Polymele Mutual Events

Update as of June 30, 2026 - Jump to original post

After receiving additional photometric observations of Polymele from the Lucy spacecraft, the Lucy team no longer believes that the previously reported potential mutual event was, in fact, a mutual event between Polymele and its satellite. The unusual lightcurve is seen to be repeatable. The lightcurve features that led to the original hypothesis of a mutual event are likely due to Polymele’s shape seen at moderately high solar phase angles. While it remains possible that mutual events are occurring during this period, the predictions for event timings previously presented by the Lucy team are no longer valid.

Update as of May 26, 2026 - Jump to original post

The table of predicted events has been updated to include occultation events and to correct for an error in the previous calculations.

Update as of May 21, 2026 - Jump to original post

Additional Notes:

• Polymele’s rotational lightcurve is not well understood, but the period appears to be 11.5 hours, with low-phase peak-to-peak amplitude of ~0.15 magnitudes (e.g. Buie et al. 2018, DOI 10.3847/1538-3881/aabd81). Observations on successive days will be almost 2 rotations apart, so that with good relative photometric control, night-to-night comparisons should allow identification of mutual events even if they are not completely observed on the night of the event.
• Current stellar occultation constraints on the relative areas of Polymele and its satellite give predicted total mutual event depths of about 10%. This is much smaller than the 25% depth of the event (likely an eclipse of the moon by Polymele) observed by Lucy in March 2026. The difference may be due to the high (40 degree) phase angle of the Lucy observations, in which case even total events would be shallower as seen from Earth. But if the difference is due to e.g. an albedo difference between Polymele and its moon, total eclipses of the moon by Polymele may be equally deep as seen from the earth (but moon shadow transits across Polymele would be shallower).
• Transits and occultations of Polymele and is moon should be occurring in addition to the shadow events. We’re working on predictions of these, but based on the phase angle, the occultation/transit event centers should occur about 8 hours after the shadow events in late May, increasing to about 10 hours after the shadow events by late June.

Original Post - May 15, 2026

The Lucy spacecraft likely detected of a mutual shadowing event between (15094) Polymele and its unnamed satellite from 2026 March 29.441 to March 29.757 UT, with maximum depth of 25%. Orbit fits to this event and the previous occultation detections imply that the event was a shadow event, likely an eclipse of the satellite by Polymele, rather than an occultation. Therefore subsequent similar events, and their counterparts (likely transits of the satellite’s shadow across Polymele) should also currently be detectable from Earth. Additional observations of these events are strongly encouraged, as they would greatly improve the knowledge of the size and orbit of Polymele’s satellite prior to the Lucy encounter. This knowledge will not only allow for improved science planning, but will mitigate the risk of the satellite interfering with Lucy’s autonomous target tracking.

The available orbital constraints do not uniquely determine the satellite orbital period, but constrain the period to two likely values near 13.1 and 15.0 days, allowing probabilistic predictions of the mid-times of future events, given below. Probabilities decrease with time due to the expected end of the eclipse season. Event durations may currently be up to 8 hours, but will decrease as the season advances. Event depths will also decrease from the 25% depth observed by Lucy on March 29. Mid-time uncertainties are always less than 15 minutes.

Polymele’s V magnitude is currently 19.1, fading to 20.0 by mid-August.

Predicted Mutual Event Midpoints

Expand Table of Predicted Events

Original Announcement

Updated: May 26, 2026