Pre-perihelion detection of a wobbling high-latitude jet in the interstellar comet 3I/ATLAS

TL;DR

The paper reports the first detection of periodic jet-angle modulation in the interstellar comet 3I/ATLAS, interpreted as precessional motion of a high-latitude jet around the sky-projected spin axis. Using 37 nights of imaging with Laplacian filtering, the authors detect a faint jet on seven nights and measure its position angle (PA) at $r=6000$ km from the nucleus, finding a PA modulation centered near $PA\approx280^{\circ}$. Phase-dispersion analysis yields a jet-PA period of $7.74 \pm 0.35$ h, which implies a nucleus rotation period of $P_{\mathrm{rot}} = 15.48 \pm 0.70$ h if the source is near a rotational pole, slightly shorter than concurrent photometric estimates. The sky-projected spin axis orientation is determined as $PA = 280.7 \pm 0.2^{\circ}$, and the results provide a purely morphological method to constrain spin-axis orientation in an extraterrestrial object, informing models of activity and nongravitational torques in interstellar comets.

Abstract

We present observations of the detection of a faint high-latitude jet in the inner coma of comet 3I/ATLAS that coincides with the broad plume detected in visible images along PA $280 \pm 10^{\circ}$. A detailed analysis shows that the jet was clearly detected on seven nights (2025, August~3, 5, 18, 19, 21, 24, and~29). The jet maintains an almost, though not perfectly, constant position angle (PA) throughout these epochs. High-precision PA measurements at a projected distance of 6000~km from the cometary optocenter reveal a periodic modulation centered at ~ 280 degrees, consistent with a high-latitude jet undergoing precessional motion around the sky-projected spin axis of the nucleus. This is the first periodic jet-angle modulation detected in an interstellar comet. The derived periodicity of $7.74 \pm 0.35$ h may imply a nucleus rotation period of $P_{\mathrm{rot}} = 15.48 \pm 0.70$ h if the jet originates from a single active source near one of the poles. This value is slightly shorter than the period of $P_{\mathrm{rot}} = 16.79 \pm 0.23$ h derived from the photometric time series. From the measured PA range, the sky-projected orientation of the spin axis is derived as $\mathrm{PA} = 280.7 \pm 0.2^{\circ}$

Figures (4)

• Figure 1: Laplacian-filtered images of the inner coma of the interstellar comet 3I/ATLAS (C/2025 N1). Thin purple lines indicate the PA of the detected jet (median value when multiple measurements are available), calculated at a projected distance of 6000 km from the comet’s optocenter (dotted red circle). For each frame, the observation date (YYYY–MM–DD) and the start and end times in UTC are shown above the panels, together with the total number of sidereal-tracking exposures and the cumulative integration time. The projected velocity vector (red arrow) and the antisolar direction (yellow arrow) are marked, as well as the image scale and orientation. Degraded orange lines denote the projected spin-axis direction (PA = $280.7\pm0.2^{\circ}$), derived from mean value of the measured jet PAs, while yellow lines trace the tail direction. Red crosshairs mark the comet’s optocenter. Isophotal contours of the original (unfiltered) images are overplotted using ten logarithmically spaced levels between the 20th and 95th percentiles of pixel intensity in each frame. The jet refers to the narrow, linear, black feature extending roughly northwest in the filtered images. It originates on the sunward side of the nucleus, in contrast to the dust tail, which points in the antisolar direction.
• Figure 2: Phase angle of the PA of jet in the inner coma of 3I, measured at 6000 km from the cometary optocenter and phased with the calculated $7.74 \pm 0.35$ h period. The horizontal red line is the mean value of PA angles representing the sky-projected orientation of the spin axis.
• Figure 3: Unprocessed image (left) and Laplacian-filtered image (right) of the inner coma of the interstellar comet 3I/ATLAS (C/2025 N1). The observation date (YYYY–MM–DD), start and end times (UTC), number of sidereal-tracking exposures, and total integration time are indicated above the panels. The projected velocity vector (red arrow) and antisolar direction (yellow arrow) are shown, together with the image scale and orientation. Red crosshairs mark the comet’s optocenter. Isophotal contours of the original (unfiltered) frame are overplotted on the filtered image (right) using ten logarithmically spaced levels between the 20th and 95th percentiles of the pixel-intensity distribution. The jet refers to the narrow, linear, black feature extending roughly northwest in the filtered image. It originates on the sunward side of the nucleus, in contrast to the dust tail, which points in the antisolar direction. The red dotted circle marks a projected radial distance of 6000 km from the optocenter, at which the jet PA is measured.
• Figure 4: Left: Polar Laplacian-filtered image of 3I from the August 19 observations (the inset shows the corresponding sky-plane filtered image). From the polar-filtered frames, PA profiles were extracted at several radial distances within $\pm250$ km (dotted red lines) around 6000 km from the optocenter (solid red line). At each radius, the PA of the jet was measured by fitting a parabola to the local intensity minimum in the jet region. Right: Linear regression of the measured PAs as a function of radius yields the final PA jet value (yellow line) with errors (dotted yellow lines) at $r = 6000$ km.