Nucleus and Postperihelion Activity of Interstellar Object 3I/ATLAS Observed by Hubble Space Telescope

TL;DR

This work reports the first detection of the nucleus of the interstellar object 3I/ATLAS using postperihelion HST imaging and nucleus-extraction techniques. It combines a conservative upper bound from coma subtraction with a direct nucleus extraction to determine a nucleus radius of $R_{\rm n} = 1.3 \pm 0.2$ km (assuming $p_{V}=0.04$) and cross-validates this with a nongravitational-acceleration-based estimate of $R_{\rm n} \approx 1.5$ km. The study also characterizes postperihelion dust scattering, finding an opposition surge of about $0.2$ mag with a width of $3^{\circ}\pm1^{\circ}$ and a linear phase slope $\beta_{\alpha} = 0.026 \pm 0.006$ mag deg$^{-1}$, and reports a postperihelion activity index of $n = 4.5 \pm 0.3$. Together, these results imply that 3I-like interstellar objects may be more common and detectable than previously thought and suggest that several such objects could have passed through the inner solar system undetected prior to current wide-field surveys.

Abstract

We report the successful detection of the nucleus of interstellar object 3I/ATLAS, achieved by applying the nucleus extraction technique to our Hubble Space Telescope (HST) observations from December 2025 to January 2026. The product of the V-band geometric albedo, $p_V$, with the physical cross-section of the nucleus is $0.22 \pm 0.07$ km$^{2}$, which corresponds to an effective radius of $1.3 \pm 0.2$ km if assuming $p_{V} = 0.04$, as is typical for cometary nuclei in the solar system. This size is in agreement with our estimate derived from the reported nongravitational effect and activity of the interstellar object. If the measured photometric variations are solely due to the rotation of an aspherical nucleus, the axis ratio must be $2:1$ or greater, and the rotation period $\gtrsim\!1$ hr. Leveraging the range of covered phase angles, we identified a significant opposition surge of $\sim\!0.2$ mag with a width of $3^\circ \pm 1^\circ$, which may include concurrent contributions from orbital plane crossing and tail projection, and determined a linear phase slope of $0.026 \pm 0.006$ mag degree$^{-1}$ for the coma dust. Compared to the preperihelion brightening trend, 3I faded more rapidly on the outbound leg, following an activity index of $4.5 \pm 0.3$, not unusual in the context of solar system comets. This activity asymmetry is further corroborated by a postperihelion coma surface brightness profile that is significantly shallower than its preperihelion counterpart. From the statistics, we infer that multiple interstellar objects resembling 3I likely went undetected even before the discovery of 1I/`Oumuamua.

Figures (5)

• Figure 1: Best-fit parameters for the surface brightness profile of the coma as functions of azimuth (measured from J2000 celestial equatorial north in an anticlockwise direction). The fits were performed within an annular region between 6 pixels (024) and 30 pixels (120) from the photocenter of 3I/ATLAS. Panels represent results from (a) Visit 2 on 12 December 2025, (b) Visit 3 on 27 December 2025, (c) Visit 4 on 7 January 2026, (d) Visit 5 on 14 January 2026, and (e) Visit 6 on 21 January 2026. Data from individual exposures are color coded. Solid lines represent the results of a median smoothing applied to the best fits with an azimuthal window of 30.
• Figure 2: Detection of the nucleus of 3I (third panel from the left) via subtraction of the best-fit coma model (second panel) from the observed HST images (first panel) for each HST visit (indicated at right). For illustrative purposes, the shown images are median-combined from individual exposures within each visit and displayed with a logarithmic intensity stretch. In each row, the red and magenta arrows indicate local north and east, respectively, in the J2000 equatorial coordinate system, with the projected antisolar direction and the negative heliocentric velocity of 3I represented by the yellow and cyan arrows, respectively. The horizontal white bar near the bottom marks a scale of 1 in apparent length.
• Figure 3: Comparison between the observed radial surface brightness profile (dark red diamonds) and the best-fit model (light blue dashed-dotted line) for the first exposure of 3I/ATLAS from (a) Visits 2 on 12 December 2025, (b) Visit 3 on 27 December 2025, (c) Visit 4 on 7 January 2026, (d) Visit 5 on 14 January 2026, and (e) Visit 6 on 21 January 2026. The best-fit coma and nucleus components are shown as pink solid and dark red dashed-dotted curves, respectively. Formal $1\sigma$ uncertainties are plotted as well but are largely obscured by the markers at the displayed scales. In each panel, the top and bottom sub-panels display the same data on log-log and linear scales, respectively. Grey vertical dotted lines mark the inner and outer boundaries (6 and 30 pixels, or 024 and 120, respectively) of the annular region used for fitting the coma. Results for the subsequent exposures are visually consistent and are therefore omitted for clarity.
• Figure 4: Temporal apparent magnitude variations of the nucleus of 3I across the HST visits. The horizontal dashed line in each panel marks the mean value computed from the respective visit.
• Figure 5: Left: Apparent $V$-band magnitude of postperihelion 3I/ATLAS as a function of heliocentric distance across different apertures of fixed linear radii (colour coded and labelled). Right: $V$-band magnitude correction versus phase angle, highlighting the opposition effect. Best-fit models incorporating the linear-exponential phase function are drawn as dotted curves.