Polymele Mutual Events

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

Table of predicted midpoints for shadowing and occultation mutual events. Events are labeled as I or S to indicate Inferior (moon in front of primary) or Superior (moon behind primary) events, and E or T to indicate Eclipse (shadow) or Transit (solid body transit or occultation) events.   Note that currently many of the Transit (T) events are occurring with higher probability than the shadow (E) events.

    JD             Date        Probability   Type
 2461174.915   May  14.415       0.104     I E
 2461175.123   May  14.623       0.179     I T
 2461181.455   May  20.955       0.089     S E
 2461181.464   May  20.964      0.507     I E
 2461181.699   May  21.199       0.178     S T
 2461181.745   May  21.245       0.789     I T
 2461187.995   May  27.495       0.075     I E
 2461188.272   May  27.772       0.176     I T
 2461188.940   May  28.440       0.439     S E
 2461189.262   May  28.762       0.788     S T
 2461194.536   Jun. 03.036       0.057     S E
 2461194.840   Jun. 03.340       0.170     S T
 2461196.416   Jun. 04.916       0.362     I E
 2461196.774   Jun. 05.274       0.779     I T
 2461201.076   Jun. 09.576       0.048     I E
 2461201.404   Jun. 09.904       0.163     I T
 2461203.892   Jun. 12.392       0.304     S E
 2461204.278   Jun. 12.778       0.760     S T
 2461207.616   Jun. 16.116       0.036     S E
 2461207.964   Jun. 16.464       0.151     S T
 2461211.368   Jun. 19.868       0.249     I E
 2461211.776   Jun. 20.276       0.722     I T
 2461214.156   Jun. 22.656       0.028     I E
 2461214.518   Jun. 23.018       0.134     I T
 2461218.844   Jun. 27.344       0.191     S E
 2461219.267   Jun. 27.767       0.655     S T
 2461220.697   Jun. 29.197       0.021     S E
 2461221.069   Jun. 29.569       0.116     S T
 2461226.321   Jul. 04.821       0.141     I E
 2461226.751   Jul. 05.251       0.539     I T
 2461227.237   Jul. 05.737       0.016     I E
 2461227.614   Jul. 06.114       0.084     I T
 2461233.777   Jul. 12.277       0.012     S E
 2461233.797   Jul. 12.297       0.103     S E
 2461234.156   Jul. 12.656       0.056     S T
 2461234.230   Jul. 12.730       0.407     S T
 2461240.317   Jul. 18.817       0.010     I E
 2461240.694   Jul. 19.194       0.036     I T
 2461241.273   Jul. 19.773       0.073     I E
 2461241.702   Jul. 20.202       0.275     I T
 2461246.858   Jul. 25.358       0.008     S E
 2461247.228   Jul. 25.728       0.019     S T
 2461248.749   Jul. 27.249       0.054     S E
 2461249.169   Jul. 27.669       0.158     S T
 2461253.399   Jul. 31.899       0.006     I E
 2461253.758   Aug. 01.258       0.012     I T
 2461256.225   Aug. 03.725       0.036     I E
 2461256.631   Aug. 04.131       0.087     I T
 2461259.937   Aug. 07.437       0.004     S E
 2461260.286   Aug. 07.786       0.007     S T
 2461263.701   Aug. 11.201       0.025     S E
 2461264.088   Aug. 11.588       0.039     S T
 2461266.477   Aug. 13.977       0.003     I E
 2461266.809   Aug. 14.309       0.004     I T
 2461271.177   Aug. 18.677       0.019     I E
 2461271.541   Aug. 19.041       0.022     I T
 2461273.013   Aug. 20.513       0.001     S E
 2461273.325   Aug. 20.825       0.001     S T
 2461278.653   Aug. 26.153       0.013     S E
 2461278.990   Aug. 26.490       0.010     S T
 2461279.551   Aug. 27.051       0.001     I E
 2461279.842   Aug. 27.342       0.000     I T

Original Announcement

Updated: May 26, 2026