Trans-Neptunian Binary Mutual Events in the 2020s and 2030s

Abstract

Mutual events of trans-Neptunian binaries (TNBs) provide rare opportunities to measure the physical and orbital properties of small bodies in the outer solar system. However, successful observations of these events have been limited by uncertain predictions. Here, we present probabilistic predictions of TNB mutual events occurring through the 2030s, using high-precision non-Keplerian orbit solutions from the Beyond Point Masses project combined with a Bayesian framework that propagates orbital and size uncertainties. Our methods generate distributions of event timing, duration, depth, and probability of occurrence, enabling direct assessment of observability. We provide predictions for five systems with ongoing or imminent mutual event seasons, including (38628) Huya, (58534) Logos-Zoe, (148780) Altjira, (469705) Kágára and !Hãunu, and (524366) 2001 XR$_{254}$. Preparing for upcoming events with long-baseline light curve monitoring is vital, as events may be difficult to distinguish from a regular rotational light curve. Rapid dissemination of event detections will benefit the entire community, allowing predictions to be updated, ensuring that these rare mutual event opportunities can be fully exploited.

Figures (4)

• Figure 1: Mutual events of Huya throughout its 2033-2043 mutual event season. Huya's pole orientation is assumed to be the same as the satellite's orbit plane rommel2025huya. This schematic does not show the effects of shadowing, which can modestly increase the depth of superior mutual events where the secondary is in the foreground.
• Figure 2: Our prediction of the light curves of Altjira's 2025-10-02 mutual event. Faint gray lines show the model light curves of posterior draws which show a mutual event (96% of the 500). Blue lines show the median light curve of the ensemble of posterior draws (including those which produce no mutual event). On the left, light curves are displayed as a function of UTC date/time, where the median light curve corresponds to the expected magnitude drop at any given UTC time. Where the median light curve is non-zero are times at which observations are more likely than not (i.e. $p>0.5$) to catch a mutual event. On the right, model light curves are indexed to the time of minimum on-sky separation, where the median light curve corresponds to the typical light curve morphology. We note the median light curve does not necessarily represent a self-consistent mutual event light curve, but rather a statistical average incorporating uncertainties/differences in event timing, depth, duration, and morphology.
• Figure 3: The predicted light curves of Altjira's 2026-09-01 mutual event, in the same style as Figure \ref{['fig:lc_good']}, though the range of potential mutual event times is much larger (several days compared to hours in Figure \ref{['fig:lc_good']}). Notably, in the left panel, there are no times at which observations are more likely than not (i.e. $p>0.5$) to catch an ongoing mutual event, despite $\sim97\%$ of statistical samples producing an observable mutual event. In the right panel, when aligning the events to sample's time of minimum separation, a distinct double-peaked mutual event is apparent. This occurs as the viewing geometry provides significant separation between the foreground component and its shadow.
• Figure 4: An example of the light curve of Altjira's 2025-10-02 mutual event if the system has a contact-binary-like light curve. In contrast with Figure \ref{['fig:lc_good']}, we display all modeled light curves---even if they do not produce a mutual event. For this date, 96% of the 500 samples produce a mutual event.