The third known interstellar object will reach its closest point to the Sun this week. Astronomers are racing to analyse its unusual features, odd behavior and what it might reveal about the cosmos.
What Is 3I/ATLAS?
- 3I/ATLAS (also designated C/2025 N1 (ATLAS)) was discovered on July 1, 2025 by NASA’s ATLAS (Asteroid Terrestrial-impact Last Alert System) telescope in Chile.
- The “3I” prefix marks it as the third confirmed interstellar object (objects not bound by the Sun’s gravity) ever observed, following 1I/ʻOumuamua in 2017 and 2I/Borisov in 2019.
- Its orbit is strongly hyperbolic with an eccentricity around 6 or more — much higher than previous interstellar objects.
- At perihelion (closest approach to the Sun), it is projected to be about 1.36 AU (~203 million km) from the Sun — between the orbits of Earth and Mars.
Why Scientists Are Intrigued
Several key anomalies have made 3I/ATLAS a standout object worthy of intense study:
- Unusual composition: Observations with the James Webb Space Telescope and other facilities indicate a coma dominated by carbon dioxide and a ratio of CO₂ to H₂O far higher than any comet seen in our Solar System.
- Early activity: It began releasing water vapour (detected via hydroxyl emissions) while still far from the Sun — a region where typical comets are inactive.
- Tail and anti-tail behaviour: Initially, 3I/ATLAS developed an “anti-tail”—a tail pointed toward the Sun rather than away, which is rare. Later observations show a more typical tail configuration.
- Trajectory alignment: Its incoming direction and orbital plane are unusually aligned with Earth’s ecliptic (solar system plane), which some researchers say is improbable by chance.
What’s Special About October 29?
- On October 29, 2025, 3I/ATLAS is expected to reach perihelion, its closest point to the Sun.
- At this time, the object will experience intense solar radiation — estimated at up to ~33 gigawatts of power hitting its surface.
- This makes the date a critical observational window: scientists hope to see how an interstellar object behaves under solar heating, how its composition changes, and whether it fragments, outgasses, or shows anomalous manoeuvres.
- Observations are more difficult at perihelion because 3I/ATLAS will be near solar conjunction (i.e., roughly behind the Sun from Earth’s viewpoint), making direct ground-based viewing challenging.
What Could Happen — Natural vs. Extraordinary Scenarios
Natural comet scenario:
- As the object heats up, we may see dramatic outgassing, coma growth, tail extension, possibly fragmentation.
- The high CO₂ content may reveal different formation regions than Solar-System comets, which could inform planetary-formation science.
Extraordinary/artificial possibilities (speculative):
- Some scientists (notably Avi Loeb) propose a small but non-zero chance (up to 40 %) that 3I/ATLAS could be artificial or carry technological signatures (e.g., engineered materials, unexplained manoeuvres).
- Features such as the anti-tail, unusual composition and alignment provoke the “Black Swan” framing (rare, high-impact event).
Bottom line: The most likely explanation remains that 3I/ATLAS is a natural interstellar comet — but its remarkable properties ensure major scientific interest.
What It Means for Science & Humanity
- Galactic material insights: Being from outside our Solar System, 3I/ATLAS offers a rare sample of other stellar systems’ building blocks.
- Expansion of comet classification: If its properties diverge greatly from known comets, we may need to revise models of how comets form and evolve.
- Search for extraterrestrial intelligence (SETI implications): If artificial components were found (though no conclusive evidence yet), it would be profound—though experts caution against premature conclusions.
- Public curiosity: The unusual nature has captured media attention, expanding public interest in space science.
What to Watch in the Coming Weeks
- Monitoring of coma brightness, tail growth and fragmentation as perihelion approaches.
- Spectral analysis to refine chemical composition (especially nickel alloys, CO₂/H₂O ratios).
- Observations post-perihelion (November/December) when the object re-emerges from solar conjunction.
- Potential close passes by Venus, Earth and Jupiter in subsequent months (e.g., ~1.8 AU from Earth on December 19, 2025).
- Data releases from agencies like NASA, ESA, ground-based observatories and space telescopes.
