The properties of the (617) Patroclus binary system derived from the mutualevents of 2017–2018 and 2024–2025

The (617) Patroclus is a binary asteroid system located in the trailing Trojan group (L5) of

Jupiter. The two components, Patroclus and Menoetius (the satellite), orbit a common center of

mass and are nearly equal in size. Studies suggest that Jupiter’s Trojan asteroids, including

Patroclus and Menoetius, may have originated from the outer solar system and were later captured

into their current orbits during the migration of the giant planets (e.g., Morbidelli et al., 2005). The

binary nature of this system adds another layer of importance, as such systems are thought to form

under specific conditions. Their mutual orbit provides valuable insights into the distribution of

mass, size, and material in the early solar system (e.g., Merline et al., 2001). The Lucy spacecraft is

expected to fly by this system in 2033, marking the final encounter of its 12-year journey.

 

A particularly useful technique for studying binary asteroids involves observing mutual

events—eclipses or occultations—that occur when the orbital plane of the system aligns with both

the Sun and the observer. This alignment allows for the accurate determination of key orbital

parameters and the physical characteristics of the system, including mutual orbital elements and the

shape of the bodies. As a result, we initiated an observing campaign that ran during both the 2017–

2018 season and the 2024–2025 season.

 

We observed a total of 7 events using various telescopes across the Earth during 2017-2018.

During 2024-2025 we took advantage of the cutting edge instruments provide by Light Bridges, the

Twin Two Mtere telescopes and we were able to capture 15 complex events (occultations and

eclipses). These telescopes are locate at at the Teide Observatory of the Instituto de Astrofisica de

Canarias (IAC), that Light Bridges operates in the island of Tenerife, Canary Islands (Spain). The

observations made with TTT were performed using the Luminance filter. The log of all these

observations is shown in Table 1.

 

We retrieved the right ascension and declination of (617) Patroclus from the Minor Planet

Center. We modeled the two components, Patroclus and Menoetius, as ellipsoids and ran a two-

body problem code for various orbital parameters. To determine the best orbital solution, we

implemented a grid search algorithm. The light curves were computed by considering three

elements in the plane of sight: the projections of Patroclus, Menoetius, and the shadow cast by the

body in front. We then calculated the flux contribution from the foreground body and the portion of

its shadow covering the background body at each timestamp using the following formula:

 

 

Thanks to the new model implemented in this study, we obtained a refined orbital solution
by trying to match all the 22 events. The best match between (Figure 1) the model and the
observations was achieved using the following orbital parameters: a (semi-major axis) = 692.4 ± 5
km, e (eccentricity) = 0.018 ± 0.018, i (inclination) = 164.8° ± 0.2, L (mean longitude) = 114.13° ±
5, Ω (longitude of the ascending node) = 270.04° ± 2.3, T (orbital period) = 4.272797 ± 0.00003
days. The estimated dimensions are 118.76 × 107.3 × 104.78 km for Patroclus, and 110.7 × 96.88 ×
94.24 km for Menoetius.

Observation log: N.images refers to the number of frames acquired for each event; UT start and UT
end indicate the start and end times of each observation in UTC; details on the telescope,
instrument, filter, and exposure time (Texp) used for each observation night are also included.

 

As noted by Pinilla-Alonso et al. (2022), there is a discrepancy in the superior events when
attempting to match the model to the upper occultation data. This difference is attributed to a crater
located at the south pole of Menoetius, as proposed in that study. Our new observations are
consistent with this interpretation (e.g. Figure 2).

 

Figure 1: Example of an observed (blue) and simulated (black) light curve for an inferior mutual
event in the Patroclus–Menoetius system. The simulated lightcurve was obtained using the best
solution we found

 

Figure 2: Example of an observed (blue) and simulated (black) light curve for an superior mutual
event in the Patroclus–Menoetius system.The simulated lightcurve was obtained using the best
solution we found