
Detailed rotational properties: e.g., rotation period, axial tilt — early photometry hasn’t yet revealed strong rotational signals.
Its exact provenance: While suspected to originate from a different stellar system or region, the precise source region (and process of ejection) remain speculative.
Long-term evolution: As it passes through our Solar System and leaves, how its outgassing, dust release, and surface changes occur will provide data but many details remain to be gathered.
3I/ATLAS
Here’s a summary of the latest orbital data for 3I/ATLAS and what this implies for observing it from Chandigarh/India — plus some tips if you’d like to try.
🔭 Orbital / visibility details
The comet’s perihelion (closest approach to the Sun) is around 1.36 AU (about 203 million km) from the Sun.
Its orbit is strongly hyperbolic (eccentricity ~6.3) indicating interstellar origin.
It will not come extremely close to Earth; the closest approach to our planet is about 1.8 AU (≈270 million km) away.
According to current data, the comet should be visible to ground-based telescopes through September 2025, after which it will pass too close to the Sun’s direction to be easily observed.
It is expected to reappear from the Sun’s glare by early December 2025 for renewed observations.
📍 What this means for India / Chandigarh region
Because the comet never comes very close to Earth and its elongation from the Sun will shrink as it approaches perihelion, it will be challenging to observe from India with naked eye. A good-sized amateur telescope will likely be needed.
Timing matters: when the comet gets too close to the Sun’s apparent path/sky, observing becomes very difficult (sunlight, low elevation, sky brightness). For Chandigarh (≈30° N latitude) you’d want to check for times when the comet has sufficient elongation (angle from the Sun) and is above horizon in dark sky.
The best window looks like it was before late September 2025, per the data. After that the “too close to Sun” issue kicks in. It then becomes visible again after Sun-conjunction in December 2025.
So if you’re planning now, December onward looks more favourable for India, assuming the object is still suitably bright and positioned above your horizon at a good angle from sunset/sunrise.
✅ Useful tips for observation
Use a planetarium/sky-map software (enter comet designation „C/2025 N1 (ATLAS)“) to check nightly RA/Dec, altitude/azimuth for your location.
Choose a dark-sky site (minimal light pollution) and check that the comet is sufficiently away from twilight/sun glare.
A telescope with good tracking and possibly a camera (CCD or DSLR) will help — the comet is likely faint compared to ordinary bright comets.
Monitor updates: as it moves, brightness may drop, and sky location will change rapidly because of its high velocity and unusual orbit.
Safety: avoid pointing near the Sun or twilight very near Sun position — risk of damage to eyes or equipment.
posted by Arshpreet Singh @ November 05, 2025
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