Lead
New astrometric and photometric reports from the Minor Planet Center and NASA’s Jet Propulsion Laboratory covering October 31–November 4, 2025 update measurements for interstellar object 3I/ATLAS after its October 29, 2025 perihelion passage. Compared with early-October data, the object brightened by roughly a factor of five in the Green band (centered at 0.464 μm) and retains a measurable non-gravitational acceleration whose net statistical significance is now about 3.7σ. The updated acceleration is somewhat reduced relative to the October 29 value but remains detectable, and the new numbers sharpen estimates of the mass-loss required if cometary outgassing is responsible.
Key Takeaways
- New post-perihelion reports cover observations from October 31 through November 4, 2025 and were released by the Minor Planet Center and NASA JPL.
- Photometry indicates a brightening of ≈5× in the Green band (0.464 μm) compared with October 5–8 measurements.
- The fitted non-gravitational signal has been reduced by about one-third from the value reported on October 29 but remains significant at ~3.7 standard deviations.
- Non-gravitational components normalized at 1 au are: radial = 1.1×10⁻⁶ au/day² and transverse = 3.7×10⁻⁷ au/day², with no measurable out-of-plane component.
- At perihelion (≈1.38 au, about 203–206 million km) the acceleration magnitude is near 9×10¹ km/day², implying strong momentum change over a passage time of order one month (perihelion speed ≈68 km/s).
- Under a natural cometary outgassing model with ejection speed v ≈300 m/s, the implied mass loss near perihelion would exceed ~13% of the nucleus mass; higher exhaust speeds (e.g., a rocket engine) reduce the required mass fraction.
- Several previously noted anomalies persist — composition, polarization, timing, trajectory alignment and brightness behavior — and remain under active scrutiny.
Background
3I/ATLAS arrived in 2025 as the third confirmed interstellar object, following 1I/ʻOumuamua and 2I/Borisov, but it differs from both in a number of notable ways. It followed a retrograde orbit whose inclination lies unusually close to the ecliptic plane (reported alignment within ~5°), and its nucleus mass estimates place it far above the previous interstellar objects while its heliocentric speed at perihelion was larger than those predecessors.
Throughout mid-2025, observers tracked photometric, polarimetric and spectroscopic signals that diverged from typical comet behavior. Between July and September 2025 there was no statistically significant non-gravitational acceleration in a dataset of 4,022 astrometric points from 227 observatories, yet other signatures — an unexpectedly blue color, a strong brightening near perihelion, and unusual compositional hints reported from emission lines — raised questions about the nature of the object and the mechanisms producing its motion and light.
Main Event
Following perihelion on October 29, 2025, the Minor Planet Center and JPL released astrometry that extends coverage through November 4. Comparing these new points to earlier October measurements shows the Green-band brightness increased by about fivefold relative to Oct 5–8 data. This photometric change is one driver behind renewed interest in the object’s activity near perihelion.
The orbit fit now includes a non-gravitational acceleration model parametrized as an inverse-square function of heliocentric distance. At 1 au the fitted parameters are a radial term of 1.1×10⁻⁶ au/day² and a transverse term of 3.7×10⁻⁷ au/day², with no significant normal (out-of-plane) component detected. When scaled to perihelion (≈1.38 au), those parameters produce a net acceleration on the order of 9×10¹ km/day², consistent with the published perihelion acceleration estimate within uncertainties.
Interpreting that acceleration as driven by anisotropic mass loss (the rocket effect) requires applying momentum conservation to connect ejected mass, exhaust speed and the observed impulse. Using the perihelion distance and measured heliocentric speed (~68 km/s) gives a characteristic perihelion passage time of order one month; combined with the measured acceleration, that yields the mass-fraction expressions discussed below.
Analysis & Implications
Under the common cometary assumption that gas leaves the nucleus at thermal speeds of order v ≈300 m/s near perihelion, the simple relation mass_fraction ≈ t·(a/v) implies a mass loss exceeding ~13% of the nucleus during the perihelion interval. A loss of that magnitude would be expected to produce a conspicuous, massive coma and extended gas cloud observable after perihelion.
However, if the expelled material had much higher exhaust velocity— for example, orders of magnitude larger than thermal speeds as would occur for high-velocity engineered propulsion— the same momentum change could be produced while expelling a far smaller fraction of the object’s mass. That logical alternative is why some observers frame the measurement as a discriminator between natural and non-natural hypotheses rather than a definitive proof of one or the other.
The observed strong blueward brightening and compositional reports (including an unusually high nickel-to-iron signature and low water fraction in some analyses) are consistent with either a specific volatile chemistry (for instance strong CO+ emission) or with material and temperature conditions that differ from typical solar system comets. Definitive compositional determination requires high-quality spectroscopy of the post-perihelion coma; planned or proposed Webb Telescope observations in December could be decisive if a dense ejecta cloud is present.
Comparison & Data
| Parameter | Value (normalized at 1 au) | Value (at perihelion ≈1.38 au) |
|---|---|---|
| Radial component | 1.1×10⁻⁶ au/day² | ≈5.8×10⁻⁷ au/day² (≈86 km/day²) |
| Transverse component | 3.7×10⁻⁷ au/day² | ≈1.94×10⁻⁷ au/day² (≈29 km/day²) |
| Net magnitude (vector) | — | ≈91–94 km/day² (depends on adopted perihelion distance) |
The table shows the two fitted components at 1 au and scaled to the reported perihelion distance of ≈1.38 au (roughly 203–206 million km). Converting au/day² to km/day² uses 1 au = 149,597,870.7 km. Small differences between published perihelion acceleration values (for example 91 km/day² vs. 94 km/day²) stem from rounding and slightly different adopted perihelion distances in separate analyses.
Reactions & Quotes
Observers and teams are treating the updated fit as a stronger but not definitive measurement. The detection significance has increased while the fitted magnitude decreased modestly, which is expected as more data points refine the solution.
The non-gravitational signal is now more robust and stands at about 3.7 standard deviations.
Avi Loeb / analysis summary
Community responses range from calls for immediate spectroscopy to requests for cautious interpretation until Webb-quality spectra are available.
I have been drawn to this topic by a spirit of wondrous curiosity and hope for open scientific exchange.
Dr. Sukanto Bhattacharya (email excerpt)
Unconfirmed
- Claims that the post-perihelion gas cloud already observed is massive enough to account for a >13% mass loss remain unconfirmed pending deep spectroscopic and imaging data.
- Reported unusually high nickel-to-iron ratios and extreme nickel-to-cyanide abundances need independent, high signal-to-noise spectroscopic confirmation to rule out calibration or line-identification issues.
- Statistical coincidences cited (alignment with the Wow! radio signal, arrival timing near planetary distances) are intriguing but remain probabilistic and do not imply causal connection without further evidence.
- Any interpretation invoking non-natural propulsion mechanisms is not supported by direct, unambiguous observational signatures at this stage and requires additional targeted observations.
Bottom Line
The newest MPC and JPL data refine but do not overturn earlier conclusions: 3I/ATLAS shows a measurable non-gravitational acceleration after perihelion and an unusually strong blue brightening. If standard cometary physics with thermal-speed outgassing applies, the implied mass loss is large (≳13% for v≈300 m/s) and should produce an obvious, massive coma accessible to spectroscopic study.
Alternatively, if follow-up observations do not find a proportionally large ejecta cloud, the community will need to consider other mechanisms — from atypical volatile chemistry and geometry to higher exhaust speeds — to reconcile the momentum change with the absence of a dense post-perihelion coma. Planned high-sensitivity spectroscopy (including possible James Webb observations) in the coming weeks will be the most direct test to distinguish these scenarios.
Sources
- Avi Loeb — Medium (analysis/opinion)
- Minor Planet Center — astrometric data repository (official observatory data)
- NASA JPL Small-Body Database / orbit solutions (official analysis)
- NASA James Webb Space Telescope — capabilities and observation planning (observatory/mission)
- Galileo Project — Harvard (research group background)