Rare Near-Opposition Alignment of 3I/ATLAS on 22 January, 2026

TL;DR

The paper identifies a rare near-opposition geometry for the interstellar object 3I/ATLAS on $22$ January $2026$ with phase angle $\alpha = 0.69^\circ$, creating an optimal opportunity to measure the opposition surge and polarimetric properties of interstellar dust. It discusses the physical origins of the surge—shadow-hiding and coherent backscatter—and articulates observational requirements to extract constraints on dust albedo $\omega_0$ and grain microstructure. By comparing with the well-studied case of 67P/C-G and noting the lack of near-opposition data for 2I/Borisov, the paper argues that 3I/ATLAS can provide novel information on interstellar dust composition and grain architecture. The work outlines a practical observing plan spanning a ±4 day window, emphasizing high-precision, multi-band photometry and polarimetry to yield unprecedented insights into the albedo, structure, and composition of interstellar matter.

Abstract

We point out that on 22 January 2026, the interstellar object 3I/ATLAS will align to within an exceptionally small angle, alpha= 0.69 degrees, with the Earth-Sun axis. This rare alignment provides unique circumstances for measuring the opposition surge and polarimetric properties of interstellar cometary dust. We characterize the alignment geometry, outline key scientific opportunities, and define the observational requirements for data collection. Observations before and after the alignment time offer an unprecedented opportunity which may not repeat for decades, for characterizing the albedo, structure, and composition of interstellar matter.

Figures (1)

• Figure 1: The angle between the Sun-Earth axis and the Sun-3I/Atlas axis in degrees, as a function of date.