2026-04-21 00:00:00 UTC (Delta: 2026-04-20)
INTERSTELLAR VISITOR: 3I/ATLAS
- Barycentric speed: 60.37 km/s (-0.01 km/s)
- Crossing the Saturn's orbit in: 80 days
🌎 Relative to Earth
- Distance (km): 982,511,073.01 km (+7,618,319.22 km)
- Distance (AU): 6.57 (+0.05)
- Light travel time: 54 min 37.40 s (+25.41 s)
2026-04-20 00:00:00 UTC (Delta: 2026-04-19)
INTERSTELLAR VISITOR: 3I/ATLAS
- Barycentric speed: 60.39 km/s (-0.01 km/s)
- Crossing the Saturn's orbit in: 81 days
🌎 Relative to Earth
- Distance (km): 974,892,753.79 km (+7,633,809.04 km)
- Distance (AU): 6.52 (+0.05)
- Light travel time: 54 min 11.99 s (+25.46 s)
"We estimated an outflow from the comet's nucleus of about two tons per second, equivalent to approximately 70 Olympic swimming pools of water vapor ejected into space every day."
2026-04-19 00:00:00 UTC (Delta: 2026-04-18)
INTERSTELLAR VISITOR: 3I/ATLAS
- Barycentric speed: 60.40 km/s (-0.01 km/s)
- Crossing the Saturn's orbit in: 82 days
🌎 Relative to Earth
- Distance (km): 967,258,944.75 km (+7,648,595.61 km)
- Distance (AU): 6.47 (+0.05)
- Light travel time: 53 min 46.53 s (+25.51 s)
2026-04-18 00:00:00 UTC (Delta: 2026-04-17)
INTERSTELLAR VISITOR: 3I/ATLAS
- Barycentric speed: 60.41 km/s (-0.01 km/s)
- Crossing the Saturn's orbit in: 83 days
🌎 Relative to Earth
- Distance (km): 959,610,349.14 km (+7,662,657.49 km)
- Distance (AU): 6.41 (+0.05)
- Light travel time: 53 min 21.01 s (+25.56 s)
2026-04-17 00:00:00 UTC (Delta: 2026-04-16)
INTERSTELLAR VISITOR: 3I/ATLAS
- Barycentric speed: 60.43 km/s (-0.01 km/s)
- Crossing the Saturn's orbit in: 84 days
🌎 Relative to Earth
- Distance (km): 951,947,691.65 km (+7,675,979.58 km)
- Distance (AU): 6.36 (+0.05)
- Light travel time: 52 min 55.45 s (+25.61 s)
2026-04-16 00:00:00 UTC (Delta: 2026-04-15)
INTERSTELLAR VISITOR: 3I/ATLAS
- Barycentric speed: 60.44 km/s (-0.01 km/s)
- Crossing the Saturn's orbit in: 85 days
🌎 Relative to Earth
- Distance (km): 944,271,712.07 km (+7,688,553.48 km)
- Distance (AU): 6.31 (+0.05)
- Light travel time: 52 min 29.85 s (+25.65 s)
We present high-resolution optical spectroscopy of interstellar comet 3I/ATLAS (C/2025 N1) obtained with the High Dispersion Spectrograph mounted on the Subaru Telescope on UT 2026 January 7, when the comet was on its outbound trajectory at a heliocentric distance of $r_{\mathrm{h}} = 2.87$ au. The spectra cover the forbidden atomic oxygen lines, [O~I], at 557.7, 630.0, and 636.4 nm. The [O~I] red-doublet intensity ratio $I_{630.0}/I_{636.4} = 2.91 \pm 0.21$ matches the optically thin branching ratio ($\sim$3; \citealt{StoreyZeippen2000}), indicating that optical-depth effects are small and that our relative flux calibration is reliable. We measure a green-to-red [O~I] intensity ratio of $G/R = I_{557.7}/(I_{630.0} + I_{636.4}) = 0.339 \pm 0.027$. This value is higher than those of most Solar System comets at similar heliocentric distances, but comparable to that of the interstellar comet 2I/Borisov. From the measured $G/R$ ratio in 3I/ATLAS, we estimate the CO$_2$/H$_2$O abundance ratio under the assumption that H$_2$O and CO$_2$ are the dominant parents of O($^1$S) and O($^1$D), with other oxygen-bearing species expected to have a smaller influence under typical conditions (e.g., \citealt{FestouFeldman1981}). The derived ratio is significantly lower than the extremely CO$_2$-rich composition reported from infrared observations on the inbound trajectory at $r_{\mathrm{h}} \sim 3.3$ au, yet higher than typical values measured for comets in the Solar System. Together with published pre- and post-perihelion measurements, our result indicates that the CO$_2$/H$_2$O ratio decreased substantially across perihelion.
Futura
Where are we now?
Jack C.M
Daniel Fischer