CAMBRIDGE, Mass., Jan 16, 2026, 05:23 (EST)
- A new research note says 3I/ATLAS will line up unusually closely with the Earth-Sun axis on Jan. 22
- Separate analysis puts the comet’s post-perihelion rotation period at about 7.1 hours
- NASA says the interstellar comet poses no danger to Earth
Astronomers say interstellar comet 3I/ATLAS will slip into a rare viewing geometry on Jan. 22, putting it nearly in line with the Sun and Earth and giving researchers a tight window to probe the dust it sheds.
The timing matters because the geometry can amplify subtle light-scattering effects that are hard to measure at other angles, and because 3I/ATLAS is already outbound and fading. The goal is to use the alignment to tease out basic properties of the dust — how reflective it is and how it is packed — from ground-based observations.
A second new analysis, released separately, estimates the comet’s rotation at roughly seven hours, a result that could help observers separate changes driven by spin from shifts caused by the viewing angle.
In a note posted online, Harvard astronomer Abraham Loeb and co-author Mauro Barbieri said 3I/ATLAS will reach a near-opposition alignment — meaning Earth sits almost between the Sun and the object — at about 13:00 UTC on Jan. 22. The phase angle, a measure of that alignment, drops to about 0.69 degrees, and stays below 2 degrees for roughly a week, they wrote. Loeb called it “a narrow but well-defined observational window,” though the object is expected to be faint at around magnitude 16.7. (Medium)
Loeb also reported that a separate paper he co-authored with Italian observer Toni Scarmato found the comet’s post-perihelion rotation period is about 7.1 hours, using changes in jet orientation and brightness. “The combined data supports a post-perihelion rotation period of about 7.1 hours,” Loeb wrote. (Medium)
NASA says 3I/ATLAS is the third known object confirmed to have entered the solar system from interstellar space, after 1I/‘Oumuamua in 2017 and 2I/Borisov in 2019. The space agency says the comet poses no threat to Earth and will come no closer than about 1.8 astronomical units — roughly 170 million miles (270 million km) — while it continues out of the solar system on a hyperbolic path. (NASA Science)
The attraction for scientists is simple: interstellar comets carry material formed around other stars. Dust grains, in particular, can hold clues about chemistry and how planets form in different environments.
The Jan. 22 geometry may help because objects near opposition can show an “opposition surge” — a jump in brightness when the Sun is almost directly behind the observer — and because polarimetry, which measures how light waves line up, can hint at grain size and structure. Those signals are usually tangled up with changing activity and viewing angles.
But the window comes with problems. The object is faint, the surge can be subtle, and poor weather or patchy coverage can ruin time-series measurements. In their draft paper, Scarmato and Loeb said “future dense and contemporaneous monitoring is required to reduce systematic uncertainties,” noting possible aliasing — false periods caused by gaps in data.
For observers, the practical task is to watch for changes in brightness and polarization around the alignment and compare them with what the comet does on nearby nights, when the phase angle is larger. That is the only clean way to separate the geometry-driven spike from normal swings in comet activity.
If the measurements work, they could add one more hard data point on a class of objects astronomers have only just started to catch — and only briefly, before they head back into the dark.
